Advances in spray drying of sugar-rich products

Abstract

Recent developments in the spray drying of sugar-rich products such as fruit juices, pulp and honey with and without the application of carriers, e.g., carbohydrate carriers (maltodextrin, gum arabic), prebiotic dietary fibers, proteins and natural carriers have been presented in this paper. The effect of the carrier type, carrier concentration and the spray drying process parameters on the product yield and selected final product properties are analyzed and discussed. Recent studies have proved that prebiotic dietary fibers and proteins may substitute conventional carriers, i.e., refined carbohydrates, providing high product yield and additional nutritional value at lower carrier concentration in the final product.

Keywords:

• Carriers
• prebiotic dietary fiber
• product yield
• sugar-rich food
• functional food

1. Introduction

Developments within current trends in the food industry are guided by a growing awareness of consumers with regard to a healthy lifestyle and the increasing demand for healthy and functional food products.^[1] Within the last years consumer expectations regarding food products have changed considerably: nowadays the majority of consumers consider food products not only as a source of necessary nutrients, but also as a means of avoiding certain diseases and improve their physical condition.^[2] The Word Health Organization has pointed out that nutrition-related chronic diseases are directly connected with an increased intake of sugar, starches or fat, these disease related conditions being: obesity, diabetes, cardiovascular disease, several forms of cancer, osteoporosis and dental diseases.^[3] Healthy nutrition trends in the food industry promote the development of functional foods and the improvement of food processing technology such as spray drying in order to obtain products, which apart from providing traditional nutrients may bring additional health benefits.^[1]

The spray drying process is widely applied in the production of food products in the form of powder, for example sugar-rich food products – fruit juices and honey. Spray drying of these types of products allows for increase the products shelf-life significantly reducing the moisture content and minimizing the risk of spoilage. An additional advantage of fruit juice and honey powder is a decrease in the expenditure on packaging, storage and transportation. The benefits of the powder form such as enhanced flowability, easiness of mixing with other ingredients gives possibilities for the production of new products with complex compositions, which might be applied in food, pharmaceuticals and cosmetic industries.^[4] Nowadays, healthy nutrition trends are also observed in the spray drying of food products, research attempts are made not only to preserve products such as fruit juices or honey, but also to produce powders with added value.

2. Problems in spray drying of sugar-rich products

The product properties of spray dried sugar-rich powders, for example, honey and fruit juices depends on the following process parameters: drying temperature, drying air flow rate, feed flow rate, atomization pressure, type and concentration of carrier used as shown in Figure 1.^[4]

Figure 1. Schematic of spray drying of sugar-rich food (based on^[4]).

The difficulties in spray drying of sugar-rich products such as fruit juices and honey arises from the high contents of low molecular weight compounds with a low glass transition temperature (T_g): fructose (T_g = 5°С), glucose (T_g = 31°С), sucrose (T_g = 62°С), malic acid (T_g = 11°С), citric acid (T_g = 16°С) and tartaric acid (T_g = 21°С).^[5,6] Glass transition temperatures of honey depend on the type and composition of the product and vary from 34°С to 51°С.^[7,8] Khalloufi et al. reported the glass transition temperature of pure anhydrous fruit powders such as blueberry (T_g = 15°C), blackberry (T_g = 22°C), strawberry (T_g = 29°C) and raspberry (T_g = 41°C).^[9] The glass transition temperature of 5.7°C was determined for anhydrous orange juice concentrate powder obtained without application of carriers.^[10] Moisture in the material decreases the glass transition temperature since T_g of pure water is equal to −135°С. At the temperature 10–20°C above T_g the material state is changing from amorphous to rubbery. The detailed description of glass transition temperature phenomenon has been provided by Bhandari and Howes,^[11] Balasubramanian et al.^[12] and Roos.^[13] Spray drying of materials with a low T_g is difficult due to sticking, caking and increased powder deposition at the dryer walls, which significantly reduces product yield and may deteriorate product quality.

The product yield or product recovery is the key parameter in the evaluation of the performance of the spray drying process. During sugar-rich food drying the product yield may be decreased due to powder loses caused by particle deposition on the dryer walls. The product yield is expressed as a ratio of weight of dry powder collected after drying to the initial mass of solids in the feed solution. For laboratory scale spray dryer, product yield for sugar-rich products should be above 50%.^[14] Low product yield indicates that the sticky properties of the material deny production of the free-flowing powder in spray drying.

The problems with particles adhesion to the dryer walls may be avoided if the product temperature during spray drying process does not exceed 10–20°C above the T_g.^[14] To overcome the stickiness problem during spray drying of sugar-rich materials the following methods can be applied: material-based, process-based and combination of both methods. In the material-based method the T_g of fruit juice and honey is increased by the addition of a carrier with a high T_g. The process-based methods are oriented to adjust the spray drying process parameters to avoid powder stickiness and include the following solutions: application of drying chamber surface scrapping, cooling of the spray dryer walls, decreasing of outlet air temperature to keep the product temperature at the final part of drying process below the sticky point and application of dehumidified air.

2.1. Material-based methods – application of carriers and drying aids

The addition of carriers to the feed solution is the most common practice applied in spray drying of sugar-rich foods to increase T_g and avoid the stickiness problem.

The application of a carrier might reduce the stickiness of sugar-rich powders due to the high T_g of the carrier. Figure 2 shows the comparison of T_g of the main components of sugar-rich products with T_g of different carrier types: conventional maltodextrin with different dextrose equivalent (DE) as a carrier, prebiotics with different degree of polymerization (DP) and selected proteins. The T_g of maltodextrin, which is a common drying carrier applied in spray drying of juices ranges from 100°C to 188°C and decreases for higher DE.^[6] The T_g of prebiotics depends on the type and DP: for inulin T_g is between 111–145°C,^[15] for resistant dextrin T_g varies from 115°C^[16] to 171°C,^[17] for fructooligosacharides (FOS) is in the range of 91°C to 121°C^[18] whereas for whey protein concentrate and sodium caseinate is equal to 127°C and 132°C.^[19]

Figure 2. Glass transition temperature of selected food materials (elaborated on the basis of^{[5,6,15–19]}).

The comparative characteristic of spray drying of sugar-rich foods without the carrier and with the application of different types of carriers: conventional carbohydrates, prebiotic dietary fibers and proteins are summarized in Table 1. Table 1 shows the range of carrier concentrations needed to achieve a high product yield as well as the main benefits and disadvantages of spray drying with and without the application of carriers.

Table 1. The comparative characteristic of spray drying of sugar-rich food performed without the carrier and with the application of different types of carriers and drying aids.

Carriers	Juice/honey solids to carrier solids content ratio	Product yield	Advantages	Disadvantages
No carrier	100:0	up to 66%^[^20^]	Bioactive compounds are not diluted. Natural color and taste of product are not altered. Elimination of costs related to carrier using.	Available only for limited range of products and limited range of process parameters: low air humidity and outlet temperature. Loss of volatile compounds, no encapsulation.
Conventional carriers:
Maltodextrin	From ca. 1.85 (65:35)^[^21^] to ca. 0.33 (25:75)^[^22^]	up to 85%^[^23^]	Applicable for wide range of sugar-rich products. Good solubility and low viscosity of the carrier. Retention of bioactive volatile compounds via microencapsulation.	To obtain high product yield the concentration of carrier should exceed 50% in the final product. Carrier affects the natural color and taste of food products. Not desirable for health glycemic load increase during maltodextrin digestion. Drying costs increase due to carrier using.
Gum arabic	From ca. 1.85 (65:35) to ca. 0.53 (35:65)^[^21^]	up to 82%^[^24^]	Applicable for a wide range of sugar-rich products. Retention of bioactive volatile compounds via microencapsulation.	Lower solubility of the powder due to hydrophobic characteristics of gum arabic. Lower availability.
Prebiotics:
Inulin, fructooligo-saccharides (FOS), oligofructose, resistant dextrin	From ca. 4.0 (80:20)^[^25^] to ca. 0.42 (30:70)^[^26^]	up to 75% for standard spray drying^[^26^], up to 95% for dehumidified air spray drying^[^25^]	Applicable for a wide range of sugar-rich products. Retention of bioactive volatile compounds via microencapsulation. Deliver additional nutritional value. Element of functional food.	Product yield depends on the type of prebiotic applied. For high product yield high concentration of carrier in the powder is required.
Proteins:
Whey protein isolates, whey protein concentrate, sodium caseinate, isolated soya protein (ISP), soya protein powder (SPP), soya milk powder (SMP), pea protein isolate (PPI).	From ca. 199 (99.5:0.5) to ca. 2.34 (70:30)^[^27^]	82%^[^27^,^28^]	Applicable for a wide range of sugar-rich products. Costs reduction due to decreased amount of carrier used. Retention of bioactive volatile compounds via microencapsulation. Reduced change of taste and color. Proteins are more favorable in diet than refined carbohydrates.	Product yield and reconstitution properties (wettability, solubility) of the dry powders depends on the solubility of the carrier. Proteins as carriers significantly increase the viscosity of the feed solution comparing to conventional carriers like maltodextrin and gum arabic^[^29^] which might affect atomization of feed. Carrier affects the natural taste of food products.

Spray drying without carriers allows to avoid dilution of the bioactive compounds, the changes in the natural color and taste of the food product and to reduce the process costs related to the carrier. Product yields of up to 66% for spray drying of jussara pulp without application of the carrier are reported in the literature,^[20] but spray drying without the carrier may only be performed for a limited number of products and needs adhering to restrictive process parameters: low air humidity and outlet air temperature.

Application of conventional carriers, i.e., refined carbohydrates such as maltodextrin and gum arabic has found numerous applications in spray drying of sugar-rich products due to its good solubility, low viscosity and excellent encapsulation properties. Application of carriers such as maltodextrin has favorable effect on the final product properties: increase of maltodextrin content in the powder results in decrease of particles moisture content and water activity due to increase of solids content in the feed.^[30] Additionally, higher maltodextrin content in the powder provide enhanced solubility, flowability and reduced hygroscopicity due to reduced adhesion and ambient water sorption of the carrier.^[30] The disadvantages of refined carbohydrates as the carriers are: high content of the carrier in the final product, change of natural taste and aroma of the product and reduction of the concentration of key bioactive nutrients. Moreover, increased consumption of refined carbohydrates is not desirable for human health, since glucose produced during digestion of refined carbohydrates is rapidly absorbed in the small intestine increasing the glycemic load. The increased intake of sugars found in food and beverages is directly related to nutrition associated diseases such as obesity, diabetes and high level of blood cholesterol.^[3]

Prebiotics as carriers may substitute conventional carbohydrates and deliver additional nutritional value to the final product. Prebiotics may also be applied in the production of functional food^[31] based on spray dried fruit juices and honey. Application of prebiotics as a drying carriers gives following advantages: high product yield,^[26] a better retention of the total phenolic compounds^[32] and anthocyanin^[33] in comparison to conventional maltodextrin applied as a carrier, enhanced wettability^[34] and solubility^[26] of dry powder.

Proteins as film forming materials allow one to achieve a high product yield significantly reducing the content of the carrier in the final product, which minimizes the alteration of color and taste of the food powder. One of the disadvantages of proteins being applied as drying carriers is an increase in the feed viscosity in comparison to using conventional carbohydrate carriers. For example, the viscosity of dates (Phoenix dactylifera L.) pulp with addition of gum arabic and maltodextrin as a carrier determined at temperature 33°C was ca. 2.5 Pa·s and 3 Pa·s, respectively, whereas for protein carriers added to the date pulp viscosity increased to ca. 7 Pa·s.^[29] The high viscosity of the feed solution might affect the atomization process.

The examples of application of different carrier types for spray drying of fruit juices, concentrates and pulp as well as the analysis of the effect of carrier type on the product properties are presented in the section 4.

2.2. Process-based methods – optimization of drying process

Process-based methods may be applied to prevent sugar-rich materials from sticking during spray drying by scrapping the surface of drying chamber, cooling of the spray dryer walls, application of dehumidified air at high and low temperature.

To increase product yield during spray drying of sugar-rich materials the design of dryer can be modified, for example scraped surface of drying chamber may be applied. The disadvantages of this method are additional investment costs, more complex dryer design and still high amount of carrier is required. Additionally, poor reconstitution properties of the powder after scrapped surface spray drying has been reported.^[35]

Another method to reduce particles adhesion to the dryer wall during spray drying process is application of double wall spray dryer, which enable decrease of dryer wall temperature below product T_g. Application of dryer wall cooling enable the reduction of wall deposits below 15%, however the application of carriers is also required.^[36] The disadvantages of dryer wall cooling are increase of air relative humidity in the vicinity of the wall and additional investment costs related to application of double wall drying chamber.

Application of spray drying by dehumidified air allows to reduce significantly the amount of carrier added to the feed solution and increase juice/honey solids to carrier solids content ratio up to 80:20 to achieve high product yield from 65% to 90%.^[10] The disadvantage of this method is additional investment and energy costs required for air dehumidification.

The dehumidified air spray drying may be optimized by reduction of inlet and outlet drying air temperatures to about 75°C and 50°C respectively, which substantially increase the product yield up to 84–95%,^[25] keeping high juice/honey solids to carrier solids content ratio up to 80:20. Application of low drying air temperature enable the increase in retention in bioactive compounds^[37] and reduction of energy consumption during the drying process. The disadvantages of application of low temperature spray drying combined with air dehumidification are additional investment and energy cost required to remove the moisture from the drying air as well as limited range of process parameters.^[25]

There are a limited number of publications, which focus on the improvement of spray drying performance by optimization of initial droplet atomization parameters and evaluate the effect of initial droplet size distribution on the product yield and dry powder properties.^[5] The initial droplet size distribution may be controlled in different way depending on the type of atomization method. For disk atomization initial droplet sizes may be controlled by change of rotational speed or feed rate, for pressure nozzles only feed rate may be changed for the same nozzle orifice diameter, application of two-fluid nozzle enables the control of droplet size in the spray via change of both feed rate and atomizing air flow rate. In general, large initial droplet size reduce the drying yield due to lower drying rate of large droplets, which might adhere to the dryer wall, when they are not completely dry and have low T_g due to high moisture content.^[5,23,38,39] Truong et al.^[40] investigated experimentally and theoretically the effect of initial droplet size on the maltodextrin-sucrose droplet glass transition temperature profile along the drying chamber height. For droplets below 70 µm drying time did not exceed 1 s, therefore, small particles reach T_g of ca. 60°C at the distance from the nozzle about 0.2 m. The large droplets with the size about 150 µm at the end of the drying had T_g below 20°C, which is significantly lower than particle temperature at the dryer outlet and will result in large particles adhesion to the dryer walls.^[40]

Table 2 summaries the advantages and disadvantages of process-based approach of sugar-rich products spray drying. Analysis of the Table 2 shows that the low temperature spray drying combined with dehumidified air drying provides high product yield allowing for significant reduction of carrier content in the final juice/honey powder. Below examples of application of process-based methods used to avoid sugar-rich products particles adhesion to the dryer wall and minimize particles cohesion to each other are presented and effect of drying method on the final product properties is discussed.

Table 2. The comparative characteristic of the process-based approach of sugar-rich products spray drying.

Drying method	Drying air temperature	Juice/honey solids to carrier solids content ratio	Product yield	Advantages	Disadvantages
Spray drying at scraped surface drying chamber	Inlet/Outlet temperature: 140–160°C/85–65°C^[^35^]	1 (50:50)^[^35^]	nd	Production of free-flowing powder	Additional investment cost related with complex drier design Poor reconstitution properties of the powder, additional agglomeration step is required^[^35^].
Cooling of spray dryer walls	Inlet/Outlet air temperature: 110–190°C/81.4°C; Dryer wall temperature: 44°C (below the Tg of the powder)^[^36^]	Carriers has been applied, however no data on juice solids to carrier solids content ratio provided	65% to 90%^[^36^]	Decrease of the dryer walls below Tg allows to reduce particles stickiness and reduce wall deposits below 15%^[^36^]	Lower temperature of the dryer walls results in local increase of air relative humidity, which might intensify particles deposition on the walls^[^10^] Additional investment and energy costs related to application of double wall drying chamber and walls cooling. Application of carriers is required
Spray drying by dehumidified air	Inlet/Outlet air temperature from 110°C/46–56°C to 140°C/61–70°C^[^10^]	From 4.00 (80:20) to 0.25 (20:80)^[^10^]	From 86% to 95%^[^10^]	Increase in product yield comparing to standard spray drying Possibility to reduce content of carrier in the final product.	Additional investment and energy costs required for air dehumidification.
Application of low drying temperature combined with air dehumidification	Inlet/outlet air temperature: 75°C/50°C^[^25^]	4.00 (80:20) and 1.00 (50:50)^[^25^]	From 84% to 95%^[^25^]	Increase in product yield comparing to high temperature spray drying Possibility to reduce content of carrier in the final product. Decrease of drying temperature minimizes the deterioration of product quality and decreases the loss of bioactive compounds during drying^[^25^]. Energy savings due to low drying agent temperature may compensate the increased energy demand for air dehumidification^[^25^]	Additional investment and energy costs required for air dehumidification. Spray drying at low drying temperatures can be performed only at limited range of process parameters: the moisture content in the feed and the feed rate have to be decreased to reduce the amount of water supplied to the drier for evaporation, because it is difficult to remove large amounts of water at low drying temperatures^[^37^].
Standard high temperature spray drying	Inlet/outlet air temperature: 180°C/85°C^[^25^], 130–170°C/75 °C^[^22^], 124–143°C/48–76°C^[^41^]	≤1.00 (50:50)^[^25^] From 1.00 (50:50) to 0.33 (25:75)^[^22^] From 0.48 (32:68) to 0.26 (21:79)^[^41^]	From 52.5% to 87%^[^25^] From 42.6 to 70.2%^[^22^] From 17% to 25%^[^41^]	No auxiliary equipment needed Applicable for wide range of process parameters	To obtain acceptable product yield carrier content in the final fruit or honey powder might exceed 50% (product solids to carrier solids content ratio ≤1) Product deterioration due to loss of valuable bioactive compounds during high temperature drying

To obtain bayberry juice powder Gong et al. applied a Wuxi Changming spray dryer equipped with a scrapped surface chamber at the following process parameters: inlet air temperature 140–160°C, outlet air temperature 85–65°C, juice solids to carrier (maltodextrin) solids ratio—50:50.^[35] Although, data on product yield has not been reported, the authors found that the bayberry powder has poor reconstitution properties with wettability time about 120 s, which required application of additional agglomeration stage in the fluidized bed granulator (after agglomeration wettability time decreased to 15 s).

Chegini et al. obtained concentrated orange juice powder in a co-current double spray drying chamber to decrease the wall temperature below the T_g of the product (T_g = 44°C).^[36] The authors applied inlet air temperatures ranging from 110°C to 190°C whereas outlet air temperature was ca. 81.4°C. Concentrated orange juice with addition of carriers such as maltodextrin, liquid glucose, and methylcellulose was atomized in a rotary spray dryer with disk speed 10,000–25,000 rpm and feed flow rate 150–450 ml/min. Application of cooling of the dryer walls allowed for the reduction of the powder deposition on the dryer walls below 15% and gave a product yield ranging from 65% to 90%. However, the aforementioned drying techniques have not found wide application in the industry due to low energy efficiency and high investment costs. Moreover, cooling of the spray dryer walls caused a local increase of drying air humidity resulting in the reabsorption of moisture by the dry particles.^[10]

To overcome the problem of high air humidity during spray drying at low temperatures, Goula and Adamopoulos dehumidified the air before supplying it to the drying unit. In the modified spray drying installation, air passed through an air dryer before entering the drying chamber.^[10] Application of dehumidified air made it possible to carry out the spray drying of concentrated orange juice at inlet temperatures in range from 110°C to 140°C and outlet temperatures from 46°C to 70°C, whereas T_g of obtained juice powders ranged from 31°C to 126°C. The authors reported high product recovery up to 88% even at high juice solids content to carrier solids content ratio – 4.0 (80:20).

Jedlińska et al. compared standard high temperature spray drying with low temperature spray drying in dehumidified air for drying honey solutions with resistant dextrin (“Nutriose”) as a carrier.^[25] Low inlet/outlet temperatures (75°C/50°C) of dehumidified air, increased concentration of the feed (60% of solid) and a lower feed rate resulted in the production of free flowing dry powder. The authors reported that spray drying at low drying temperatures can be performed only within a limited range of process parameters: the moisture content in the feed and the feed rate have to be decreased to reduce the amount of water supplied to the dryer for evaporation, because it was difficult to remove large amounts of water at low drying temperatures.^[25] High honey product yield (from 84.4% to 95.8%) was obtained for dehumidified air spray drying, whereas for standard high temperature spray drying (inlet/outlet air temperatures – 185°C/80°C) product yield was 52.5% and 87.5%. The same research group found that low temperature spray drying by dehumidified air has favorable effect on retention of bioactive compounds in rapeseed honey.^[37] For instance, retention of phenolic compounds in rapeseed honey powder dried with a “Nutriose” carrier (product solids to carrier solids content 80:20) was about 86.3% and antioxidant activity decreased only by ca. 22.1% after low temperature spray drying.^[37]

In the standard high-temperature spray drying process in order to obtain acceptable product yields of fruit or honey powder, concentration of the drying carrier in the final product might exceed 50% (product solids to carrier solids content ratio ≤1.00). For example, Aragüez-Fortes et al. obtained spray dried guava powder (Psidium guajava L.) with product yield 42.6–70.2% applying inlet/outlet air temperatures 130–170°C/75°C and adding maltodextrin as a carrier in the amount 50:50, 37.5:62.5 and 25:75 juice solids to carrier solids content ratio.^[22] Jafari et al. reported lower product yield in the range from 17% to 25%^[41] for pomegranate juice (Punica granatum L.) spray drying at the inlet/outlet air temperatures 124–143°C/48–76°C.

3. Quality parameters of honey and fruit juice powders

There are a number of review papers in the literature on the spray drying of sugar-rich food, i.e., fruit juices, pulps, concentrates and honey.^{[4,5,7,42,43]} The aforementioned papers are focused on general information about spray drying of food: design of dryers, process parameters, type of carriers applied and final product properties. This review demonstrates how the quality of spray dried food products might be improved to meet the changes in consumer behavior and requirements for healthy nutrition. The reduced application of conventional carriers, i.e., refined carbohydrates, in spray drying of food products opens new perspectives for production of functional food enriched with components providing additional nutritional value. In this paper the effect of substitution of conventional carriers (maltodextrin and gum arabic) by prebiotics and proteins or drying without carriers on the product yield and selected powder properties: hygroscopicity, solubility, wettability, flowability and retention of bioactive compounds is reviewed and critically commented.

Powders obtained from fruit juices and honey are highly hygroscopic, which affects the final stages of drying, i.e., separation of powder and drying air streams, product collection, transportation and handling.^[44] High powder hygroscopicity causes the deterioration of product quality due to particles caking and lumping. Increased moisture sorption during sugar-rich powder storage may also result in formation of liquid bridges between particles, collapse of the powder structure and further deliquescence, which promote microbiological growth in the final product.^[45,46] The caking of amorphous powders stored at high ambient humidity occurs via following mechanism: at the initial stage of moisture sorption, water starts to fill in the free space between particles, particles adhere to each other and the sinter bridges formation between particles takes place. Further increase of the relative humidity results in the increase of sinter bridges size and decrease of the bridges viscosity, which is followed by collapse of particle structure and deliquescence.^[47] Moreover, as ambient moisture is absorbed by amorphous powders, the T_g of the powder is reducing, which increase the plastifying effect of the water on the amorphous structure of the particles.^[47]

Hygroscopicity is defined as the mass of moisture (g) absorbed by 100 g of the powder subjected to the environment of relative humidity 75% (saturated NaCl solution) and 25°C until the equilibrium is reached.^[48–50] For fruit juice powder hygroscopicity values reported in the literature are from ca. 10 to 14 g H₂O/100 g for jussara powder,^[49] 22–28 g H₂O/100 g for persimmon powders,^[51] 12.48–15.79 g H₂O/100 g for acai powder,^[52] from 18.77 to 27.33 g H₂O/100 g for blackcurrant powder.^[53] For honey powders hygroscopicity ranges from ca. 20 to 25 g H₂O/100 g for Capilano honey powder,^[54] from 20.1 to 26.4 g H₂O/100 g sample for honeydew honey powder, from 22.0 to 27.4 g H₂O/100 g sample for rapeseed honey^[25] and 13.2–20.3 g H₂O/100 g for Perhutani’s honey.^[55]

Wettability which determines the instant characteristics of the dry powder is affected by particle surface composition and porosity. Wettability is determined as time needed to wet the powder sample spread on a distilled water surface. The wettability time measurement starts when the powder sample is allowed to drop onto the water surface and time measurement is stopped when all the particles in the sample are visually completely wetted.^[56–58] The wettability times reported in the literature for spray dried fruit juices/honey powders are in the range: of 41 s to 91 s for jussara pulp powder,^[34] from ca. 277 s to 304 s for sweet orange juice powder^[59] and from 9 s to 120 s for multifloral honey powder.^[56]

Flowability determines the powders characteristics which are important for packaging, transportation, storage, dosing, and mixing. The flowability may be evaluated by Carr Index (CI) or Hausner Ratio (HR). Based on the Carr Index powders flowability might be classified as very good (< 15), good (15–20), fair (20–35), bad (35–45), and very bad (>45).^[60] According to Hausner ratio powder cohesiveness may be classified as low (high flowability) for HR <1.2, medium 1.2–1.4 and high for HR >1.4 (low flowability).^[60] The Hausner Ratio is determined as ratio of tapped density to bulk density. For spray dried fruit juice the following values of Carr Index (CI) have been reported in the literature: 13.4 for spray dried maqui berries powder^[61] and from 13 to 19 for pink guava powder.^[62] Hausner Ratio for typical spray dried products are within the range of: 1.15–1.24 for pink guava powder,^[62] 1.27–1.52 for tamarind pulp powder^[63] and 1.05–1.29 for multifloral honey powder.^[56]

Retention of bioactive compounds characterizes the spray drying process performance in terms of retention of valuable biocomponents. The minimal loss of antioxidants, phenolic compounds, carotenoids, etc. is required during spray drying of honey and fruit juices.

In the next section the application of conventional carriers and prebiotics, proteins, novel natural carriers or carrier-free spray drying of fruit juice and honey is discussed and the effect of the carrier type on the powder properties and process characteristics is analyzed.

4. Spray drying of sugar-rich foods

4.1. Application of conventional carriers

Two carriers: gum arabic and maltodextrin have found wide application in spray drying of sugar-rich products.^[64–68]

Gum arabic is a natural gum comprised of complex heteropolysaccharide and is commonly produced from Acacia trees. Gum arabic has found wide application as a carrier due to its low viscosity, high solubility and good film-forming properties. Application of gum arabic is however limited due to high cost, short supply production and contamination by impurities.^[4]

Maltodextrin is a hydrolyzed starch with DE below 36^[6,11] which may be produced by the hydrolysis of starch using acid or enzymes. As a carrier maltodextrin shows the following advantages: good solubility, low viscosity, neutral aroma and low cost. The ability of maltodextrin to improve anti-sticking properties of sugar-rich products is based on the high T_g of maltodextrin, which varies from 141°C to 188°C^[6] (Figure 2). Glass transition temperature of the mixture is affected by glass transition temperatures of the components and the ratio of juice solid to carrier solid content, therefore, to avoid the problem of stickiness the concentration of the carrier in the initial solution might be equal or even exceed the concentration of juice solids. For example, to obtain free flowing pomegranate powder an optimal ratio of juice solids to maltodextrin solids content was ca. 50:50 – 33.4:66.6,^[66] for unclarified pomegranate juice to obtain product yield of 74%, juice solids to maltodextrin solids content 40:60 must have been applied,^[69] whereas for guava pulp powder the ratio of juice solids to maltodextrin solids content was even lower – ca. 25:75 (product yield – 70.2%).^[22] Thus, the content of a carrier in the final product varies from ca. 35% to 75% in the fruit powders^[21,22] and exceeds 50% in honey powder^[7] which means that the product contains more than half of low nutritional value additive, causing decrease of the bioactive compounds concentration in the product and undesirable change of the natural color^[70] and taste^[44] of food powders.

Moreover, studies carried out on the digestion of maltodextrin in the human body showed, that glucose from the digested maltodextrins is rapidly absorbed in the small intestine, which may lead to an increased glycemic load which is not desirable for health. The substitution of foods rich in dietary fibers, proteins and other nutrients by products contain refined carbohydrates in the diet might be related to health risks such as weight gain, diabetes and high level of blood cholesterol.^[71]

Application of refined carbohydrates for spray drying of fruit juice is not in line with current trends in healthy and functional foods.

4.2. Application of prebiotic dietary fiber as carriers

Recently, in the spray drying field a trend is observed for substitution of the conventional carbohydrates as carriers by substances belonging to the group of prebiotic dietary fibers. Prebiotic dietary fibers are specific substances that play a key role as source of carbon required for the growth of Bifidobacteria, which are valuable for human health and help prevent many lifestyle related diseases.^[72] The positive effect of prebiotic dietary fibers on human health covers: increase in Bifidobacteria and Lactobacilli growth, metabolism improvement, increased minerals absorption, reduced protein fermentation, reduced pathogenic bacteria populations, reduced allergy risk and enhanced immune system defense.^[73] The prebiotic dietary fibers are biopolymeric compounds consisting of β[1, 2]-fructans molecules with different degree of polymerization (DP):

• inulin (DP of 3–60 fructan monomers^[73]),

• fructooligosaccharides (FOSs) (DP of 2–4^[73]),

• oligofructose (DP of 2–20^[73]),

• resistant dextrins (DP of ca. 18^[26]).

In the last decade several studies on the application of prebiotic dietary fibers as additives to conventional carbohydrate carriers or as an individual carrier for the spray drying of juice have been carried out.

4.2.1. Apple concentrate

The effect of the carrier type, i.e., maltodextrin, resistant dextrin (“Nutriose”), β-cyclodextrin (“Kleptose”) and skimmed milk powder on the product yield and the physicochemical properties of the apple concentrate spray dried powder was analyzed by Samborska et al.^[16] Resistant dextrin carrier showed a higher product yield (55.8%) in comparison to maltodextrin (44.6%) and “Kleptose” (32.1%) carriers, which has been explained by the higher T_g of apple powder with resistant dextrin carrier of ca. 52°C in comparison to T_g of apple powder with maltodextrin (44.8°C) and “Kleptose” (49.0°C). Nevertheless, the application of milk powder as a carrier produced the highest product yield (81.7%) which was related to the film forming properties of proteins comprised in the skimmed milk powder (340 g/kg). The aforementioned data has been obtained for fruit solids to carrier solids content ratio equal to 50:50 and inlet/outlet temperatures of 180°C/80°C. The lowest hygroscopicity was observed for maltodextrin and skimmed milk powder as a carrier – 24.6 and 25.4 g H₂O/100 g, respectively, whereas for resistant dextrin and “Kleptose” carrier higher hygroscopicity was determined – 27.0 and 26.7 g H₂O/100 g. According to Hausner ratio ranging from 1.24 to 1.43 the apple powder was characterized by high or medium cohesiveness (low flowability). Comparing the results of total phenolic compound analysis in apple powder, resistant dextrin and “Kleptose” as carriers enabled lower retention of phenolic content than maltodextrin or milk powder.

4.2.2. Blueberry juice

The effect of inulin and maltodextrin application as a carrier on blueberry (Cyanococcus) powder T_g, powder stability during ambient moisture sorption and retention of antioxidants was analyzed by Araujo-Díaz et al.^[74] The authors applied spray drying at inlet/outlet air temperatures of 180°C/70°C and a carrier to blueberry juice ratio of 30:70. The sugar composition in blueberry juice was following: the glucose to fructose ratio was 47.5:52.5. The powder T_g was lower when inulin was applied as a carrier (96°C) in relation to maltodextrin (118°C). Moisture sorption tests and X-ray diffraction analysis showed that the powder contained maltodextrin which remained in an amorphous state in a wide range of water activity 0.07–0.83, while powder with inulin changed the microstructure from amorphous to an undesired crystalline state at water activity above 0.434. Retention of antioxidants after spray drying with inulin was 5.1% for resveratrol and 1.5% for quercetin 3-d-galactoside, whereas after drying with maltodextrin 21.1% and 28.5% for resveratrol and quercetin 3-d-galactoside, respectively, indicating that inulin shows lower encapsulation efficiency of bioactive compounds comparing to maltodextrin.

4.2.3. Cactus pear pulp

Cactus pear pulp (Opuntia ficus-indica), which is valued for its high content of natural colorants (betalains and carotenes) as well as phenols and antioxidant compounds was spray dried by Saénz et al.^[75] Two different carriers were applied: maltodextrin or inulin with fruit solids to carrier ratio in the range from 1:1 to 5:1, The researchers determined optimal spray drying parameters to achieve high retention of bioactive compounds: optimal juice solids to carrier solids content ratio was 3:1 for both carriers and inlet temperature – 140°C for maltodextrin as a carrier and 120°C for inulin. At optimal drying conditions high encapsulation efficiency for betacyanin and indicaxanthins (100%) has been observed regardless of the carrier type applied. Antioxidant activity of the cactus pear powder was obtained by applying maltodextrin as a carrier (34.0 mmol TEAC/g) and was slightly higher than with the inulin carrier (24.0 mmol TEAC/g).

4.2.4. Cranberry juice

Michalska-Ciechanowska et al. applied inulin, maltodextrin or a mixture of inulin and maltodextrin (2:1) as carriers for the spray drying of cranberry (Vaccinium macrocarpon L.) juice at a ratio of juice solids to carrier solids content of 0.52 (34:66), 0.32 (24:76) and 0.22 (18:82). The authors found that the final moisture content of the powders depended on carrier type: cranberry juice powder with an inulin carrier had higher moisture content in comparison to maltodextrin as carrier, which can be explained by the different chemical structure of inulin, which includes fructose units with higher hydrogen bonding. The inulin carrier showed a better retention of the total phenolic compounds in comparison to maltodextrin as a carrier, however the type of the carrier applied had no effect on the antioxidant capacity of cranberries juice powders.^[32]

4.2.5. Jussara pulp

Lacerda et al.^[34] studied spray drying of jussara pulp (Euterpe edulis M.) by application of mixtures of carriers—maltodextrin, starch sodium octenyl succinate (SSOS) and inulin in different proportions. Experiments were carried out on concurrent spray dryer at inlet/outlet air temperatures of 140°C/60°C. The feed was sprayed by a pneumatic nozzle at atomizing air pressure of 0.03 MPa. A decrease of product yield with a subsequent increase in pulp solids to carrier solids content had been observed: for 0.5 pulp solids to carrier solids content ratio product yield varied from 61.1% to 31.9%, for 1.0 pulp solids to carrier solids content ratio lower product yield was recorded (55.8–21.5%). A further increase in pulp solids as a fraction of the powder to 2.0 of pulp solids to carrier solids content ratio resulted in a lower product yield (49.3–22.4%). Comparison of the product yields obtained for pure carriers showed that the SSOS carrier produced the highest yield 51.5%, 55.8%, 49.3% for pulp solids to carrier solids content 0.5, 1.0, 2.0, respectively. For pure maltodextrin applied as a carrier the following values of product yields had been determined: 43.5%, 40.6% and 38.8% and for inulin – 44.2%, 38.9% and 22.4% for pulp solids to carrier solids content ratio 0.5, 1.0, 2.0, respectively. The authors found that the content of hydrophilic anthocyanins and antioxidant activity of jussara fruit powder increased for carriers with hydrophilic nature, i.e., maltodextrin and inulin. Increase of inulin content in the feed resulted in higher hygroscopicity and wettability of the dry product, whereas addition of SSOS reduced powder hygroscopicity and wettability. The authors proposed an optimal composition of feed solution: pulp solids to carrier ratio – 1.0 and carrier proportions: 2/3 SSOS, 1/6 inulin and 1/6 maltodextrin.

Paim et al. also studied spray drying of jussara (Euterpe edulis M.) pulp with mixtures of maltodextrin and prebiotics as a carriers in the following proportions: maltodextrin, maltodextrin/inulin in ratio 1:1, maltodextrin/oligofructose (1:1) and maltodextrin/inulin/oligofructose (2:1:1).^[33] Spray drying tests were carried out at the following process parameters: inlet and outlet air temperature – 140°C and 65°C, feed flow rate – 0.3 L/h, atomization pressure – 0.03 MPa. The results of the study showed that for carriers containing inulin the content of anthocyanin and phenolic compounds in the powder was higher in comparison to maltodextrin as a carrier. The highest powder hygroscopicity (0.341 g H₂O/kg powder/min) was found when maltodextrin was added as a carrier. However, the addition of inulin or oligofructose to the maltodextrin decreased powder hygroscopicity to 0.171 and 0.167 g H₂O/kg powder/min, respectively.

4.2.6. Orange juice

Saavedra-Leos et al. carried out spray drying of orange juice (Citrus X sinensis) applying inulin as a carrier at juice solids to carrier solids content ratios of 0.32 (24:76) and inlet/outlet air temperature 210°C/70°C.^[76] The authors determined the T_g of orange juice powder (water activity 0.05) for 92.17°C which might be explained by the high content of the inulin carrier in the orange juice powder (76%). Low powder hygroscopicity of ca. 14 g H₂O/100 g sample was determined based on the powder sorption isotherms as the amount of water absorbed by the sample at relative humidity 71%. The hygroscopicity value determined in this work is relatively lower than values typically determined for spray dried sugar-rich powders (>20 g/100 g sample),^[55] which might also be explained by the high content of the carrier in relation to the solids in the juice powder.

4.2.7. Pomegranate juice

Miravet et al.^[26] applied two types of dietary fibers: fructooligosaccharide (FOS) and resistant dextrin (“Nutriose”) and a conventional carrier, i.e., maltodextrin as carriers for spray drying of pomegranate (Punica granatum) juice. Figure 3 shows the product yield and dissolution time for pomegranate dried with different prebiotic dietary fibers. The authors found that fructooligosaccharide due to low T_g has low product yield: 0% for juice solid to carrier solids content ratio ca. 0.5 (33:67) and 25.7% for ratio 0.41 (29:71). In the case of maltodextrin, the maximum juice solids to carrier solids content ratio which allowed for a product yield above 59.8% was 0.62 (39:61). Resistant dextrin showed higher drying yields (73.5%) for the same juice solids to carrier solids content ratio equal to 0.62 (39:61). Moreover, a lower content of resistant dextrin in the final powder (56:44 juice solids to carrier solids content ratio) in relation to maltodextrin was needed to obtain a product with a drying yield of 49.6%. Retention of bioactive compounds such as anthocyanins, total phenols and antioxidant activity was acceptable and decreased with the increase of inlet air temperature but was not affected by the type of carrier (resistant dextrin or maltodextrin). Powders dried with resistant dextrin had slightly lower dissolution times (determined as time required for complete dissolving of 2.50 g of powder in 50 mL of water) from 74 s to 100 s depending on the drying aid concentration, in comparison to the maltodextrins – from 82 s to 115 s.

Figure 3. Product yield and dissolution time for pomegranate dried with different prebiotic dietary fibers (based on^[26]).

4.2.8. Rapeseed honey

Samborska et al. combined the application of carriers, i.e., resistant dextrin (“Nutriose”), maltodextrin and skimmed milk, with low temperature spray drying by dehumidified air to obtain rapeseed honey powder.^[77] The experiments were carried out with inlet/outlet air temperatures of 75°C/50°C, feed solids concentration 60% and honey solids to carrier solids content ratio for maltodextrin – 60:40, 70:30 and 80:20 and for resistant dextrin – 80:20. The authors claimed that application of dehumidified air would achieve higher product yields than typically reported for honey spray drying, even for high honey solids to carrier solids content ratios (80:20). Despite the low T_g of the honey powder ranging from 6.5°C to 20.3°C, which is lower than outlet air temperature of 50°C, high product yield was obtained for both maltodextrin as a carrier from 75% to 93% and for resistant dextrin – 85%. The critical sticky point temperature was not reached due to carefully adjusted drying process parameters: during spray drying the temperature of the particle surface may be even 10–20°C lower than the outlet air temperature; moreover, the particles became sticky at the temperatures 10–20°C above the glass transition point. The Hausner ratio of produced honey powder ranged between 1.29 to 1.33, which is the typical range for sugar-rich products reported in the literature. The significantly higher hygroscopicity of honey powder with resistant dextrin as a carrier (28.8 g H₂O/100 g) in comparison to conventional maltodextrin as a carrier (19.3–23.3 g H₂O/100 g) was reported.

4.2.9. Summary on application of prebiotic dietary fiber as a healthy carrier

Prebiotics as carriers provide additional health benefits to spray dried products, i.e., growth of favorable intestinal bacteria, improved immune defense and metabolism. From the group of prebiotic dietary fibers, inulin and resistant dextrin have the highest potential as carriers in spray drying of sugar-rich foods allowing to obtain a high product yield: 75.6% for inulin^[78] and up to 95.8% for resistant dextrin.^[25] Addition of oligofructose and fructooligosaccharides to sprayed feed resulted in lower product yields (<50%) due to low T_g.^[26,33]

The studies on the application of prebiotics as carriers are summarized in Table 3, which demonstrates the performed experiments in the concurrent laboratory scale dryers with inlet/outlet air temperatures ranging from 75°C/50°C^[25] to 180°C/70°C.^[74] Application of low inlet/outlet drying temperatures 75°C/50°C and air dehumidification allowed to obtain a high product yield for honey powder in ranges from 84.4% to 95.8%.^[25] Prebiotics as carriers may also improve the physical properties of the powder, e.g., addition of resistant dextrin decreased the time of solubility in comparison to the addition of maltodextrin (for pomegranate powder). Prebiotic carriers show high retention of the following bioactive compounds during spray drying: betacyanin in cactus pear,^[75] phenols and antioxidants in pomegranate juice,^[26] phenolic and anthocyanin compounds in jussara pulp.^[33] However, Araujo-Díaz et al. reported increased loss of antioxidants during spray drying with inulin as a carrier indicating that further research in this field is required.^[74]

Table 3. Fruit juice and honey spray drying applying prebiotic dietary fibers as carriers.

Product	Carrier agent	Powder Tg, °C	Type of spray dryer	Process parameters	Observation	Reference
Apple concentrate (AC) Solids content: 67.0% Solids concentration in the feed solution: 30% and 60%	Maltodextrin (MD), Resistant dextrin (RD—“Nutriose”), β-cyclodextrin (“Kleptose”), Milk powder (MP) AC solids to carrier solids content ratio: 50:50; 75:25	MD carrier—44.8°C, RD carrier—52.0°C, “Kleptose” carrier—49.0°C, Milk powder carrier—31.6 °C	Laboratory spray-dryer (Mobile Minor™; GEA, Søborg, Denmark) + air	Inlet air temperature: 75°C, 180°C Outlet air temperature: 50°C, 80°C Feed rate: 0.25 mL/s Speed of atomizing disk: 26,000 rpm Air pressure: 4.5 bar	Product yield: MD carrier—44.6%, RD carrier—55.8%, “Kleptose” carrier—32.1%, MP carrier—81.7% Hausner ratio: MD carrier—1.38, RD carrier—1.41, “Kleptose” carrier—1.24, MP carrier—1.43 Hygroscopicity: MD carrier—24.6 g H2O/100 g, RD carrier—27.0 g H2O/100 g, “Kleptose” carrier—26.7 g H2O/100 g, MP carrier—25.4 g H2O/100 g. Total phenolic content: MD carrier—1141 mg/kg material solids, RD carrier—1025 mg/ kg material solids, “Kleptose” carrier—945 mg/ kg material solids, MP carrier—1233 mg/ kg material solids.	[16]
Blueberry (Cyanococcus) juice (BJ) Sugars content in juice: nd Sugars composition in juice: glucose to fructose ratio: 47.5:52.5	Inulin (I) and Maltodextrin (MD) blueberry juice ratio/carrier ratio: 70:30 wt/wt	BJ-I: 96°C BJ-MD: 118°C	MiniSpray Dryer B290 (Buchi, Switzer-land)	Inlet air temperature: 180°C Outlet air temperature: 70°C Feed temperature: 40°C Feed flow rate: 7 mL/min Atomization pressure: 1.5 bar Air flow rate: 28 m^3/h	The Tg of blueberry powder with maltodextrin was 118 °C and with inulin 96 °C. Powders with inulin change structure from amorphous to crystalline state during storage at water activity above 0.434. Retention of antioxidants after spray drying with inulin was 5.1 and 1.5%, whereas after drying with maltodextrin 21.1 and 28.5%.	[74]
Cactus pear pulp (Opuntia ficus-indica) Total sugars: 15.02% Solids content: 14.68%	Inulin or Maltodextrin Pulp solids to carrier solids content ratio: from 1:1 to 5:1	nd	Buchi B-191 spray dryer (Switzer-land)	Inlet air temperature: 120–160°C Air flow rate: 600 l/h Feed flow rate: 10 ml/min Atomization pressure: 20 psi	At the optimal drying conditions both maltodextrin and inulin showed high encapsulation efficiency 100% for betacyanin and indicaxanthins.	[75]
Cranberry juice (Vaccinium macrocarpon L.) Solids content: 7.72%	Inulin, Maltodextrin or Inulin/Maltodextrin Mixture (2:1) Juice solids to carrier solids content ratio: 0.52 (34:66) 0.32 (24:76) 0.22 (18:82)	nd	Mini spray-dryer (Mini spray-dryer, B-290, Büchi Labor Technik AG, Flawil, Switzer-land).	Inlet air temperature: 180°C and outlet temperature of 70°C.	Moisture content: for inulin carrier from 0.75 to 1.10%, for maltodextrin as a carrier—from 0.58 to 0.95% Total polyphenolic compounds: for inulin carrier—from ca. 129 to 316 mg GA/100 g, for maltodextrin as a carrier—from ca. 150 to 316 mg GA/100 g Antioxidant capacity: for inulin carrier—from ca. 0.8 to 1.7 mmol TEAC/100 g, for maltodextrin as a carrier from ca. 0.93 to 1.70 mmol TEAC/100 g	[32]
Jussara pulp (Euterpe edulis M.) Solids content: 5.5%	Starch sodium octenyl succinate, inulin and maltodextrin Pulp solids to carrier solids content ratio: 0.5 (34:66), 1.0 (50:50) and 2.0 (66:34) wt/wt	nd	Mini Spray Dryer Büchi 290 (Büchi Laboratoriums Technik, Flawil, Switzer-land)	Total solids content: 5.5 g/100 g Inlet air temperature: 140°C Outlet air temperature: ca. 60°C Feed flow rate: 0.36 L/h Atomization pressure: 0.03 MPa	Product yield from 61.1% to 21%. Anthocyanins contents ranged from 3.3 mg/g to 24.2 mg/g. Antioxidant activity (TEAC) from 555.8 mmol Trx/g to 992.9 mmol Trx/g. Powder Solubility: from 76.8 to 85.8% Powder hygroscopicity: from 10.7 to 14.3% Wettability: from 41 s to 267 s.	[34]
Jussara pulp (Euterpe edulis M.) with probiotic culture Bifido-bacterium animalis Solids content in the pulp: 2.66% Carbohydrates content in the pulp: 0.22%	Maltodextrin, inulin and oligofructose Pulp solids to carrier solids content ratio: ca. 0.18 (15:85)	nd	LabPlant SD-06 (Hudders-field, England),	Inlet air temperature: 140°C Outlet air temperature: 65°C Air speed: 4.3 m/s Feed flow rate: 0.3 L/h Atomization pressure: 0.25 MPa	Total phenolics: from 1229.4 to 1697.5 mg GAE/100 g Anthocyanins: from 574.7 to 739.2 mg cya-3-glu/100 g Hygroscopicity: from 0.167 g to0.341 g H2O/kg powder/min	[33]
Orange juice (Citrus X sinensis) Solids content: 13.9% Sugars content: 8.5% Sugars composition: 53% sucrose, 25.5% fructose, 21.5% glucose	Inulin Juice solids to carrier solids content ratio: 0.32 (24:76)	92.17°C (for orange juice powder at water activity 0.05)	Mini Spray Dryer B290 (Buchi, Switzer-land)	Inlet/Outlet air temperature: 210 °C/70 °C Feed flow rate: 7 mL/min Atomization pressure 1.5 bar Air flow: 28 m^3/h	Hygroscopicity: 14 g H2O/100 g sample	[76]
Pomegranate (Punica granatum) juice Solids content: 62.65% Soluble solids content: 64°Brix	resistant dextrin (“Nutriose”) fructooligo-saccharides (“Actilight”, “Beneo P95”) Maltodextrin Juice solids to carrier solids content ratio: From 0.42 (29:71) to 1.25 (56:44)	nd	Buchi Mini Spray Dryer B-290, Flawil, Switzer-land)	Inlet air temperature: 120–200°C Air flow rate: 32 m^3/h Feed flow rate: 0.72–1.08 L/h Atomization air flow rate: 0.47 m^3/h	FOS is not effective carrier (product yield: from 0% to 25.7%) Resistant dextrin’s increases product yield (49.6–75%) comparing to maltodextrin (0–64.3%) Powder solubility for resistant dextrin carrier (from 74 s to 100 s) and for maltodextrin (from 82 s to 115 s) anthocyanins content ca. 33.8 mg cyanidin 3-glucoside equivalents/g of powder total phenol content of 8.80 mg GAE/ g of powder	[26]
Rapeseed honey (H) Feed solids concentration: 60%	Maltodextrin (MD), Resistant dextrin (RD) (“Nutriose” FM06), Skimmed milk (SM) Honey solids to carrier solids content ratio: 60:40, 70:30 and 80:20	Powder Tg for different carriers MD: 6.5–20.3°C RD: 9.1°C Milk + RD + M: 11.8°C	Laboratory spray dryer Mobile Minor (GEA, Denmark) + dehumidification system	Inlet air temperature: 75°C Outlet air temperature: 50°C Feed rate: 0.22 mL/s Speed of atomizing disk 26,000 rpm Air pressure 4.5 bar	Product yield: MD carrier: 75–93%, RD carrier: 85%, milk + MD + RD: 91% Hausner ratio: MD carrier: 1.29–1.33, RD carrier: 1.32, milk + MD + RD: 1.29 Hygroscopicity: MD carrier: 19.3–23.3 g H2O/100 g, RD carrier: 28.8 g H2O/100 g, milk + MD + RD: 27.6 g H2O/100 g	[77]

4.3. Application of proteins as carriers and drying aids

In the literature there are several works focused on the substitution of conventional carbohydrate carriers by proteins to reduce the concentration of the carrier in the dry powder. The mechanism of stickiness prevention of carbohydrate carriers (see section 4.1.) is based on the increase of T_g of sugar-rich material; therefore, a significant amount of carrier should be added to the material being dried.

The anti-stickiness mechanism of proteins as carriers is a result of the surface-active properties of proteins which migrate to the droplet surface to form a film preventing particle stickiness. In comparison to carbohydrate carriers lower amounts of proteins are needed to avoid powder stickiness.

A detailed analysis of studies on the application of proteins as carriers for spray drying of fruit juices is presented below.

4.3.1. Elderberry juice

Murugesan and Orsat, 2011^[38] applied five different carriers for spray drying of elderberry (Sambucus nigra L.) juice: maltodextrin, gum arabic, isolated soya protein (ISP), soya protein powder (SPP), soya milk powder (SMP) at juice solids to carrier solids content ratio 5:1, 5:2, 5:3, 5:4 and 1:1. He highest product yield was obtained for maltodextrin (77.9–82.0%) and gum arabic (72.5–80.1%), whereas for soya powders product yield ranged from 46% to 72%. Soya powders were characterized, however, by lower loss of phenols during storage and enhanced color stability.

4.3.2. Fructose and sucrose solutions

Jayasundera et al. carried out spray drying of sucrose and fructose solutions with the addition of proteins, i.e., sodium caseinate and pea protein isolate as carriers.^[79] Figure 4a shows powder recovery (powder collected from the cyclone, swept from the dryer walls and total) for sucrose drying without carrier, sucrose drying with sodium caseinate and pea protein isolate (PPI) and fructose drying with sodium caseinate. Sucrose drying without carrier resulted in zero powder recovery, since all the powder was lost due to it sticking to the dryer walls. The authors reported high product yields of up to 82% when only 0.5% of sucrose was replaced by protein sodium caseinate. When proteins are applied as drying carriers, the powder recovery depends rather on the amount of proteins which covered the particle surface then amount of proteins present in the particle core.^[21] Whereas, for carbohydrate carriers product yield is related to the concentration of the carrier in both particle surface and core, therefore to obtain a high product recovery, the concentration of carbohydrate carriers in the final product typically range from 35% to 60%.

Figure 4. Powder recovery for spray drying of sugars with proteins as carriers and drying aids (based on^[79,80]).

To obtain a high product yield (ca. 80%) in spray drying of the fructose solution the amount of sodium caseinate had to be increased to 30% due to the much lower T_g of fructose (5°C) comparing to sucrose (65°C).

Pea protein isolate was found not to be an effective carrier resulting in a product yield of 47.7%, which was explained by the lower solubility of pea protein isolate limiting the film forming properties of the protein. The T_g of sucrose powder with both sodium caseinate carrier and PPI carrier was ca. 54°C, which was favorable in comparison to outlet air temperature (T_g only 10°C below the outlet air temperature), therefore the differences in the products yield for both carriers could not be related to the T_g of the powder.

The solubility of dry powders showed the following decreasing trend: fructose/sodium caseinate > sucrose/sodium caseinate > sucrose/PPI. The low solubility of sucrose powder with PPI carrier (87.05%) in comparison to powder with a sodium caseinate carrier (90.87) and has been explained by the low solubility of pure PPI (16.84%) in relation to pure sodium caseinate (91.55%).

Adhikari et al.^[80] reported high product recovery of 84.7% and 84.1% when 0.5% of sucrose was substituted with sodium caseinate and whey protein isolate – WPI (Figure 4b). When the amount of protein in the powder was increased to 1%, the total powder recovery did not increase further and was about 85% for both sodium caseinate and WPI.

4.3.3. Honey

Shi et al.^[54] investigated spray drying of honey with the addition of WPI and maltodextrin solely or mixed in different proportions. A high product yield of 63.5% was obtained where the honey/WPI ratio was 70:30, whereas for maltodextrin at a ratio honey/maltodextrin 40:60 the product yield was 52.8%. Combining maltodextrin with small amounts of WPI (Honey/MD/WPI 60:39.5:0.5) as carriers significantly reduced the amount of the carrier in the product with a product recovery of 57.4%. Analysis of the dry powder hygroscopicity showed that lowest moisture absorption was observed for honey/maltodextrin powders, which was explained by the lower amount of hydrophilic groups in the maltodextrin (DE10).

The enhanced powder recovery due to the addition of sodium caseinate to the honey/gum arabic solution has also reported by Samborska et al.^[81] Honey powder recovery for gum arabic as a drying carrier was ca. 67–69%, whereas addition of 1% (wt/wt) of sodium caseinate to the carrier resulted in a powder recovery increase to ca. 75% due to the film forming properties of protein. For powder with gum arabic lower hygroscopicity was observed (ca. 1.8–2.0 g moisture/100 g of product) in relation to powder with sodium caseinate (ca. 2.0–2.8 g/100 g). The authors explained the higher hygroscopicity of powders containing sodium caseinate due to smaller particles size, folded and rough particle surface which increased the surface area available for moisture sorption. Addition of sodium caseinate resulted in significantly lower Hausner ratio: 1.11 for buckwheat honey and 1.14 for rape honey (enhanced flowability) comparing to honey powder with Gum arabic – 1.13 for buckwheat honey and 1.16 for rapeseed honey.

4.3.4. Sour cherry juice

Sarabandi et al. applied maltodextrin and gum arabic carrier with addition of whey protein concentrate for spray drying of sour cherry juice.^[45] The product yield for “pure” maltodextrin was about 42% and for “pure” gum arabic was 56%, whereas addition of 5% of WPC significantly increased the product yield to ca. 54%, when WPC was added to maltodextrin and to ca. 55%, when WPC was added to gum arabic. This effect was explained by the film forming properties of proteins, which covers the outer surface of particles preventing the adhesion of particles to dryer walls. The highest powder solubility has been determined for maltodextrin applied as a carrier (95.55%). Application of WPC resulted in lower powder solubility, ca. 94%, which reduced to 86.23% with increase of WPC concentration in the powder to 20%. The hygroscopicity of spray dried sour cherry powder has been also affected by the type and composition of the carrier. The lowest hygroscopicity of the powders has been determined for powders with addition of WPC (from ca. 19% to ca. 20%) due to low cohesiveness and hygroscopicity of protein film formed at the particles surface. Type and composition of carrier affected particles size and morphology. Maltodextrin applied as a carrier resulted in formation of particles with average particle diameter 42.65 µm and shrunk particle surface, when gum arabic was applied as a carrier, the produced particles had more smoothed surface with average particle diameter 51.43 µm. Combination of WPC with maltodextrin or gum Arabic resulted in formation of irregular particles with concave surface with particle size in the range from 48.69 µm to 57.88 µm. High shrinkage of the particles caused by application of WPC as a carrier was explained by rapid formation of protein crust in the beginning of drying and increase of particle size due to WPC application was caused by higher viscosity of the feed solution containing proteins and higher initial particles diameters.^[45]

4.3.5. Tamarind pulp

The performance of whey protein concentrate (WPC) as a carrier in spray drying of tamarind pulp (Tamarindus indica L.) was compared with maltodextrin and gum arabic by Bhusari et al.^[63] Highest product yields of up to ca. 76% was observed if WPC was added in concentrations from 10% to 30%, whereas for maltodextrin in higher concentrations 40–60% lower product yield was observed (ca. 46–60%). Powders produced with WPC has higher particle size, lower bulk density and increased porosity, which was explained by higher viscosity of the WPC in comparison to maltodextrin and gum arabic, which resulted in higher initial particle size.

Hygroscopicity of all the samples was in the range from 16.61% to 28.96%. Application of WPC resulted in slightly lower powder hygroscopicity (ca. 16–25%) and the degree of caking (30–37%) in comparison to maltodextrin and gum arabic with hygroscopicity ca. 20–29% and the degree of caking from ca. 30% to 47%. Due to the lower concentration of the carrier in the tamarind powder color change was negligible for powders with WPC.

4.3.6. Summary on application of proteins as carriers

Results of literature analysis have showed that proteins may be a good alternative for conventional carbohydrate carriers such as maltodextrin and gum arabic. Due to high surface activity and film forming properties proteins migrate to the particles surface protecting them from sticking. Application of proteins as a carrier for spray drying of sugar-rich foods allows one to obtain a high product yield at low carrier concentrations (from 0.5:99.5 to 30:70 juice solids to carrier solids content ratios), reducing the amount of additive material in the final product.

Table 4 summarizes the experimental studies on protein applications as carriers for spray drying of fruit and vegetable juices. In the analyzed papers the following process parameters were applied: inlet air temperatures in the range from 130°C to 180°C, outlet air temperatures from 65°C to 85°C. The authors used laboratory or pilot scale concurrent spray dryers equipped with pressure nozzles or atomizing disk^[81] for feed atomization.

Table 4. Fruit juice and honey spray drying applying proteins as carriers.

Product	Carrier agent	Powder Tg, °C	Type of spray dryer	Process parameters	Observation	Reference
Elderberry (Sambucus nigra L.) juice Total soluble solids: 10–13°Brix	Maltodextrin, gum arabic, isolated soya protein (ISP), soya protein powder (SPP), soya milk powder (SMP) Juice solids to carrier solids content ratio: 5:1, 5:2, 5:3, 5:4 and 1:1	nd	Mini spray dryer (Buchi-B290)	Inlet temperature: from 70°C to 130°C Atomization pressure:5.5 bar Air flow rate: 35 m^3/h Feed flow rate: 180 mL/h or 300 mL/h	Product recovery: gum arabic—from 72.5% to 80.1%, maltodextrin—from 77.9% to 82%, ISP—from ca. 53% to 72%, SPP—from ca. 46% to 68%, SMP—from ca. 60% to 68%. Actual phenolic content: gum arabic—from ca. 28 to 35 mg GAE/g, maltodextrin—from ca. 31 to 34 mg GAE/g, ISP—from ca. 21 to 26 mg GAE/g, SPP—ca. 20–24 mg GAE/g, SMP—ca. 25–27 mg GAE/g.	[38]
Fructose and sucrose solutions Feed solids content: 25% (wt/wt)	Pea protein isolate (PPI), Sodium caseinate (SC) Product solids to carrier solids content ratio: 99.5:0.5 and 99:1 for sucrose and 70:30 for fructose	Sucrose/sodium caseinate: 54.3°C Sucrose/PPI: 54.2°C Fructose/sodium caseinate: 53.43°C	Buchi B-290, Buchi, Switzerland	Inlet temperature: 165°C Outlet temperature: 65°C Air flow rate: 36 m^3/h Feed solids to carrier ratio: 70:30 (fructose/SC), 99:1 (sucrose/PPI) and 99.5:0.5 (sucrose/SC)	Small addition of sodium caseinate (0.13% (wt/wt) in feed solution and 99.5:0.5 sugar to protein ratio) allowed to achieve product yield of 82% due to preferential migration of protein to the particles surface.	[79,82]
Honey Total soluble solids: 80.2°Brix, Sugar content: fructose 47.04 g/100 g, glucose 31.78 g/100 g, and sucrose 1.25 g/100 g	Whey protein isolate (WPI) and maltodextrin (MD) Honey solids to carrier solids content ratio (honey:MD: WPI): 50:50:0, 40:60:0, 70:0:30, 70:10:20, etc.	From ca. 47°C to 82°C	Buchi B-290 mini spray dryer (Buchi Labortechnik AG, Switzerland)	Feed solids: 10% Inlet temperature: 150°C Outlet temperature: 85°C Air flow rate: 36 m^3/h	Product yield: for maltodextrin as a carrier: 20.6% and 52.8%, for WPI: 63.5% and ca. 75%, for honey:MD:WPI ratio 70:10:20 product recovery was ca. 55%. Hygroscopicity: from 20.13 to 25.29 g H2O/100 g sample	[54]
Rape and buckwheat honey Solids content: 83.6% and 81.6%	Gum arabic and sodium caseinate Honey to carrier solids content ratio: 50:50 for gum arabic and 1–2% wt/wt of sodium caseinate	nd	Laboratory-scale spray dryer (Anhydro, Soeborg, Denmark)	Feed Solids: 30% Inlet temperature: 180°C Outlet temperature: 80°C Atomization disk speed: 35,000 rpm, Feed rate: 1 mL/s Air flow rate: 4 m^3/h	Powder recovery for gum arabic as drying carrier was ca. 67% - 69%, addition of sodium caseinate increased powder recovery to ca. 75%. For powder with gum arabic lower hygroscopicity was observed (ca. 1.8–2.0 g moisture/ 100 g of product) in relation to powder with sodium caseinate (ca. 2.0–2.8 g/100 g).	[81]
Sucrose Feed solids content: 25% (wt/wt)	Whey protein isolate (WPI) and Sodium caseinate Feed solids to carrier solids content ratio: 99.5:0.5 and 99:1	nd	Pilot scale spray dryer (SL20, Saurin Company, Victoria, Australia)	Inlet temperature: 170°C Outlet temperature: 70°C	Addition of 0.125% protein to the solution (sucrose:protein = 99.5:0.5) the powder recovery was 84–85%. Further increase of protein concentration has no effect on product recovery.	[80]
Sour cherry (Prunus cerasus L.) Soluble solids content: 61%	Maltodextrin (DE 18–20), gum arabic (GA), whey protein concentrate (WPC) Juice solids to carrier solids ratio: ca. 37.5:62.5	nd	pilot-scale spray dryer (Maham sanat, Neyshabur, Iran)	Inlet air temperature: 170°C, Atomizer rotational speed: 18,000 rpm, Feed flow rate: 12 mL/min, Feed temperature: 25°C, Atomizer pressure: 4.2 bar.	Product yield for MD: ca. 42%, for GA: ca. 56%. Addition of 5% of WPC increased the product yield: for MD + WPC product yield was ca. 54%, for GA + WPC product yield was above 55%. Solubility: for MD—ca. 95%, GA—ca. 94%. Addition of 20% of WPC decreased the powder solubility: for MD + WPC solubility was ca. 86.23%, for GA + WPC solubility was ca. 83.38%. Hygroscopicity: for MD—ca. 22%, GA—ca. 25%. Addition of 5% of WPC decreased the powder hygroscopicity: for MD + WPC solubility was ca. 20%, for GA + WPC solubility was above 24%.	[45]
Tamarind (Tamarindus indica L.) pulp Solids content in the pulp: 10%	Maltodextrin-DE 20, gum arabic and whey protein concentrate (WPC) Pulp solids to carrier solids content ratio: 40:60–60:40 for gum arabic and maltodextrin; 90:10—70:30 for WPC	nd	Laboratory scale spray dryer (S.M. Scientech, Calcutta, India)	Feed solids: 20°Bx Inlet air temperature: 180°C Outlet air temperature: 80°C Feed flow rate: 600 mL/h Atomization pressure: 50 psi	Product yield: maltodextrin—from ca. 46% to 60%, gum arabic—from ca. 60% to 70%, WPC—from ca. 70% to 76% Carr index: from ca. 19 to 34 Hausner ratio: from ca. 1.26 to 1.51 Hygroscopicity: from 16.61% to 28.96%	[63]

Whey protein isolate, whey protein concentrate and sodium caseinate show the highest product yield of up to 85% and improved dry powder properties: reduced caking, higher flowability and enhanced reconstitution properties.

4.4. Application of natural carriers and spray drying of sugar-rich products without carriers

In recent years the application of natural carriers, which could be a healthy alternative to conventional carriers, or spray drying of fruit juices without addition of the carriers has become an attractive area of research. The process of spray drying and the properties of the selected fruit juices without the addition of the carrier and the application of natural carriers obtained from fibers extracted from barley, agave and orange by-products will be described in this section.

4.4.1. Jussara pulp

Pereira et al.^[20] has carried out spray drying of jussara (Euterpe edulis M.) pulp without a carrier. Jussara is palm tree native to the Brazilian Atlantic Forest commonly cultivated for fruit juice and palm heart. Jussara fruit is rich in anthocyanin pigments which have anticancer, antimutagenic, antimicrobial, anti-inflammatory properties.^[83,84] The authors applied a standard spray drying process at inlet and outlet air temperatures 160°C and 86°C. Product recovery was 66% calculated from powder collected from both the drying chamber and the cyclone collector. The researchers explained the high product yield as a result of the low content of carbohydrates (52.54%, solids) and organic acids (0.31% citric acid, wet basis) in the jussara fruit. Application of spray drying without a carrier allowed them to produce a powder with a high content of anthocyanins 7079.19 mg/100 g cyanidin-3-glucoside (solids), which was not diluted by the carrier. The diameters of the jussara particles in the powder were ranged widely between (2.3–1,674 µm) with a high volumetric mean diameter (D_4,3) of 124.5 µm which can be explained by the particles agglomeration due to sugar content in the pulp. The high stability of the bioactive compounds and antioxidant capacity for 103 days has been proved in a storage test. The carrier free jussara powder had a hygroscopicity of 11.62%, which is in the same range (10.7% to 14.3%) as jussara pulp spray dried powder with starch sodium octenyl succinate, inulin and maltodextrin as carriers.^[34] Powder solubility was slightly lower – 72.9% then previously reported in the literature for jussara pulp spray dried powder with carriers – from 76.8% to 85.8%.

4.4.2. Durian pulp

In the work of Chin et al.^[85] spray drying of durian pulp—Durio zibethinus (originated from Raub, Pahang, Malaysia) without the carrier was carried out at the air inlet/outlet temperature – 160°C/85°C. The paper focused on the detailed evaluation of volatile compound retention after spray drying. Durian fruit is valued for its organoleptic properties (taste and aroma) related to the amount and composition of volatiles present in the pulp. The authors found that the retention of volatile compounds after spray drying was about 1% without carrier addition. There was no shell formed on the surface of the carrier-free durian pulp to enable water diffusion and volatile encapsulation in the particle interior.

4.4.3. Mango pulp

Zotarelly et al. applied dehumidified air in spray drying of mango pulp without a carrier and with conventional maltodextrin as a carrier for pulp solids to a carrier solids content ratio of 74:26.^[86] The authors determined the T_g of carrier free mango powder of 26.74°C and powder with maltodextrin as a carrier of 32.4°C, indicating that addition of carrier increased the powder T_g for inlet air temperature 150°C (outlet air temperature and product yield were not provided). The results of the study showed that addition of a carrier decreases the mango powder hygroscopicity from 26.9 g H₂O/100 g to 23.9 g H₂O/100 g for drying with maltodextrin as a carrier. Spray drying of mango powder without the carrier allowed for a decrease in the loss of carotenoids to 60% in comparison to 65% loss when maltodextrin as a carrier was applied.

4.4.4. Red cactus pears pulp

Ruiz-Gutiérrez et al. applied soluble fiber ((1-3)(1-4) β-D glucan) obtained from barley as a drying carrier for spray drying of red cactus pears (Opuntia ficus indica).^[87] Application of soluble fiber with T_g 63.06°C as a drying carrier allowed to increase the T_g of red cactus pears pulp from 10.14°C before drying to ca. 32–38°C after spray drying with carrier. The minimal proportion of the carrier in the final product needed to obtain red cactus pear powder was 43:57 (juice solids to carrier solids content ratio), however the product yield has not been reported.^[87] The water solubility index decreased with an increase of the carrier proportion in the dry powder due to the lower solubility of the fiber carrier in comparison to red cactus pear pulp solids which were comprised of high amounts of low molecular carbohydrates. Polyphenol content was affected by the juice solids to carrier solids content ratio, lower juice solids to carrier solids content ratio resulted in a decrease of polyphenol content in the dry powder due to the dilution of the bioactive compound by the addition of the carrier.

4.4.5. Pineapple juice

The natural fructans obtained from agave (Agáve) were applied as a carrier in order to spray dry pineapple juice.^[88] Application of low fructans concentration in pineapple juice of 2% and 4% (m/v) with 10% (m/v) maltodextrin concentration in the juice allowed for an increase in the pineapple powder T_g from 42.45°C when only maltodextrin as a carrier was applied to 52.68°C and to 55.24°C when the fructans were combined with maltodextrin as a carrier. Addition of fructans to maltodextrin as a carrier allowed for the reduction of the difference between powder T_g and the outlet air temperature – T_out (T_g on 30–19°C lower then T_out) in comparison to maltodextrin being applied alone (T_g on 43–32°C lower then T_out), which is more favorable in order to avoid reaching the sticky point temperature by the dry powder. Application of natural fructans also allowed for an increase of pineapple powder solubility to 97.34% in comparison to powder without fructans as a carrier – 95.93%.

4.4.6. Orange by-products

The application of natural carriers rich in dietary fiber obtained from the orange juice industry by-products has been reported by Kaderides and Goula and Kaderides et al.^[89,90] The authors claimed that the novel carrier, i.e., orange waste powder has a total dietary fiber content of 65 g/100 g solid basis and T_g of 98.63°C (for anhydrous powder), which indicates that orange waste powder might be an alternative to conventional carriers rich in refined carbohydrates. The novel orange powder carrier has not been applied for spray drying of a sugar-rich products yet, nevertheless a recent study on the microencapsulation of pomegranate peel extract with an orange waste powder carrier showed that a high encapsulation efficiency of 99% might be obtained, but with a low product yield of 12.99% leaving the gap in the knowledge for further process improvement and optimization.^[91]

4.5. Summary on application of natural carriers and spray drying sugar rich products without carriers

The studies on sugar-rich spray drying products without carrier addition are limited to products with a relatively low content of sugar and acid, for instance jussara pulp (52.54% of sugars and 5% of acids in dry matter) has been dried with a product yield of 66%.^[20] However, a zero product yield for watermelon juice^[70] and for persimmon pulp^[51] while spray drying without a carrier was reported. Thus, the effectiveness of spray drying without the addition of a carrier is directly related to the type and composition of the food material, the T_g of the powder and the outlet air temperature during the drying process. For spray drying without carriers the following range of process parameters was typically applied: inlet air temperatures ca. 150–160°C and outlet air temperatures ca. 84–85°C. In the analyzed papers low solids content in the initial solutions (from ca. 6.5 wt% to ca. 16 wt%) were reported, which in industrial practice will generate a high energy demand in order to evaporate ca. 80 wt% of the water content from the feed solution.

Recently, application of natural carriers produced from orange by-products for food material spray drying has been reported in the literature.^[89,90] Application of fruit processing by-products for production of carriers for spray drying might resolve the problem of food processing waste utilization and, additionally, allow for the use of the rich bioactive compounds, which fruit waste contains.^[92]

Another example of the application of natural carriers in spray drying of fruit juices includes the use of fructans obtained from agave^[88] and soluble fiber (glucan) obtained from barley.^[87]

The spray drying process parameters and their effect on the quality characteristic of the fruit powders for the papers discussed above is summarized in Table 5.

Table 5. Spray drying of sugar-rich food without carrier addition and applying natural carriers.

Product	Carrier	Powder Tg, °C	Type of spray dryer	Process parameters	Observation	Reference
Carrier-free powder
Durian pulp (Durio zibethinus) Solids content: 38%	–	nd	Niro model 2000A, Denmark	Feed solids content: ca. 9.5 mass% Inlet air temperature: 160°C Outlet air temperature: 85°C Atomizing air pressure: 3 bar	About 99% of volatile compounds has been lost during spray drying. Thermal degradation of pulp components occurred due to Maillard reaction, indicated by presence of aldehyde compounds in the spray dried durian powder.	[85]
Jussara pulp (Euterpe edulis M.) Solids content: 6.53%, Total carbohydrates in pulp solids: 52.54%	–	nd	Spray dryer LabPlantTM SD-06 (Huddersfield, England)	Feed solids content: 6.5 mass% Sugar content in feed: 5.73°Bx Inlet air temperature: 160°C Outlet air temperature: 86°C Feed flow rate: 9 g/min Drying air speed: 4.3 m/s Atomizing air pressure: 0.25 MPa	Products yield: 66%. Anthocyanin content retention: 100%. Phenolic compounds retention: 90% High anthocyanin concentration: 7079.19 mg/100 g cyanidin-3-glucoside dry basis. Solubility: 72.93% Hygroscopicity: 11.62%	[20]
Mango pulp (Mangifera indica) Pulp sugar content: 14–15°Brix, Pulp total solids: 16%	Without carrier or maltodextrin Pulp solids to carrier solids content ratio: 2.87 (74:26)	Powder Tg without carrier—26.74°C and with maltodextrin—32.4°C	A lab scale spray dryer (Buchi, B-290, Switzer-land) with dehumidifi-er (B-296, Switzer-land)	Intel air temperature: 150°C Feed flow rate: 0.42 L/h Air flow rate: 35 m^3/h	Hygroscopicity: without carrier—26.9 g H2O/100 g, maltodextrin as a carrier—23.9 g H2O/100 g Carotenoid content: without carrier—113 µg/g, maltodextrin as a carrier - 98 µg/g Carotenoid content loss: without carrier—60%, maltodextrin as a carrier—65%	[86]
Natural carriers
Red cactus pears juice (Opuntia ficus indica) Juice solids content: 11.35% Carbohydrates content in solids: 90.06%	Soluble fiber ((1-3)(1-4) β-D glucan) Juice solids to carrier solids content ratio: 0.75 (43:57) 0.50 (34:66) 0.37 (27:73)	Powder Tg: from 32°C to 38°C	Pilot plant spray dryer (Niro A/S Mobile Minor DK-2860 2001; GEA Company, Søborg, Denmark)	Inlet air temperature: 160–200°C Outlet air temperature: 85°C Rotary atomizer pressure: 4.5 bar	Water solubility index: juice solids to carrier solids content ratio: 0.75—from 0.741 to 0.766, 0.37—from 0.64 to 0.671. Retention of betalain: from 40.66% to 56.92% Retention of indicaxanthin: from 36.66% to 56.39% Polyphenol content: from ca. 14 to 16.5 mg GA/g	[87]
Pineapple (Ananas comosus) juice Juice solids content: nd Juice sugar content: nd	Natural agave fructans and maltodextrin Carrier concentration in the juice volume: 10% (m/v) maltodextrin and 0%, 2% and 4% fructans	Powder Tg: for maltodextrin as a carrier only—42.45°C, for maltodextrin + fructans—52.68°C and 55.24°C	Pilot spray dryer (model LPG-5, CIMA Industries Inc., Shangai, China)	Inlet air temperature: 120°C Outlet air temperature: 74–85°C Feed rate: 15 mL/min Air speed: 9.4 m/s Atomization pressure 2.5 bar	Solubility: maltodextrin as a carrier: 95.93%, maltodextrin + 2% fructans: 96.88%, maltodextrin + 4% fructans: 97.34%	[88]

5. Conclusions

Spray drying of sugar-rich food such as fruit juices and honey requires the application of carriers or the modification of the spray drying process (application of scrapped surface of drying chamber, cooling of the spray dryer walls, application of air dehumidification at high and low temperatures) in order to avoid particle sticking caused by the low T_g.

Literature reports on spray drying of fruit juices without a carrier underline the high concentration of valuable components in the dry powder with a product yield of above 50%, however only for products with low sugar content such as jussara pulp. Moreover, spray drying of sugar-rich foods without the addition of carriers may only be carried out at the low air inlet/outlet temperature or with application of dehumidified air.

Carbohydrate carriers such as maltodextrin and gum arabic are commonly applied to increase the T_g of spray dried foods. The application of conventional carriers provides the following advantages: retention of bioactive volatile compounds due to microencapsulation, good solubility and low viscosity of the carrier, which improves feed atomization. To achieve a high product yield (up to 85%) the content of the carbohydrate carrier in the final product might exceed 50% to reduce particle stickiness. Moreover, recent studies directly link diet and nutrition with diseases such as: obesity, diabetes, cardiovascular disease, etc. which might be the result of the consumption of substantial amounts of refined carbohydrates. One of the recommendations for healthy nutrition is the reduction of refined carbohydrates in the diet and an increased consumption of proteins and dietary fibers.

Prebiotic dietary fibers such as inulin and resistant dextrin demonstrate great potential in their application as carriers, delivering additional nutritional value. A few studies have appeared in recent years, which have been focused on the application of pure dietary fibers as carriers (inulin, resistant dextrin). For instance, pomegranate juice and honey were spray dried with a high product yield ca. 70–95% applying resistant dextrin as the carrier. In comparison to maltodextrin, prebiotic carriers may improve the powders properties: powders with resistant dextrin have better solubility, whereas oligofructose and inulin carriers show lower hygroscopicity. The negative effect of the application of prebiotic carriers is the high concentration of the carrier material in the final product which is required in order to reach a high product yield.

Substitution of carbohydrate carriers with proteins allows for the significant reduction of the amount of carrier needed to produce a non-sticking powder. Proteins, due to their surface activity and film forming properties cover the surface of the dried particles preventing coalescence and sticking. Product yield during sugar-rich food spray drying depends on the concentration of proteins on the surface of the particles and is not affected by the content of protein at the core of the particle which allows for a significant reduction of additive material in spray dried foods. Literature analysis shows that whey protein isolate, whey protein concentrate and sodium caseinate demonstrate highest product yields (ca. 80%) at low concentrations of carrier (protein) in the final product (in the range from 0.5% to 30%). Additionally, the application of proteins improves the properties of the dry powder: reduces caking, enhances flowability and its reconstitution properties. The effectiveness of the protein carrier, i.e., product yield, and the final reconstitution properties of the powder depend on the type and solubility of the protein applied.

Replacement of maltodextrin as a conventional carrier by novel carriers obtained from fruit waste such as orange juice by-products is important advance in sugar-rich products spray drying. Application of fruit by-products for production of carriers enable to solve the problem of food waste utilization, moreover fruit by-products are rich in dietary fibers and phenolic compounds.

Literature review on the recent developments in spray drying of sugar-rich foods has showed that conventional carbohydrate carriers may by successfully substituted by prebiotic dietary fibers or proteins which will provide a high product yield and additional nutritional value to the final product.

The perspectives of sugar-rich products spray drying are oriented to development and evaluation of novel sustainable and healthy carriers and drying aids based on the food processing industry by-products such as fruits and vegetables peels and pomace.

Extensive research on the evaluation of effect of initial droplet size distribution and initial atomization parameters on the product yield and properties during sugar-rich products spray drying is still needed.

Abbreviations
AC	=	apple concentrate
CI	=	Carr Index
DE	=	Dextrose equivalent
DP	=	degree of polymerization
DPPH	=	2,2-diphenyl-1-picrylhydrazyl radical
GA	=	gallic acid
HPLC	=	high pressure liquid chromatography
HR	=	Hausner ratio
ISP	=	isolated soya protein
MD	=	maltodextrin
PPI	=	pea protein isolate
RD	=	resistant dextrin
SMP	=	soya milk powder
SPP	=	soya protein powder
TEAC	=	trolox equivalent antioxidant capacity
WPI	=	whey protein isolate

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.