Survival of some food-borne bacteria in kefir produced by microbial levan and pullulan

Abstract

Kefir is a fermented milk product produced by kefir grains traditionally or lyophilized starter culture. Some quality characteristics of kefir produced at 22 °C and 30 °C using kefir grains after addition of microbial levan and pullulan were investigated for 14 days. Additionally, the survival of different Gram-positive and Gram-negative bacteria strains which were inoculated before and after fermentation were investigated during storage. The pH and titratable acidity were in the ranges of 4.67–4.39 and 1.10–0.87%, respectively. The highest exopolysaccharide values (458.3–465.3 mg/kg) were found in the samples produced with the addition of 1% (w/v) of microbial levan at 30 °C. Yeast counts were in the ranges of 6.73–5.11 log CFU/mL while the lactobacilli and lactic streptococci counts were in the ranges of 9.64–7.91 and 9.64–8.69 log CFU/mL, respectively. The microbial polysaccharide addition did not show any significant differences in the enumeration of lactic acid bacteria in kefir (p > 0.05). All Gram-positive and Gram-negative bacteria grew/survived during fermentation however Listeria monocytogenes was the most susceptible test bacterium to metabolites during storage. The highest reduction in the counts of L. monocytogenes was determined as 2.39 log units after 24 h of fermentation at 30 °C in the samples which were inoculated before fermentation and produced with the addition of pullulan. Kefir samples produced at 30 °C without polysaccharide addition were considered as the best with respect to appearance, taste, consistency and overall acceptability.