Rheological Properties of Reconstituted Hot Salep Beverage

Abstract

In this study, some physicochemical (solubility index, apparent density and dry matter content) and rheological properties of the hot salep, an indigenous Turkish beverage, were determined. The samples were prepared by adding water or milk to the commerical instant salep powder. The apparent viscosity of samples was measured in the temperature range of 10–50 °C, with the rotational speed of 20–200 rpm. The empirical power law model was used for the determination of the flow behavior and consistency indices of the samples. Dry matter content and apparent density of salp-milk beverage were significantly (p < 0.05) higher than those of salep-water counterpart. Apparent viscosity, consistency and flow behavior indices were also higher in the salep-milk samples. Salep-water sample was more pseudoplastic with lower flow behavior index. Since there is a close relationship between the rehological properties of beverages and consumer preference, the results might be useful in the formulation of instant hot salep beverages.

Keywords:

• Salep
• Hot beverage
• Rheological properties

Introduction

Salep is a well known indigenous Turkish drink whose main ingredient is also called with the same name (salep powder). Hot salep, a viscous dairy beverage, is generally home-prepared and preferred particularly in winter. It is made by boiling salep powder and milk with sugar and served with cinnamon sprinkled on top. In recent years, high demand for hot salep beverage received the attention of manufacturers, and its instant form became available in the market.

Salep (powder) is white-colored flour obtained by milling of dried tubers of certain wild orchids (orchis species). It contains glucomannose, starch, moisture, and mineral matter.1 The most important constituent of salep is glucomannose due to its stabilizing function.2 Therefore, it has been mainly used as a stabilizer in some food formulations; however, it is also preferred for its characteristic flavor which plays an important role in the final product. In addition to its use in the preparation of hot salep beverage, powdered salep is also an essential ingredient for ice cream.

Instant salep which has recently been on the market is manufactured using various formulations. The basic ingredients of instant salep are sugar, milk powder, salep, stabilizer, and cinnamon. For the preparation of beverage from instant salep, hot water, or milk in the amount instructed on the package is added to the beverage powder and then stirred. The amount of sugar, milk powder, type, and concentration of hydrocolloids and flavor as well as processing conditions of the product affect the physical and sensory properties of the final product.3 4

Knowledge on the rheology of beverages is very valuable tool in the processing and quality control of the fluid foods. Its significance in product development in the food industry has been extensively discussed in literature.5 6 7 Consumer preference is influenced from the flow behaviors of a beverage. For example, optimum consistency for a good mouthfeel is a desired quality criterion in the hot salep beverage. Liquid foods are classified as Newtonian and non-Newtonian fluids in terms of rheology according to the relationship between shear rate and shear stress of the solution.8 Shear stress of Newtonian fluids is not depended on the shear rate while that of non-Newtonian fluids changes with varying shear rate. Various models such as Newton, Herschel-Bulkley and Power Law models are used to describe rheological properties of fluid food products.

Although the rheological properties of various dairy foods such as yogurt,9 10 concentrated yogurt (labneh),11 12 ayran,13 ice cream mix,14 15 cheese,16 and chocolate milk3 have been studied, up to our knowledge there is currently no information available in the literature about rheological properties of instant salep beverage. In this study, the rheological and certain physicochemical properties (total solids, apparent density, and solubility index) of instant hot salep beverage were determined, and the applicability of empirical power-law model to describe the rheology of the product was investigated.

Materials and Methods

Samples

Instant salep powder with the moisture content of 5.11% (Dr. Oetker, Piyale, Co, Turkey) and milk (Yorsan Co, Turkey) were obtained from a local market. The main ingredients of instant salep were sugar, modified starch, nonfat dry milk, salep powder, and cinnamon as stated on the label of product. The label on the package specified that 100 mL of milk contained 3.1 g protein, 4.5 g carbohydrate, and 3.0 g fat. In this study, two forms of salep beverage were prepared using milk or water. The first one was prepared by addition of boiled milk to instant salep, and the boiled water was used instead of milk in the second form. For instance, 40 g instant salep and 300 mL liquid (milk or water) were vigorously stirred for about 5 min using a glass bar in a beaker until all solid particles were thoroughly dissolved. Physicochemical properties of samples were measured at ambient temperature. All analysis was carried out in triplicate.

Solubility Index

The solubility index (SI) was determined using a method described by American Dairy Products Institute.17 Three drops of diglycol laurate were added to the sample to prevent foaming, and after leaving samples undisturbed for 15 min, it was stirred with a spoon for 5 s. It was then poured into a conical centrifuge tubes up to the 50 mL mark, and centrifuged (Nuve, Ankara) at 330 g (900 rpm, 360 mm radius) for 5 min, and the supernatant was discarded. The sediments were resuspended in distilled water, and centrifuged one more time for 5 min. The SI of the sample was determined from the volume (mL) of sediment left in each tube after the second centrifugation.

Apparent Density

Apparent density (AD) was determined according to the method described in literature.18 A few drops of diglycol laurate were also added into the sample to prevent foam formation. The prepared sample was manually mixed in the cylinder for 3 min, held undisturbed for 1 h and mixed again for 1 min before the measurement of apparent density. The tared cylinder filled with sample was weight by GR200 balance (ANND, Japan) and the same cylinder was weight after filling it with distilled water. The ratio of sample to water gave the AD value.

Proximal Analysis

Dry matter (DM) content of samples were measured by weighing 10 g of sample in a petry dish and drying in an oven for 3 h at 102 ± 3°C.19 Protein content was calculated from the percentage of nitrogen which was determined by UDK140 Kjeldahl analysis system (Velp, Italy). Ash contents of samples were determined by incineration of predried samples in oven at 500°C for 8 h. Fat content was measured using Gerber method.19 A pH meter (Inolab, Germany) was used to determine the pH of samples.

Viscosity Measurement

The apparent viscosities (AV) of samples were determined using J.P. Selecta, s.a. Brand Viscometer (Viscostar-R, ST-TEMP, Spain,) at various temperatures (10, 20, 30, 40, and 50°C). Appropriate spindle (number 4) was used to obtain correct readings of AV in the shear rate range of 0.33–3.33 rps. Three readings from the same batch (150 mL) for each speed were taken 1 min interval and average value vs. speed was recorded as AV of the sample.

Statistical Analysis

All statistical calculations were completed using the SAS system.20 One way Analysis of Variance (ANOVA) was carried out using the general linear model (GLM) procedure.

Results and Discussion

Physico-Chemical Properties

Some physico-chemical properties of the reconstituted salep samples were given in Table 1. Although the solubility index of milk-salep was higher than that of water-salep due to the certain amount of dry matter from milk, the difference was not statistically significant (p > 0.05). The apparent density (AD) also showed the similar trend and the value for salep–milk samples was significantly higher (p < 0.05) compared to that of salep–water. The average AD values were determined as 1.035 and 1.061 for reconstituted salep–water and salep–milk samples, respectively. Dry matter content of sample prepared with water was 12.30 g 100 g⁻¹ sample and 23.58 g 100 g⁻¹ sample for the sample prepared with milk. Protein content of salep–water sample was 9.08% in dry basis while that of salep milk was 18.49%. Fat content was 0.200% for salep–water and 2.20% for salep–milk. It was determined that total ash was 0.156 and 0.757% for reconstituted salep–water and salep–milk samples, respectively. Salep-water pH was 6.105 and salep–milk pH was 6.049.

Table 1 Physico-chemical properties of reconstituted salep beverages samples

Properties	Salep–water	Salep–milk
Dry matter (g/100 g sample)	12.30	23.58
Solubilty index (mL)	4.30	4.50
Apparent density (g mL^−1 sample/g mL^−1 water)	1.035	1.061
Ash content (%)	0.156	0.757
pH	6.105	6.049
Fat content (%)	0.200	2.20
Protein content (% dry basis)	9.08	18.49

Rheological Properties

The apparent viscosity of the hot salep beverage decreased with increasing shear rate for all samples as the sample showed pseudoplastic behavior. Shear-thinning behavior of sample was expected since weak physical bonds, electrostatic, and hydrophobic interactions affect the rheology of a milk product.10 21 Changes in apparent viscosity with respect to shear rate at 20°C was illustrated in Fig. 1. As shown in the figure, the apparent viscosity of salep–milk sample was higher than that of salep–water sample, because the reconstituted milk-salep evidently had higher solid content. Similar results were also observed at the other temperatures. These results agreed with published studies in the literature for various food samples such as pekmez (grape molasses)/tahin (sesame paste) blend, ice cream mix, and sunflower oil–water emulsions.12 22 23 It was stated that the viscosity increase with higher solid content could be as a result of molecular movements and interfacial film formation.24 Additionally, it should be noted that higher solid content in the solution causes the molecules come closer to each other; thus, some changes in structural network and ionic strength might occur and affect the intermolecular interactions.4 As a result of these interactions, rheological behavior of sample changes; in relation, the apparent viscosity of sample increased. It could also be speculated that in the sample prepared with milk, caseins, and salep (hydrocolloid) may form a network of which could have consequences in the rheological behavior of the sample. It was reported that the viscosity of carrageenan added milk samples was dependent on carrageenan concentration and its tendency for the formation of a network with casein.25 Similar results were also reported by other researchers.3 26

Figure 1. The relationship between viscosity and rotational speed at 20°C.

The apparent viscosity of samples in both forms of reconstituted salep–water and salep–milk increased with decreasing temperature as shown in Figs. 2 and 3. A substantial viscosity increase was observed when the temperature decreased from 20 to 10°C and with increasing shear rate, the effect of temperature on the apparent viscosity became less prominent. This finding might be related to the behavior of stabilizer present in the instant salep in low temperature.

Figure 2. Effect of temperature on the apparent viscosity of salep–water samples.

Figure 3. Effect of temperature on the apparent viscosity of salep–milk samples.

The power law model was used to describe the rheological properties of instant salep beverage. The flow behavior index (n) and consistency index (K) values were obtained by fitting the rotational speed vs. apparent viscosity values to the power law model.

where η _a is the apparent viscosity (Pa s), K (Pa s ⁿ ) is the consistency index, γ is the rotational speed (s⁻¹) and n (dimensionless) is flow behavior index. Table 2 illustrates the rheological properties of samples. The flow behavior index ranged from 0.25 to 0.44 for the salep–water samples and 0.44 to 0.57 for salep–milk samples; the consistency index ranged between 107–407 for salep–water and 564–1453 mPa s for salep–milk samples. The R ² values from fitting of the data were generally higher than 0.984. The results obtained from the rheological properties suggested that the reconstituted salep–water samples more pseudoplastic with lower flow behavior index as compared to salep–milk counterparts. It was observed that the flow behavior index of salep–milk was generally higher than that of salep–water (Table 2). This might be related to the interaction between stabilizer in the instant salep mix and the fat and protein present in milk. The consistency index linearly decreased with increasing temperature (R ² = 0.987 for salep–milk, R ² = 0.926 for salep–water). The decrease in consistency index with increasing temperature was reported by several other researchers in the literature for various samples.21 27 The salep–milk samples had higher consistency index than salep–water samples which could be related to the greater solid content in the salep–milk samples. Similar results were also reported in ice-cream mixes in which they determined that with increasing solid content in ice-cream mix, the consistency index increased.12

Table 2 Effect of temperature on rheological parameters of reconstituted salep beverage

Sample	Temperature (°C)	K (mPa s^n)	n	R ^2
Salep–water	10	407	0.44	0.994
	20	346	0.37	0.996
	30	281	0.25	0.989
	40	183	0.26	0.984
	50	107	0.38	0.999
Salep–milk	10	1,453	0.44	0.999
	20	1,026	0.47	0.999
	30	946	0.48	0.997
	40	762	0.48	0.998
	50	564	0.57	0.993

Conclusion

In this study it was determined that the rheological behavior of hot salep beverage prepared from commercial instant salep was depended on whether it was made by addition of milk or water. The temperature of the samples also affected the rheological properties. It was found that hot salep beverage prepared from commercial instant salep powder exhibited pseudoplastic behavior. Those samples prepared with milk had higher apparent viscosity, consistency, and flow behavior indices. It is known that rheological properties of liquid affect the consumer preferences; hence, the results obtained in this study could be used in the development of new instant salep beverage formulations. Also, the results in this study revealed that milk is recommended for the preparation of instant hot salep beverage in terms of the rheological properties of the product.

Acknowledgment

This work has been supported as financial by Erciyes University Scientific Research Project Unit (Project No: EUAP 01-51-7; Kayseri, Turkey).