Hamid Pourbaba - Department of Food Hygiene, Science and Research Branch, Islamic Azad University, Tehran, Iran
Amir Ali Anvar - Department of Food Hygiene, Science and Research Branch, Islamic Azad University, Tehran, Iran
Rezvan Pourahmad - Department of Food Science and Technology, College of Agriculture, Varamin-Pishva Branch, Islamic Azad University, Varamin, Iran
Hamed Ahari - Department of Food Science and Technology, Science and Research Branch, Islamic Azad University, Tehran, Iran.

 BACKGROUND: Kefir is a fermented milk product containing some anticarcinogenic organic compounds with nutritional benefits, which made it one of the natural dairy beverages extensively consumed. OBJECTIVES: The present study was conducted to assess the effect of two selective probiotics on the values of acidic criteria and probiotic survival in the kefir produced in this study. METHODS: In the first step, the cow milk, preheated at ۹۰oC for ۵ min, was inoculated with the commercial starter and divided into two groups. They were complemented with L. acidophilus LA-۵ and L. paracasei ۴۳۱ and incu-bated at ۳۰oC for ۶ h. They were then preserved at refrigerated temperature up to ۱۴ days and then sampling was carried out to evaluate the changes of values of organic acids (lactic acid and acetic acid), pH, titratable acidity and survival of probiotic complemented bacteria on the ۱st, ۷th, and ۱۴th days. RESULTS: The pH values of L. acidophilus LA-۵ and L. paracasei ۴۳۱ were ۴.۳۴ and ۴.۳۶ at the beginning of the cold storage and reached ۴.۲۷ and ۴.۳۱ at day ۱۴. The acidity of L. acidophilus LA-۵-complemented kefir on the ۱st day was ۰.۸۰ gr/۱۰۰ gr higher than L. paracasei ۴۳۱-complemented kefir which showed ۰.۷۲ gr/۱۰۰ gr. Lactic acid was ranging from ۱.۵۷ to ۲.۴۰ gr/۱۰۰ mL or ۲.۱۷ to ۲.۴۲ gr/۱۰۰ mL (from the ۱st to the ۱۴th day) in the kefirs complemented with L. acidophilus LA-۵ and L. paracasei ۴۳۱, respectively. In the kefirs complemented with L. acidophilus LA-۵ and L. paracasei ۴۳۱, the acetic acid was stable (from ۰.۱۱ to ۰.۱۳ gr/۱۰۰ mL) during ۱۴ days but increased in the later (from ۰.۱۱ to ۰.۲۳ gr/۱۰۰ mL) .The survival of both bacteria was higher than ۷ logs CFU/gr in the kefir. CONCLUSIONS: Adding L. acidophilus LA-۵ and L. paracacei ۴۳۱ can moderate the acidity of the kefir and extend the survival of complementary probiotics at a standard level during two weeks of cold preservation.

Acidic parameters, kefir, lactobacillus acidophilus, Lactobacillus paracasei, Survival