Evaluation of bacterial viability and product properties of probiotic yogurt with stevia

In this study, the ability utilising stevia added to non-carbohydrate containing TPY (Tryptone Peptone Yeast Extract) and MRS (Man, Rogosa and Sharpe) basal media under in vitro conditions by probiotic strains of Bifidobacterium animalis subsp. and Lactobacillus acidophilus was investigated. The basal media were supplemented with 0.025% and 1% stevia, 0.025% stevia + 1% inulin, %1 stevia + 1% inulin and 2% culture and incubated for 48 hours at 37 ° C under anaerobic conditions. The medium which contained no carbohydrate was designated as negativecontrol, whereas the media containing 1% glucose or inulin were evaluated as positive. Samples were collected from the cultures developed in basal media on the 0, 12, 24, 36 and 48 hours of fermentation. The samples were analysed for pH, cell density (OD600), probiotic bacteria counts, prebiotic activity score (PAS), lactic acid and short chain fatty acids (SCFA) content. In the second stage of the study probiotic yoğurt supplemeted with stevia was manufactured and the potential prebiotic activity of stevia and its effect on yogurt characteristcs as sugar substitute were evaluated. Depending on the acidity development during fermentation, the cell density (OD600) values of B. animalis subsp. lactis and L. acidophilus showed an increase and the highest bacterial fermentation was observed in L. acidophilus yogurt. The best growth medium for probiotic bacteria was 0.025% stevia and 0.025% stevia + 1% inulin. B. animalis subsp. lactis and L. acidophilus were able to ferment stevia whilst addition of inulin increased the bacterial growth. Microbiological, physicochemical (titritable acidity, syneresis, color (L, a and b), textural (firmness, consistency, cohesiveness and viscosity index) and sensorial (appearance, structure and texture, odor, color, aroma density, taste, sensory acidity and general acceptability) properties of yogurt samples were determined on the 1, 7, 14, 21 and 28 days of storage. The counts of probiotic bacteria remained within the biotherapeutic level (> 7 log kob/mL) during storage in yogurt samples containing stevia and inulin. Yoghurts containing B. animalis subsp. lactis have improved the instrumental textural properties, whereas yoghurts containing L. acidophilus had higher scores for sensorial attributes. Consequently, it was found that stevia improved the survival and activity of B. animalis subsp. lactis and L. acidophilus species in vitro, and could be assigned as a potential prebiotic source and sugar substitute for manufacturing of sugar-reduced dairy products.