Research on the usage of heat stress adapted L. acidophilus at different time-temperature combinations in manufacture of ice cream

To manufacture of probiotic ice cream produced by using Lactobacillus acidophilus, which was adapted to heat stress at its diffrerent growth phases (log and stationary phases) and with different manufacturing methods was aimed in this study. Probiotic ice cream was stored at -20C for 90 days and physicochemical properties of the probiotic ice cream samples, and microbiological and sensorial properties of probiotic ice cream samples were determined on day 1 and on days 1, 30, 60 and 90 of the storage, respectively. In the first stage of the study, L. acidophilus at different growth phases (log and stationary phases) was adapted to heat stress, which at different time-temperature combinations, then stored at -20ºC for 30 days. According to the microbiological analysis results, heat adaptation conditions were determined by the highest survival rate of L. acidophilus in the samples during 30 days of storage to the count of heat-adapted L. acidophilus in MRS broth. Adaptation conditions of L. acidophilus, such as temperature, time and growth phase, were combination of 4ºC, 18-hour and log-phase and combination of 45ºC, 15-minute and log phase for below-optimum growth temperature and above-optimum growth temperature, respectively. Probiotic ice cream was produced by using L. acidophilus, which was adapted at specified heat adaptation conditions in milk. Probiotic ice cream was produced by the following two different methods: ice cream mix was fermented by heat-adapted L. acidophilus in milk prior to freezing (the first method) and heat-adapted L. acidophilus in milk was added to ice cream mix prior to freezing (the second method). Probiotic ice cream samples produced by using non-adapted L. acidophilus and with both of manufacturing methods were control samples of the study. Significant differences (P<0.001) were found among ice cream mixes, and the ice cream samples produced by the first method and by the second method regarding physicochemical properties. The viscosity value of ice cream mix was significantly affected by the heat adaptation treatments. At the end of the 90-day storage sufficient numbers (≥ 107 cfu/g) of probiotic bacteria was found in all probiotic ice cream samples. The lowest reduction ratio of number of L. acidophilus in the samples at the end of the storage was determined in the ice cream samples produced with the first method and by using L. acidophilus, which was adapted at 4ºC for 18 hours. According to the sensory evaluation results, probiotic ice cream can be produced by using L. acidophilus, which was adapted to heat stress. However, it was determined that probiotic ice cream produced from non-fermented ice cream mix was more preferred by panelists.