Study on the Physicochemical, Microbial and Sensory Properties of Functional Soy Kefir Containing Soluble Soybean Polysaccharides during Cold Storage

Introduction: Kefir is a fermented dairy beverage with a sensory attributes similar to that of doogh, but with the higher nutritional and therapeutic properties due to the microorganisms used during manufacturing process. Nowadays, advantageous effects of functional dairy products such as kefir have been proven with many researches (Kesenkaş 2011; Prado et al. 2015; Yilmaz-Ersan et al. 2018; Gamba et al. 2020). It is confirmed that kefir is useful for anorexia, vigilance and nervous syndromes (because of presence of B vitamins, calcium and magnesium) and is helpful for preventing of blood pressure, bronchitis and biliary complaints (Yilmaz-Ersan et al. 2018). Furthermore, tryptophan presented in kefir has a relaxing effect on the nervous coordination. The phosphorus of kefir is also potent for operating of the important organs such as kidneys and liver throughout the function of phosphorus on fat, carbohydrate and energy metabolisms. High lactic acid bacteria count in kefir capable to quandary the cholesterol which consequently decreases cholesterol in the gastrointestinal tract. In addition, kefir has bacteriostatic and bactericide effects due to its competition for nutrients against other microorganisms and also due to different produced metabolites by kefir microorganisms such as lactic acid, acetic acid, H2O2, antibiotics and bacteriocins (Vardjan et al. 2018). Soybean soluble polysaccharide (SSPS) extracted from soybean are acidic polysaccharides having a pectin-like structure with 18% galacturonic acid (Nakamura et al. 2003). The SSPS has the property of being able to disperse and stabilize proteins under acidic conditions, and has recently been used in acidic beverages (Nakamura 2011). On the other hand, soy products particularly at its fermented form are good sources of proteins and isoflavones (Jooyandeh 2011). These products have attained substantial deliberation for their vital function in cultivating health risks such as hazard elements for vascular syndrome. Therefore, in case of using soymilk instead of dairy milk, a functional kefir with noticeable nutritional-pharmaceutical properties can be produced.
Materials and methods: In this research, soy-kefir samples produced from soymilk and kefir grains. Concisely, after heat treatment (90 oC for 10 min), soymilk cooled and inoculated with 3% kefir grain (w/v) and incubated for 24 h at 25°C (Ghasabnezhad et al. 2020). Four kefir samples containing different levels of soluble soybean polysaccharide (SSPS; levels of 0, 0.5, 1 and 1.5%) was produced and some physicochemical characteristics (pH, acidity and viscosity), color parameters (a*, b* and l* values), microbial counts (lactic acid bacteria (LAB) and yeast counts) and organoleptic properties (taste, odor and overall acceptability) of the product were studied during one-month cold storage. Furthermore, the fatty acid compositions of soymilk and soy-kefir sample containing 1% SSPS were also determined using gas chromatography (GC) method after 30 days of storage (Metcalfe et al. 1966).
Results and discussion: It was observed that by increasing the amount of SSPS, the acidity of kefir samples increased and the pH decreased significantly (p < 0.05). Also, until 20th days of the storage, the acidity increased but thereafter it decreased significantly (p < 0.05). The highest acidity (0.5% lactic acid) was recorded for sample containing 1.5% SSPS at 20th days of storage while the lowest acidity (0.23% lactic acid) was verified for control sample (without SSPS) at the beginning of the storage. Similarly, the lowest pH (4.18) was recorded for sample containing 1.5% SSPS at 20th days of storage while the highest pH (4.92) was noted for control sample at the initial days of storage. The results also showed that both experimental variables (SSPS concentration and storage period) had significant effect on the viscosity, color indexes and microbial populations. By increasing the level of SSPS and the storage period, the viscosity was noticeably augmented in kefir samples. The viscosity of soy-kefir chaged from 27.19 cP (in control kefir without SSPS at the first day of storage) to 37.29 cP (in kefir containing 1.5% SSPS at the end of storage). The count of bacteria and yeasts were also depended on SSPS percentage and storage. By increasing the SSPS value and the storage, the bacteria and yeasts counts increased significantly. However, like the acidity, the maximum populations of LAB at different SSPS concentrations were observed at 20th days of storage. By increasing the amount of SSPS and storage, all sensory properties i.e. odor, taste and overall acceptability increased significantly (p < 0.05). The tested two variables also caused an increase in b* value (p < 0.05) and decrease in l* (p < 0.05) and a* (p>0.05) values. The a* values of kefir samples were increased with increasing the SSPS level and storage time. Based on obtained results from different tested parameters (physicochemical, viscosity, microbial quality/yeasts and LAB counts and sensory attributes), no differences were found between kefir samples containing 1% and 1.5% SSPS. Therefore, soy kefir sample with 1% SPSS was selected as the best sample and its fatty acid composition was compared with soy milk after 30 days of storage at refrigerator (4 oC). Determination of fatty acids composition of soymilk and its prepared kefir (containing 1% SSPS) showed that soy kefir had significantly the lower amount of saturated fatty acids (SFA) and the higher amount of unsaturated fatty acids (UFA) as compare with soymilk (p < 0.05).
Conclusion: Today, food is not contracted to only fulfill starvation and to afford essential nutrients for people, but also it considers to inhibit nutrition-linked disorders and to promote physical and mental well-being of purchasers. Even, consumers nowadays are more concerned about mental well-being than physical fitness. The distinct well-being situation is an essential consumer aspect that affects its personal entanglement. Therefore, consumers are looking for functional ingredients/foods. In the other word, functional foods deliver nutraceutical ingredients that impart health benefits to consumers. The advantageous effects of functional dairy products such as kefir have been proven with many researches. By producing kefir from the soymilk, a functional beverage with the higher nutraceutical properties could be produced. On the other hand, by utilizing soluble soybean polysaccharides (SSPS) as prebiotic component, the growth of probiotic microorganisms in kefir may be promoted. Therefore, in the current research SSPS was incorporated in kefir and its effect on kefir characteristics were evaluated. The results showed that by using soymilk containing 1% SSPS, a kefir with acceptable physicochemical, sensorial and microbial properties might be produced and this product can be introduced as a functional food. The count of LAB and yeasts in all the treatments containing the higher level of SSPS (1 and 1.5% SSPS) was more than 108 logcfu/ml at the end of cold storage period.