Effect of Soymilk Substitution on the Rheological and Sensory Properties of Salep (Traditional Turkish Milk Beverage)

Abstract

Soymilk mixed at a proportion of 0%, 25%, 50%, and 75% (g/100 g total milk) with cow's milk to make a salep, a beverage prepared from the dried tubers of Orchis mascula. The values of the flow behaviour index, n, of salep samples decreased as the soymilk content in the beverage increased. All salep beverages exhibited a pseudoplastic behaviour. Among the samples, salep beverage containing 25% soymilk was scored the highest in terms of overall acceptance by sensory panellists. Statistically significant (p < 0.05) differences were determined for colour, odour, taste, mouthfeel, and overall acceptance as the concentration of soymilk increased in salep beverage. This study produced a beverage from cow's milk and soymilk to offer a promising nutritious and healthy alternative to the consumers with a reasonable acceptance. The rheological parameters of the cow's milk-soymilk salep blends obtained in this study may have useful implications for design and processing.

Keywords:

• Salep
• Beverage
• Soymilk
• Milk
• Viscosity

INTRODUCTION

The dried tubers of Orchis mascula are used to make a salep, which is a milk beverage of subtle flavour and consistency enjoyed throughout the Eastern Mediterranean. Salep powder is white powder obtained by milling of dried tubers. It has characteristic flavour, which plays an important role in making beverage. For the preparation of salep beverage, cow's milk, sugar, and powder of the salep are mixed together; after boiling, it is consumed as a hot beverage. Salep powder contains glucomannose (16–55 g/100 g), starch (2.7 g/100 g), moisture (12 g/100 g), and mineral matter (2.4 g/100 g).[1]

Recently, soy-foods have received much attention for their preventive effects on chronic diseases such as arteriosclerosis, cancer, osteoporosis, and menopausal disorder.[2,3] Soymilk is an excellent source of high-quality protein, B-vitamins, iron, unsaturated fatty acids, lecithin and isoflavones, contain no cholesterol or lactose, and may be consumed by those suffering from lactose intolerance.[4,5] Despite these merits, many consumers find the taste of plain soymilk to be unappealing.[6,7]

The food industry is moving toward developing new products by using innovative ingredients with health benefits. Beverages based on fruits and milk products are currently receiving considerable attention as their market potential is growing.[8] Development of soymilk-based beverage would be beneficial in response to the increased need for an alternative to the dairy-based beverages. In the design of a beverage, the possible effects of added thickener on the acceptance must be taken into consideration. It is known that agents affecting viscosity will also alter both the mouthfeel and flavour properties of the beverage[9] and could also influence consumer attitudes and expectations towards the product.[10] Consumer preference is influenced from the flow behaviors of a beverage. The rheological properties of salep beverages have been studied,[11,12] however, there is currently no information available in the literature on the properties of soymilk salep beverage.

In the food industry, reliable rheological data is essential for design, process and quality control, sensory assessment, stability, and consumer acceptance of a product.[13,14] The flavour of soymilk is the main factor limiting its consumption by the western population. Flavour and aroma of soymilk are formed by a complex combination and interaction of chemical compounds.[15] The characteristic flavour of salep would compensate with unpleasant soymilk flavour leading to more preferable plant base drink. In terms of healthy diet, the replacement of cow's milk with soymilk seems alternative way of formulation for salep beverage. The objectives of this study were to prepare salep by blending cow's milk with soymilk, to test rheological and sensory properties of the blends and to evaluate consumer acceptance of the product.

MATERIALS AND METHODS

Soybean variety, A3935, was obtained from Dept. of Crop Science, Harran Univ, Sanliurfa, Turkey. Salep, as a powder, was purchased from Mado Co., Malatya, Turkey. Cow's milk was purchased from Pınar Co, Turkey, in which the label of on the package specified that 100 mL of milk contained 3 g protein, 4.7 g carbohydrate, and 2.2 g fat. Table sugar used in salep preparation was obtained from local markets in Malatya.

Soymilk Preparation

Whole soybeans were washed and soaked overnight in distilled water. After decanting the water, the soybeans (100 g) were soaked in excess water overnight, drained, and washed with fresh water. The hydrated beans were combined with tap water (1 L) and homogenized in a Waring blender at low speed for 5 min. The slurry was filtered through double layers of nylon cheesecloth. Total solid content of soymilk was determined according to AOAC.[16] Total soluble solids content of the soymilk was standardised to 9° Brix.

Preparation of Salep Beverage

The soymilk was heat-treated individually in water baths at 90°C for 1 h. After heat treatment and cooling to room temperature, soymilk mixed at a proportion of 0%, 25%, 50%, and 75% (g/100 g total milk) with cow's milk. A 20 g salep and 50 g table sugar are mixed well in an open pan. Milk-soymilk blends (1.5 L) added gradually with constant stirring to prevent lumping and cooked over low heat for 10 minutes until thickened and smooth in consistency.

Viscosity Measurement

The viscosity of soymilk salep beverages were determined at 40°C using a Brookfield rotational viscometer (Model RVDV-I+, Brookfield Engineering Laboratories, Stoughton, MA) equipped with spindle 4 at the speed of 5, 10, 20, 30, and 60 rpm. Enough sample in a 600 mL beaker was used to immerse the groove on the spindle with guard leg.[17] Temperature was maintained using a thermostatically controlled water bath. The readings of the viscometer were converted into average shear stress (mPa) and average shear rate (1/s) values.[18]

Sensory Analysis

The panelists (4 female and 5 male aged from 19 to 45), who were familiar with salep beverage, were also exposed to soymilk samples for performing descriptive analysis to evaluate sensory attributes of salep beverages. Prepared salep samples were served at hot drink temperature (40°C) in 100 mL white porcelain mugs labelled with three-digit random numbers. Samples for sensory evaluation were kept in a water bath to maintain serving temperature. Panelists were required to stir each sample for 15 seconds. before evaluation and instructed to visually observe, smell, and drink at least four sips of each sample before finishing the evaluation. Water and unsalted crackers were provided for panelists to cleanse their palate between samples. After palate cleaning, a pause (15 s) was imposed before the panellists could assess the next sample.

Panelists evaluated beverage colour (the intensity or strength of a color from whiteness to yellowness), odour (aromatics associated with soymilk and salep evaluated orthonasally), taste (aromatics associated with soymilk and salep evaluated retronasally), and mouthfeel (mouthcoatness and thickness). Attributes were scored on a 9-point hedonic scale where 1  =  absence of attribute or dislike extremely; 5  =  neither dislike nor like; and 9  =  very high intensity of the attribute or like extremely.

The consumer evaluation panel consisted of randomly selected 47 university students, staff and members, 27 male and 20 female, ranging in age from 18 to older than 50 years. Consumers were provided with 100 mL beverage (40°C) samples in white porcelain mugs. Each consumer evaluated acceptance and purchase intent of beverage using a yes/no scale.

Statistical Analysis

All determinations were made in triplicate, unless otherwise specified. Data were analysed one way analysis of variance. The mean comparison was carried out with Duncan's multiple range test using SPSS Institute, software version 9.0.

RESULTS AND DISCUSSION

Sensory Evaluation

Table 1 shows that the greater the soymilk content, the lower the sensory score. In fact, control was the better evaluated sample, although no significant differences were detected from the statistical analysis carried out. The substitutions of cow milk by soy milk at a ratio lower than 50% gives to obtain an acceptable sensory product. The mean scores for colour ranged from 4.5 to 7.8. The control sample had highest colour score followed by the containing of 50% and 25% soymilk salep sample and the 100% soymilk salep had the lowest colour score. The taste scores of the soymilk salep beverage concentration of 100% and 75% soymilk were lowest, and highest for the beverage substituted with 25% soymilk. Sensory evaluation results suggest substitution of the cow's milk with 25 and 50% soymilk produced acceptable salep beverages. In consumer tests, 60 to 69% of the consumers rated these beverages acceptable and 53 to 61% responded purchase intent question positively. However, consumers were less willing to buy formulations having higher amounts of soymilk (32–44% saying yes) than low-soymilk salep formulations.

Table 1 Sensory properties, consistency index (K, Pasⁿ), and flow behaviour index (n, dimensionless) of milk-soymilk salep beverages. ^A

Sample	Colour	Odour	Taste	Mouthfeel	Overall acceptability	Consumer acceptance ^B (%)	Consumer purchase intent ^B (%)	K	n
Control ^C	7.8^a (0.4)	7.3^a (0.5)	6.8^a (0.5)	7.0^a (0.6)	8.3^a (0.3)	73	65	0.147^a	0.7303^a
25% soymilk	6.8^ab(0.2)	6.8^a (0.4)	6.5^a (0.3)	6.3^ab(0.1)	7.0^a (0.2)	69	61	0.236^b	0.621^b
50% soymilk	7.0^ab(0.2)	6.3^ab(0.2)	5.5^ab(0.2)	6.0^ab(0.3)	6.3^a (0.2)	60	53	0.328^c	0.6059^c
75% soymilk	5.0^bc(0.3)	4.3^bc(0.2)	3.5^b (0.3)	4.5^ab(0.4)	4.3^b (0.1)	50	44	0.471^d	0.5774^d
100% soymilk	4.5^c (0.5)	3.0^c (0.2)	3.5^b (0.3)	3.8^b (0.3)	3.5^b (0.3)	45	32	1.276^e	0.4094^e
^ANumbers in parentheses refer to standard deviation of consumer responses. A 9-point hedonic scale was used (1 = dislike extremely, 5 = neither like nor dislike, and 9 = like extremely). Mean values in the same column not followed by the same letter are significantly different (p < 0.05).
^BConsumer acceptance and purchase intent were evaluated using a yes/no scale.
^CControl: no soymilk added, and 25, 50, 75, and 100% (g/100 g total milk) are the soymilk substitution levels.

The ratings of colour, odour, taste, mouthfeel, and overall acceptability were positively correlated with all the treatments and overall correlations are given in Table 2. Taste had a positive correlation with overall acceptability, with r = 0.915, and the colour attribute had positive correlations with taste (r = 0.660).

Table 2 Correlation between rheological parameters and certain sensory attributes

	Colour	Odour	Taste	Mouthfeel	Overall acceptability
Colour	1.000
Odour	0.477∗	1.000
Taste	0.660∗∗	0.552∗	1.000
Mouthfeel	0.365	0.728∗∗	0.409	1.000
Overall acceptability	0.651∗∗	0.651∗∗	0.915∗∗	0.610∗∗	1.000
N	0.637	0.730	0.620	0.573	0.760
K	−0.604	−0.727	−0.596	−0.551	−0.708
∗∗Correlation is significant at the 0.01 level;
∗Correlation is significant at the 0.05 level;
n: flow behaviour index (dimensionless);
K: consistency index (Pa s^n).

Rheological Behaviour

The flow behaviour index (n) and consistency index (K) values, obtained by fitting the rotational speed versus apparent viscosity data to a power law model (Eq. (1)), are presented in Table 1. Chhinnan et al.[13] reported that fundamental rheological information could be achieved from viscosity-rotational speed data of Brookfield viscometers.

(1)

Where η_a is the viscosity (Pa s), K is the consistency index (Pa s ⁿ ); γ is the rotational speed (s⁻¹); and n is the flow behaviour index (dimensionless).

Soymilk salep beverages exhibited a pseudoplastic behaviour because the values of flow behaviour index (n), a measure of the departure from Newtonian flow,[13] were less than 1.[17,19] The smaller the n values the greater the departure from Newtonian behavior. Similar results were also reported in milk-salep mixtures[13] and in other beverage studies.[19,20] Kaya and Tekin[21] reported that the power-law index values (n) of milk-salep-sugar and water-salep-sugar mixes were in the range of 0.95–0.77 and 0.99–0.85, respectively.

Lower K values were obtained for soymilk salep beverages indicate a decrease in viscosity. The decrease in viscosity as the speed (shear rate) was increased may be related to the decreased alignments of constituent molecules of the tested system.[22] Shear-thinning behaviour of sample was expected since weak physical bonds, electrostatic, and hydrophobic interactions affect the rheology of a milk product.[23] The K values of soymilk salep samples were significantly different (Table 1). The greater the soymilk ratio, the greater the K value. The r ² values ranged from 0.98 to 0.99 for all samples.

Figure 1 represents typical rheological behaviour of soymilk salep beverages. The higher solid contents generally cause an increase in the viscosity resulting from mainly molecular movements and interfacial film formation.[24,25] It is known that the viscosity changes resulting from heat treatments are related to heat-induced denaturation and subsequent aggregation of proteins.[26]

Figure 1 Typical rheological behaviour of milk-soymilk salep beverage (40°C).

It is likely that shear thinning flow behaviour may result form complex involvement of partially broken-down micellar casein at the droplet surface in the milk-salep system.[27] It could also be speculated that in the beverage prepared with milk, caseins, and salep (hydrocolloid) may form a network of which could have consequences in the rheological behaviour of the sample.

CONCLUSIONS

This study developed blends from cow's milk and soymilk to offer a promising nutritious and healthy alternative product to the consumers. The 25% cow's milk-75% soymilk blend was scored the highest in terms of the overall acceptance by sensory evaluation. Salep can serve as effective material for improving the colour, taste, flavour, mouthfeel, and overall acceptability of the products in which soymilk used. This approach would be the starting point for developing novel soymilk products. The rheological parameters of the cow's milk-soymilk salep blends may have useful implications for design and processing in the food industry. As rheological properties of liquid affect the consumer preferences; the results obtained in this study could be used in the development of new salep beverage formulations. Beverages can be considered as a medium for the addition of beneficial components for enrichment of the diet. The findings of this study suggest that food manufacturers may develop acceptable functional beverages that combine healthy ingredients to enhance specific health benefits.

Notes

16. Association of Official Analytical Chemists Official Methods of Analysis. 1990, Arlington.