Physico-chemical and rheological properties of fruit and vegetable juices as affected by high pressure homogenization: A review

ABSTRACT

In recent years, the consumers’ demands for fruit and vegetable juices (F&VJ), such as higher nutritive values, minimal processing and high quality have been growing rapidly. High pressure homogenization (HPH) is a non-thermal technology that has been widely studied as a partial or total substitute for thermal fluid food processing. The present work summarized the effect of HPH on the particle size distribution (PSD), microstructure, pulp sedimentation (PS), serum cloudiness (SC) or turbidity, color change (CC), bioactive compounds (BC) and antioxidant activity (AA), rheological properties (RP) and sensorial properties. The juices considered were apple, banana, blueberry, carrot, cashew apple, grape, kiwifruit, mango, orange, clementine, pineapple, salustiana, strawberry, taro and tomato juices. HPH changed the F&VJ PSD, physical properties, RP, SC, PS behavior, color and microstructure by disrupting the suspended pulp particles. Recent progress in high-pressure homogenization technology and the design of new homogenization valves were able to stand pressures up to 400–600 MPa. This have opened new opportunities to homogenization processing in the food industry to reduce particle sizes well below the micron and, consequently, permitted the development of new products with differentiated from traditional ones by physico-chemical characteristics and RP. It caused the size reduction of juices’ particle below the visual detection limit. The successful application of HPH treatments may provide with a new generation of minimally processed products, hopefully closer to the fresh ones in relation to the organoleptic and nutritive properties as well, as safe and lasting as the pasteurized ones. In addition, it can improve viscosity, color, cloudiness and stability of suspended solids (SS) in F&VJ.

Abbreviations

AA: Antioxidant activity; AJ: Apple juice; AV: Apparent viscosity; BC: Bioactive compounds; BJ: Banana juice; CC: Color change;F&VJ: Fruit and vegetable juices; GJ: Grape juice; HP: Homogenization pressure; HPH: High pressure homogenization; KJ: Kiwifruit juice; OJ: Orange juice; PS: Pulp sedimentation; PSD: Particle size distribution; RP: Rheological properties; SC: Serum cloudiness; SS: Suspended solids; TJ: Tomato juice

KEYWORDS:

• Cloudiness
• color
• high-pressure homogenization
• size reduction
• viscosity

Introduction

Fruits and vegetables are essential parts of human nutrition. In recent years, the consumers’ demands for F&VJ, such as higher nutritive values, minimal processing and high quality, have been growing rapidly.^[1] The word “homogenization” is referred to as the ability to produce a homogeneous size distribution of particles suspended in a liquid, by forcing the liquid under the effect of pressure through a specially designed homogenization valve. Homogenizer able to process fluid matrices at the pressure ranging between 20–100 MPa are nowadays employed in the dairy, beverage, pharmaceutical, and cosmetic industries mainly to reduce particle size and consequently increase the stability of emulsions to avoid creaming and coalescence phenomena. Homogenization is a commonly used unit operation in F&VJ processing .^[2,3] Recent progress in high-pressure homogenization technology and the design of new homogenization valves (with ceramic seats and needles and more recently diamond coating) able to withstand pressures up to 400–600 MPa have opened new opportunities to homogenization processing in the food industry to reduce particle sizes well below the micron and, consequently, permitted the development of new products differentiated from traditional ones by physico-chemical characteristics and RP .^[2–4] The high pressure treatments used in food industry include: (i) Hydrodynamic treatment (High Pressure Homogenization (HPH)), (ii) Hydrostatic treatments (High Pressure Processing (HPP)). Both of them can be applied to achieve the same goal but the principle action, the pressure level, the process conditions (temperature, residence time, inlet and outlet temperature, geometry) and the structural characteristics of food matrix determine the effect of each one. HPH applies pressures from 3 to 500 MPa in continuous to fluid products, while HHP is applied in batch systems to both solid and liquid products already packaged, using a pressure between 150 and 900 MPa .^[5,6]

Viscosity is defined as the internal friction of a fluid or its tendency to resist flow .^[7,8] Table 1 shows some studies evaluating the effect of HPH treatments on physico-chemical and rheological properties of different F&VJ. HPH technology consists of pumping a fluid through a narrow gap valve using high pressure intensifiers, which significantly increases its velocity resulting in depressurization with consequent cavitation and high shear stress. HPH was firstly employed as a useful method for cell disruption and recovery of intracellular bio-products. Thus the particles, cells and macromolecules suspended in the fluid are subjected to high mechanical stress, becoming twisted and deformed .^[52]

Table 1. Effect of high pressure homogenization on the characteristics of fruit and vegetable juices.

Product	Brix	pH	Initial Temp. (°C)	Time (Min)	Pressure levels (MPa)	Results	Reference
Apple juice	-	-	4 & 20	-	0, 100, 200 & 300	HPH did not influence the original ascorbic acid and dehydroascorbic acid content in AJ.	^[Citation9]
Apple juice	13.67	3.89	4 & 20	-	0, 100, 200 & 300	UHPH processing increased shelf-life of AJ without affecting on the product quality.	^[Citation10]
Apple juice	14.03	3.56	-	-	20, 40 & 60	Use of kiwifruit puree addition and HPH resulted in an AJ with improved uniformity and cloud stability by reducing particle size and increasing viscosity and yield stress.	^[Citation11]
Apple juice	-	-	4 & 20	-	100–300	Browning index was higher in non-thermal-treated juice, and it had an inverse correlation with the severity of UHPH treatment.	^[Citation12]
Apple juice	11.1	3.72	20, 40, 60 & 80	-	100, 200 & 300	Alicyclobacillus acidoterrestris could be inactivated in clarified AJ at a level of 4.8 log10 CFU/mL by a 300 MPa treatment.	^[Citation13]
Apple juice	7.5 & 6.7	3.5	-	-	150–300	Shelf life of clear Annurca AJ can be prolonged for many weeks of storage both at 4°C and 37°C upon HPH treatment at 250 MPa.	^[Citation14]
Apple, carrot and peach	-	-	25	-	140	HPH processing decreased particle size and altered water-soluble pectin characteristics.	^[Citation15]
Banana juice	-	4.8	4	-	0, 150, 200, 300 & 400	Following HPH, BJ resulted in brighter and less viscous than the untreated one. A full inactivation of pectate lyase was obtained in BJ at pressure level ≥200 MPa.	^[Citation16]
Blueberry juice	7.4	2.95	25	5, 9 & 15	0, 200, 400 & 600	All the functional parameters (ascorbic acid, polymeric anthocyanins, total phenolic compounds, antioxidant capacity) were equal or higher in homogenized juices (92% vitamin C retention). No significant changes were observed in pH, Brix, conductivity and color measurements.	^[Citation17]
Carrot emulsion	-	-	-	-	10	HPH did not affect carotenes or ascorbic acid but increased the release and micellar incorporation of α- and β-carotene in carrot emulsions.	^[Citation18]
Carrot, broccoli and tomato dispersions	-	-	-	-	60	HPH decreased the viscosity of carrot and broccoli dispersions, while it increased the viscosity of tomato.	^[Citation19]
Cashew apple juice	10	-	-	-	0–150	Reduction in particle size of cashew apple juice caused by HPH resulted in a decrease in juice consistency, σ0 and k, and an increase in n.	^[Citation20]
Grape juice	-	-	6	-	200, 300 & 400	L. monocytogenes viable counts became undetectable in UHPH treated and also in control samples of GJ which could be attributed to the presence of natural compounds with antilisterial effect.	^[Citation21]
Kiwifruit juice	13	-	4	-	200	Application of treatment at 200 MPa for three cycles allowed to obtain a stable KJ for more than 40 days under refrigerated storage.	^[Citation22]
Mango juice	4.45	4.06	20, 40, & 60	-	40, 70, 100, 130, 160, & 190	HPH significantly enhanced AV of mango juice, meanwhile thixotropy, G′ and G″ were also altered, and those changes were related to the HPH condition applied.	^[Citation23]
Mango juice	4.37	4.22	20–60	-	40–190	HPH increased shelf-life from 12 to 60 days when stored at 4°C vs. room temperature. HPH was superior to heat treatment concerning the post-treatment level of bioactive.	^[Citation24]
Mango Nectar	14	4.05	-	-	200	Combined treatment induces less CC indicating that the HPH combined with heat shock is a potential alternative treatment to reduce sensory damage of fruit products caused by heat treatment.	^[Citation25]
Orange juice	-	-	6	-	200, 300 & 400	Pressure level had a significant impact on the lethal effect of UHPH and complete inactivation of Salmonella enterica serovar Senftenberg 775 W was achieved at 400 MPa.	^[Citation21]
Orange juice	11.59	3.37	5	1	600	HPP treatments did not differently influence sugar profile, organic acid profile, bitter compounds, vitamin C and carotenoid profile.	^[Citation26]
Orange juice	14	3.5	28	-	150	Aroma profile of the double homogenized juice was initially closer to fresh juice and presented lower concentrations in off-flavor compounds during storage.	^[Citation27]
Orange juice	-	-	10 & 20	-	0, 100, 200 & 300	UHPH treatments significantly reduced the particle size and in consequence the cloudiness and the total color value increased.	^[Citation28]
Orange juice	-	-	10 & 20	-	0, 100, 200 & 300	No significant differences were observed on the total polyphenol content and antioxidant capacity values between the fresh and the UHPH treated OJ samples, while these values were significantly lower in the thermal pasteurized one.	^[Citation29]
Orange juice	10.2	4.0	22, 35 & 45	-	0, 50, 100, 150, & 250	Cloudy appearance of OJ was maintained for 12 d of storage at 4°C. More than 30 and 80% of enzyme activity was reduced after five passes at 100 and 250 MPa.	^[Citation30]
Orange juice	-	-	-	10	500 & 900	Cloudy stability in OJ was preserved after 10 min of processing at pressures from 500 to 900 MPa.	^[Citation31]
Orange juice	66	3.8	25	-	0, 25, 50, 75, 100 & 150	HPH reduced the concentrated OJ particle size, decreasing the juice consistency (k) and increasing the n.	^[Citation32]
Orange juice	-	-	10 & 20	0.5	100, 200 or 300	Pectin methylesterase activity decreasing was higher as the pressure applied increased, achieving the maximum reduction in the samples treated at 200 and 300 MPa.	^[Citation33]
Orange juice	11.33	3.69	20	15	50‐400	Combinations of HPH and temperature effectively reduced peroxidase activity in OJ (50%) to 35°C.	^[Citation34]
Orange juice	-	3–4	-	-	170	Freshness attributes of OJ treated by warming were improved by HPH treatment.	^[Citation35]
Orange and Clementine juices	-	-	-	-	150	Inactivation of pectinmethylesterase was a function of pH, maturity and HPH temperatures.	^[Citation36]
Orange, red orange, pineapple	-	-	2–20	-	50–250	Vitamin C concentration of the homogenized samples was also similar to the fresh juice, suggesting that the HPH treatment did not significantly cause its degradation.	^[Citation37]
Ortanique juice	12.9	3.42	-	-	0, 5, 10, 15, 20, 25 & 30	HP affected the PSD and color of the ortanique juice, which made it possible to define different sample groups based on the applied pressure.	^[Citation38]
Pineapple pulp	11.2	3.76	25	-	0–70	Particle size of the homogenized pulp ranged from 400 to 100 μm for homogenization pressures of between 0 and 70 MPa.	^[Citation39]
Salustiana juice	12.7	3.50	-	-	0, 5, 10, 15, 20, 25 & 30	HP affected the PSD and color of the salustiana juice, which made it possible to define different sample groups based on the applied pressure.	^[Citation38]
Strawberry juice (Ottoman)	9.8	3.79	-	-	60 & 100	Mean particle size and CIE L*a*b* color values of the juice showed significant differences at both pressures.	^[Citation40]
Strawberry puree	8.76	3.9	20	15	50‐400	Pressurization/depressurization treatments caused significant loss of strawberry polyphenoloxidase up to 250 MPa and peroxidase activity up to 230 MPa.	^[Citation34]
Strawberry juice	10 & 4.4	3.47	-	4	600	Significant decreases in anthocyanins, ascorbic acid, total phenols and antioxidant capacity in HHP-treated cloudy and clear strawberry juices were reported as a function of the storage time and temperature, and the viscosity and cloud of cloudy juices reduced with the raise of the storage times at 4 and 25°C.	^[Citation41]
Strawberry and blackberry purées	-	-	10–30	15	400, 500 & 600	HPH at moderate temperatures can maintain the nutritional quality of purées and could be used in commercial production of high-quality products with superior characteristics than thermally processed counterparts.	^[Citation42]
Taro pulp	15	-	50	-	0, 10, 20, 30, 40, 50 & 60	HPH increased the dispersion of pulp particles, hence increasing the consistency and reducing particle sedimentation. Furthermore, HPH increased the absolute zeta potential, lightness and total soluble solids of the homogenized samples	^[Citation43]
Taro pulp	15	-	50	-	0, 10, 20, 30, 40, 50 & 60	HPH (0–60 MPa) reduced taro pulp particle size and increased the pulp consistency. HPH improved taro pulp both elastic and viscous behavior.	^[Citation44]
Tomato juice	4.9	5.6	5	-	0, 25, 50, 75 & 100	HPH increased the consistency and reduced particle sedimentation and serum separation, hence improving sensory acceptance.	^[Citation45]
Tomato pulp	-	-	4	-	0, 8.4, 21.2, 47.9, 70.7 & 132.7	Increasing the HP resulted in the breakdown of the tomato cell aggregate structures, but improved the strength of the fiber network.	^[Citation46]
Tomato juice	-	-	4	15	20, 30, 40 & 50	Increase in lycopene bioaccessibility was attributed to the isomerization of all-trans lycopene and cell breakage induced by HPH and ultrasound treatments. Also, the antioxidant activities of TJ and digesta were slightly influenced by HPH and ultrasound process.	^[Citation47]
Tomato juice	5, 7.5 & 10	4.5	10	-	20–150	HPH and ultrasound treatments were responsible for higher G′ and consistency of processed TJs than untreated samples. Increases in sample rheological characteristics were comparable for the 5.0 and 7.5 °Brix treated HPH and ultrasound samples, whereas HPH was more effective than ultrasound in the case of a10 °Brix sample.	^[Citation48]
Tomato residue fibers	-	-	-	-	100	HPH processing could change both the morphology and the structure of raw fibers.	^[Citation49]
Tomato peel	-	-	24	-	100	HPH processing caused an increased release of intracellular compounds, such as proteins, and polyphenols with a corresponding increase in AA and reduction in oil-water interfacial tension.	^[Citation50]
Tomato and pepper emulsions	-	-	-	-	10, 20, 50, 100 & 150	Release of carotenoids into oil droplets is affected by HPH pressure.	^[Citation51]

The hypothesis established is that under the pressure influence, small molecules, such as volatile compounds, pigments, amino acids and vitamins remain unaffected, due to their relatively simple structures. In contrast, larger molecules, such as proteins, enzymes, polysaccharides and nucleic acids, may be altered .^[53]

Ultra high pressure homogenization (UHPH) is an emerging technology that is currently under investigation. The principle of this technology is similar to the conventional homogenization used in the dairy industry, but implies using considerable higher pressures (up to 400 MPa). UHPH allows to process of continuous fluid foodstuffs and, its great potential to inactivate pathogenic and spoilage microorganisms in fruit juice has been demonstrated. Besides its ability to reduce the microbial activity, UHPH also minimizes the adverse effects of heat on food properties or constituents .^[10,21,29]

Several studies have evaluated the use of HPH for microbial inactivation in fruit products. In addition, HPH technology has been recently proposed as an exciting unit operation to improve food and food component properties. Moreover, HPH has been recommended for use as a valuable tool to promote desirable changes in the physical properties of food products .^[45,54,55] The use of HPH as a partial or total substitute for thermal food processing has been studied for tomato, ^[56] apple, ^[12,57–59] mango, ^[60,61] orange ^[,6263] (Figure 1), carrot, ^[58,64,65] pomelo and kiwi^[66] and apricot^[64,65] juices.

Figure 1. Effect of high pressure homogenization on the characteristics of orange juice.

The degradation of BC during storage after HPH is equal or lower but never higher than those observed in thermal treated juices. In many cases, fresh-indicative parameters such as color and aromatic compounds are maintained better in HPH juices than in thermal treated ones .^[26] The HPH was shown to be an effective process to inactivate vegetative bacteria, yeasts, and non heat-resistant mold. However, its low efficacy against bacterial spores turns the HPH into a treatment similar to thermal pasteurization .^[25] The effect of HPH on the color of banana juice (BJ), ^[67] SC, PS and the microstructure of pineapple pulp, ^[39] and carrot, broccoli and tomato dispersions and emulsions^[19,68] has been studied.

The effect of HPH on the RP of tomato juice (TJ) was recently studied. HPH processing decreased the viscosity of the juice serum^[69] and increased the consistency, thixotropy, viscous and elastic behavior of the TJ .^[70] A rheological analysis indicated that this technology could be used to increase the consistency of TJ, improving its sensory acceptance, reducing the need for adding hydrocolloids and reducing particle sedimentation and serum separation.

HPH is an emerging technology based on the dynamic application of high pressure allowing the processing of fluid foods continuously, which has been proposed as an alternative to thermal pasteurization for foods with heat-sensitive properties .^[29] The application of HPH is very promising because the resulting juices could be safe with high content in BC, and with a fresh-like flavor. In all the observed cases the pressures treatments increased, maintained or just slightly decreased compared with the thermal treatments, the BC content .^[5]

Current knowledge indicates that the use of HPH has been established as a non-thermal technology, able to maintain and improve the nutritional and quality properties of F&VJ compared with those resulting from thermal treatments. So, this study summarized the effect of HPH on the PSD, microstructure, PS, SC, CC, BC and AA, RP and sensorial properties of apple, banana, blueberry, carrot, cashew apple, grape, kiwifruit, mango, orange, clementine, pineapple, salustiana, strawberry, taro and tomato juices.

Particle size distribution (PSD)

The particle size of plant-based food dispersions are indicators of mechanical disruption that further contribute to the flow behavior of the product .^[44] The homogenization processing reduced the mean particle diameter. The effect of homogenization and HPH on the PSD of TJ, ^[45,54] highly concentrated tomato suspensions, ^[71] tomato paste, ^[72] apple, tomato, carrot and potato pulp, ^[73] carrot, broccoli and tomato dispersions, ^[19] passion fruit juice, ^[74] orange juice (OJ), ^[35,75] citrus juices, ^[38] apple juice (AJ)^[57] has been investigated. These authors reported that the homogenization and HPH processing reduced the mean particle diameter. As explained by Augusto, et al., ^[54] the homogenization process disrupts the remaining cells and breaks their fragments into small suspended particles. It is to be expected that the smaller fragments would be less susceptible to being broken during processing when compared to the bigger ones or even to the whole cells, which explains the observed effect of homogenization pressure (HP) on the suspended particle disruption behavior, which was further confirmed by the microstructure analyses. The effect of the HP on the disruption of suspended particles seems to follow an asymptotic behavior, i.e., at higher pressures an increase in HP caused smaller changes in PSD .^[45]

Effect of HPH and high power ultrasound on some physical characteristics of TJs with different concentration levels were studied by Bot, et al. .^[48] Differences in tomato appearance, mainly relevant to color and consistency, observed between the untreated and treated samples. Increasing TJ concentration to 10.0 °Brix, HPH treatments were more effective than ultrasound in changing sample RP. These changes were attributed to cell disruption and the consequent increase of inter-particle interactions.

Microstructure

Cell disruption and subsequent fragmentation not only increased the surface area of the suspended particles, but also changed the properties of the particles and serum. Cell fragmentation exposed and released wall constituents such as pectins and proteins, improving the particle–particle interactions and resulting in aggregates. The authors observed a significant increase in the thixotropy of the juices due to the HPH, which directly describes the changes in juice microstructure .^[54] Kubo, et al.^[45] reported that the suspended particles in homogenized TJ were smaller at higher HP values, highlighting the effect of HPH in disrupting the fruit pulp particles.

The microstructural changes caused by HPH process can also be observed in homogenization-treated potato pulp, various fruit or vegetable products such as cashew apple juice, ^[20] OJ^[32] and TJ .^[45]

Pulp sedimentation (PS)

HPH increased juice stability to sedimentation and changed its color due to leakage of lycopene from the disrupted cells .^[45] Stokes law describes the sedimentation velocity of spheres as a function of the properties of the particles and the dispersed medium. According to Stokes law, the particle sedimentation velocity is proportional to the particle size (diameter) and the difference between the densities of the particles and the dispersed medium. It is inversely proportional to the dispersed medium viscosity. Therefore, the reduction in particle size during homogenization can be related to the greater stability of the homogenized samples, and HPH can be seen as an essential tool in preventing PS .^[45]

Different behaviors were observed for pineapple pulp, ^[39] where even the homogenized samples showed PS after ten days of storage (25°C). This difference can be attributed to the formation of particle aggregates, whose importance is different for each vegetable. Lopez‐Sanchez, et al.^[68] reported that each vegetable cell wall had a different behavior when processed by HPH. While carrot tissue requires higher shear values to be disrupted, the cell walls of tomato cells were broken even at moderate shear values. In another study, the effect of HPH on the physical properties of taro pulp was studied by Yu, et al. .^[43] Their results showed that HPH processing improves the suspension stability of taro pulp, promoting the pulp with desirable physical property changes. In addition, HPH treatment changed the PSD, microstructure, zeta potential, PS behavior, SC, color and total soluble solids of taro pulp.

Commonly, fruit and vegetable serum consists of soluble polymeric compounds such as pectin, sugars, acids, proteins and salts. HPH was reported to induce disruption of cells, making smaller size particles including insoluble polymer clusters and tissue fragments (cell walls and organelles). In addition, HPH could cause the release of serum components, which could bind within the serum phase as well as with particles, and the dispersion becomes more stable .^[15] Kubo, et al.^[45] reported that HPH treatment could be used in preventing PS. Moreover, HPH processing could release and modify serum components including serum pectin, which could bind with SS, enhancing cloud stability .^[76]

Fruit juices are composed of an insoluble phase (the pulp) dispersed in a viscous solution (the serum). The dispersed phase, or pulp, is constituted of fruit tissue cells and their fragments, cell walls and insoluble polymer clusters and chains. The serum is an aqueous solution of soluble polysaccharides, sugars, salts and acids .^[69] In the SC analysis, a parallel beam of radiation is passed through the sample and its absorbance is obtained. The absorbance is thus directly related to the sample cloudiness/turbidity, as the suspended particles are responsible for the absorption of radiation. SC decreased with increasing HP.

Augusto, et al.^[54] described the importance of the non-hydrodynamic forces (electrostatic, Van der Waals) of juice processed by HPH, where the reduction in its size increases the particle surface area. As a result, aggregates are formed, whose behavior during centrifugation and the turbidity analysis is more complicated. Precipitation of these aggregates is a function not only of their size but also of their density, which is directly related to their porosity.

The cloudy stability in OJ was preserved after 10 min of processing at pressures in the range of 500 to 900 MPa .^[31] HPH (0–250 MPa) of OJ to inactivate pectinmethylesterase was studied by Welti-Chanes, et al. .^[30] Their results showed that the cloudy appearance of the homogenized OJ was maintained for 12 days under low temperature conditions.

Color change (CC)

In the L*a*b* space, L* is lightness/darkness that ranges from 0 to 100, a* is redness/greenness that ranges from −120 to 120 and b* is yellowness/blueness that ranges from −120 to 120 .^[77–79] HPH induced color modifications of the samples. In particular, significant color differences were reported by Calligaris, et al.^[16] between the untreated and treated BJs: an increase in the L* and b* values as well as a decrease in a* values were reported as a consequence of HPH. These changes indicate that the homogenized BJs appear brighter after processing. By contrast, lightness (L*) of the homogenized BJs tended to decrease only after 20 days of storage while, no significant changes were reported below this storage time. This result is an exciting effect since fresh fruit juices are generally expected by the consumers to have a light color due to the higher ability of small particles to scatter light leading to an increase in sample lightness.

HPH processing showed no significant impact on color variables of apple, carrot and peach mixed juices. On the other hand, the L* and b* variables were slightly enhanced by HPH processing .^[15] Saldo, et al.^[12] reported increases in L* value of AJ after HPH at 200 MPa and 300 MPa, while the a* values significantly increased after HPH at 100 MPa, then considerably decreased as the pressure increased to 200 and 300 MPa. Zhou, et al.^[23] reported a decrease in L* value of mango juice after HPH at 20, 40 and 60 MPa. At the same time, the contrary result was observed for a* values. Notably, the a* values increased as the pressure, temperature and pass number increased. That was to say, HPH resulted in luminosity loss and an increase of red-color intensity in this study. Tribst, et al.^[61] reported that the L* and a* values of mango nectar both significantly decreased after HPH at 300 MPa. So, the effects of HPH on color of juice could strongly depend on food matrices and treatments. Betoret, et al.^[38] reported that the lightness of the Salustiana juice samples increased as the HP increased. The values for a* and b* were higher in samples homogenized at 30 MPa than in unhomogenized samples and were smaller in samples homogenized at 15 MPa than in unhomogenized samples.

HPH Potential on the shelf-life and functionality of kiwifruit juice (KJ) was investigated by Patrignani, et al. .^[22] The HPH treatments caused a significant increase in color parameters in comparison to the control samples. Concerning a* and b* parameters, both samples treated at 200 MPa showed lower values than the control sample. During the storage at all considered temperatures, a slight decrease of L* and increasing of a* was observed, while b* remained almost unchanged in control and 200 MPa × 2cycles treated samples. The application of treatment at 200 MPa for three cycles allowed to obtain a stable KJ for more than 40 days under refrigerated storage and to extend the shelf-life of 1 week at room temperature concerning the control, increasing at the same time the polyphenols availability and its AA, and allowing to retain the color better.

Bioactive compounds (BC) and antioxidant activity (AA)

The influence of UHPH processing (100, 200 and 300 MPa) on BC and AA of OJ was investigated by Velázquez-Estrada, et al. .^[29] Their results showed that the flavanone content increased with the UHPH treatments and more specifically, the amounts of hesperidin, achieving its highest concentration on the samples treated at 200 and 300 MPa. In addition, no significant differences were reported on the total polyphenol content and antioxidant capacity values between the fresh and the UHPH treated OJ samples while these values were significantly lower in the thermal pasteurized one. Suárez-Jacobo, et al.^[9] studied the effect of UHPH treatments on the antioxidant capacity, polyphenol composition, vitamin C and provitamin A contents of AJ, reporting that UHPH-treatments significantly reduced the degradation of most of these compounds compared with pasteurized samples.

Welti-Chanes, et al.^[30] reported that vitamin C content of OJ was not affected by UHPH when was treated between 50–250 MPa at 22°C. HPH processing, thermal treatment and milk matrix effect the in vitro bioaccessibility of phenolics in apple, grape and orange juice to different extents were studied by He, et al. .^[80] HPH processing decreased the total phenolic bioaccessibility of AJ by 29.3% but did not significantly affect Grape juice (GJ) or OJ. Karacam, et al.^[40] observed a higher viscosity (gel-like structure) in strawberry juice treated at 100 MPa for two passes (cycles), compared to 5 passes. According to the authors, the temperature increase during the treatment at 100 MPa×2 passes reached the optimal temperature for the activation of pectinmethylesterase (43°C). In addition, their results indicated that HPH could be used to improve the physical and chemical properties in the Ottoman Strawberry juice, such as increasing total phenolic content, AA and affecting color positively resulting in a desirable high-quality juice for the consumer.

Rheological properties

The RP of F&VJ has confirmed to be influenced by many factors such as temperature, pH, sugar type and content, particle size, PSD and the other composition of dispersed particles .^[81,82] Several works have investigated the effect of HPH on the PSD and RP of many fruit or vegetable products .^[44] The research works suggest that HPH treatment caused changes in particle shape, particle size, PSD of the products, and, consequently, affected their RP. In addition, HPH processing decreased the viscosity of the juice serum^[69] and increased the consistency, thixotropy, viscous and elastic behavior of the TJ .^[54] A rheological analysis indicated that this technology could be used to increase the consistency of TJ, improving its sensory acceptance, reducing the need for adding hydrocolloids and reducing particle sedimentation and serum separation .^[45] Effect of mechanical and thermal treatments on the microstructure and RP of carrot, broccoli and tomato dispersions was investigated by Lopez‐Sanchez, et al. .^[19] HPH decreased the viscosity of carrot and broccoli dispersions, while it increased the viscosity of tomato. Carrot, tomato and broccoli dispersions had a pH of 5.9, 4.3 and 6.2, respectively. HPH had no significant effect on pH.

Some works have studied the PSD changes concerning the Herschel-Bulkley model parameters. Augusto, et al.^[54] reported that HPH reduced the particle size of TJ, and the reduction in particle size increased the juice apparent viscosity (AV, η_a), yield stress (σ₀), flow behavior index (n) and decreased the consistency index (k). Bayod, et al.^[72] and Lopez‐Sanchez, et al.^[19] observed that the homogenization of tomato products induced an increment in the percentage of small particles, hence increasing the products yield stress. The aim of Calligaris, et al.^[16] study was to evaluate the potential applicability of HPH for the production of BJs. To this purpose, a prototype equipment working up to 400 MPa and a lab-scale homogenizer working up to 150 MPa was used. Following HPH, BJ resulted in less viscous than the untreated one. In addition, increases in the HP did not induce significant changes in the AV of BJs. In Yu, et al.^[44] study, the yield stress value of taro pulp increased as the HPs increased, suggesting that the homogenized pulps required higher shear stress to initiate the pulp flow. HPH increased pulp particle specific surface area, improving the particle-particle interactions and thereby resulting in the increase of pulp interparticle bond strength. Thus, as for the homogenized pulps, the increase in pulp yield stress could be attributed to the formation of interconnected-particle networks due to attractive particle-particle interactions, giving rise to the reinforcement of taro pulp internal structures. Silva, et al.^[39] also observed that the decrease in particle size of pineapple pulp was due to homogenization promoted an increase in pulp flow behavior index and reduction in the consistency index.

The effect of HPH processing in the product RP is different for each F&VJ, since the composition of dispersed particles for each product is different. HPH processing also affected both storage modulus (G′) and loss modulus (G″). The G′ and G″ were lower in high pressure homogenized apple, carrot and peach mixed juices than in non-homogenized mixed juices, which might be attributed to lower SS and water-soluble pectin contents in HPH-treated mixed juice samples. Also, the G′ was significantly higher than G″ at all frequencies and no crossover of G′ and G″ was observed, indicating more solid-like behavior in homogenized mixed juice samples .^[15]

From an engineering point of view, a minor consistency leads to a minor friction loss during processing and distribution, thus minimizing the amount of energy required to flow. HPH technology could be used to reduce the consistency of concentrated juices with low pulp content. The use of HPH to reduce the consistency of concentrated OJ was examined by Leite, et al. .^[32] The HPH decreased the product consistency, with a reduction of up to 50% on its AV and 64% on its consistency index at −10°C. The flow behavior index increased by 11%. HPH reduced the mean particle diameter to 12% of its original value (232 to 28 μm). In another study, the effect of HPH (0 to 60 MPa) on the RP of taro (Colocasia esculenta (L). Schott) pulp was investigated .^[44] HPH reduced taro pulp particle size, increased the dispersion of pulp particles and pulp consistency. HPH increased the yield stress (σ₀) and consistency index (k) of taro pulp, and decreased the flow behavior index (n). Moreover, HPH increased taro pulp both the storage and loss modulus, showing that HPH improved pulp both elastic and viscous behavior. The effect of HPH on pH, total soluble solid, color, PSD, microstructure, ζ-potential, water-soluble pectin, AV, thixotropy and dynamic RP of mango juice were investigated by Zhou, et al. .^[23] HPH changed the content of water-soluble pectin, and the content was increased with increasing inlet temperature and passes number, whereas no change was found for ζ-potential. HPH significantly enhanced the AV of mango juice, meanwhile thixotropy, G′ and G″ were also altered, and those changes were related to the HPH condition applied.

HPH has also been proved to inactivate or modulate the activity of enzymes that cause phase separation in F&VJ, to preserve the initial juice color, flavor, and aromas and, finally, to retain the nutritional and functional features of the treated matrices .^[22] Effect of HPH combined with juice ratio on water-soluble pectin characteristics, functional properties and BC in apple, carrot and peach blended juices was investigated .^[15] HPH influenced functional properties of mixed juices, including cloud stability and RP, which were correlated with physico-chemical and particle characteristics. HPH processing enhanced polyphenol content, which showed significant positive correlations with content and degree of methylesterification of water-soluble pectin. Homogenization is useful in the citrus industry for increasing the yield of citrus juices and for improving some quality factors of citrus juices, such as viscosity, color, cloudiness, and the stability of SS .^[38] Effects of pressure homogenization (0–30 MPa) on particle size and the functional properties of citrus juices (Ortanique and Salustiana fruits) was studied by Betoret, et al. .^[38] Their results showed that HP affected the PSD and color of the juices, which made it possible to define different sample groups based on the applied pressure. In fresh juices, the contents of the flavonoids were not affected by HP but after five months stored juice the content of the flavonoid hesperidin was affected.

Sensorial properties

The food industry has shown an increased interest in the manufacture of healthier and more natural food products. The benefits of HPH include a shelf-life extension through inactivation of microorganisms and improvements in functionality due to increased emulsion capacity and stability, with minimal effects on nutritional value and sensory characteristics .^[3] It can be used as an efficient tool for the processing of F&VJ to improve the sensorial attributes of products. Homogenization converts the sensible pulp to background pulp, which modifies the flavor of the juice .^[83]

A consumer acceptance test revealed that HPH treated AJs were somehow preferred to pasteurized juice; fruity, natural, sweet, and fresh were the most common attributes given to HPH treated AJs .^[12] The influence of UHPH on the physico-chemical and sensorial properties of OJ in comparison with conventional thermal processing was studied by Velázquez-Estrada, et al. .^[28] None of the UHPH treatments caused significant differences in the °Brix, reducing sugars, pH and non‐enzymatic browning index with respect to fresh or pasteurized juice. In addition, the overall consumer acceptability of UHPH and pasteurized OJs was similar.

Conclusion

HPH process is a non-thermal technology applied in the food industry, mainly used to disrupt pathogens and spoilage microorganisms, inactivate enzymes and improve the nutritional and functional quality of F&VJ. The successful application of HPH treatments may provide with a new generation of minimally processed products, hopefully closer to the fresh ones in organoleptic and nutritive properties, but as safe and lasting as the pasteurized ones. The HPH could be used as a valuable tool to promote desirable physical property changes in fluid food products, such as increasing the consistency and reducing particle sedimentation and serum separation, hence improving sensory acceptance. The HPH treatment also delivered significant changes to the distribution of suspended particles, whose comminution had a measurable effect on the F&VJ viscosity. It is a technique specially suitable for industrial applications, because of the ease of operation, scalability, reproducibility, and high throughput. Moreover, HPH can produce a homogeneous size distribution of the vegetable particles suspended in a liquid, by forcing the liquid under the effect of pressure through a specifically designed homogenization valve. So, HPH is useful in the F&VJ industry to improve its quality.