Comparison of technological characteristics of native starter isolates from traditional Khorasan yogurts with commercial starter in yogurt production

One of the most important factors in producing yogurt is choosing the right starter. Native isolates of any country are among the genetic resources of that country, which play a major role in the production and development of the organoleptic properties of fermented products. Therefore, it seems necessary to study the industrial applications of native isolates. In this study, the production of yogurt was investigated by using native starter isolates from traditional Khorasan yogurts and its comparison with yogurts produced with two types of commercial starters.  

Six strains of Streptococcus thermophilus and three strains of Lactobacillus delbrueckii subsp. Bulgaricus isolated from traditional Khorasan yogurts were selected. The proteolytic activity of Streptococcus thermophilus strains was determined according to the method of Erkus et al(2012) and the proteolytic activity of Lactobacillus delbrueckii subsp. Bulgaricus was determined according to the method of Nespolo et al.(2010). Milk acidification activity byall examined strains was evaluated according to the method of Erkus et al.(2012). The production property of gamma-aminobutyric acid of native isolates was also evaluated using the method of Lacroix et al. (2013). Yogurt was produced using commercial starters and native isolates. Acidity and pH were measured according to Iranian National Standard No. 2852. Synersis of yogurt samples was measured using centrifugation according to Çelik (2007) method. In order to measure textural properties, a combination of backward extrusion techniques and Texture Profile Analysis (TPA) was used (Yang et al, 2010). Sensory characteristics of yogurt samples were evaluated by 20 panelists using five-point hedonic scale. This study was conducted based on factorial experiment using a completely randomized design. Statistical analysis of data was performed using Minitab Statistical Software (version 17) and ANOVA. Comparison of means was performed using Tukey’s test at the significant level (p < 0.05).  

The results of proteolytic activity of Lactobacillus delbrueckii subsp. Bulgaricus showed that the L3 code was weaker than the other two strains. All Streptococcus thermophilus strains were positive protease. All Streptococcus thermophilus strains of the study decreased the pH of the milk to 4.6 in less than five hours, and Streptococcus thermophilus strain S6 code, as the fastest acid producer, decreased the pH of the milk to 4.6within 3 and half hours (Figure 2). The results of GABA production showed that among the streptococcus thermophilus strains, S1, S5 and S6 codes had distinct blue discoloration. Among the Lactobacillus strains, except the L3 strain, two other strains produced green color. Of the 18 samples of the yoghurt, S5L2 and S2L2 samples showed the highest decrease in pH and increased in acidity compared to other samples after being stored refrigerated overnight (P <0.05). The other samples had the same pH and acidity changes as the control samples after overnight refrigeration. The results show the ability of yogurt producion by native starters to compete with those produced by commercial starters in terms of technological characteristics. Thus, samples with codes S2L2, S3L1, S3L2, and S1L1 showed less Synersis of   yogurt than commercial starters. This decrease in the Synersis of yogurt might due to the possibility of exopolysaccharide production by these strains. Textural characteristics of yogurt samples were the hardness of the samples in the range of 3 to 4 N, which was significantly different (P <0.05). The lowest hardness was observed for the S3L3, which can be attributed to the poor proteolytic activity of the strain. L3, S6L2, S3L2 and S2L2 had the highest   hardness compared to other compounds and control samples (P <0.05). This differences in the hardness of the samples can be attributed to the possible production of some metabolites by the strains. There was no statistically significant difference in the adhesion properties of the samples (p <0.05). Considering the sensory evaluation scores, it was found that in all respects, S3L3 had the lowest score among the other samples (P <0.05). This was in good correlation with the results of the textural properties of the test. Therefore, this compound was eliminated compared to the control samples. Finally, considering the proteolytic behavior of the strains and the ability to produce acid and GABA, as well as the sensory, rheological and physicochemical properties of 18 samples in comparison to the two control samples, Streptococcus thermophilus strains with codes S4, S2, S5, S1, S6, and Lactobacillus delbrueckii subsp. Bulgaricus strains with codes L1 and L2 have the potential to be used as commercial starters in the form of compounds S1L1, S2L2, S5L1, S6L2, S4L2.