Biochemical Properties and Viable Probiotic Population of Yogurt at Different Bacterial Inoculation Rates and Incubation Temperatures

Combined effects of inoculation rate [standard inoculation (S), 2S, 4S and 8S (representing two-fold, four-fold and eight-fold of standard inoculation, respectively)] and incubation temperature (40 or 44 °C) on biochemical and microbiological characteristics of yogurt milk during and immediately after fermentation were investigated. Two probiotic bacteria (Lactobacillus acidophilus LA-5 and Bifidobacterium lactis BB-12), along with the yogurt bacteria Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, were used. Acidification-related parameters during fermentation, incubation time and concentrations of lactic and acetic acids, as well as probiotic bacteria viability at the end of fermentation (final pH of 4.5), were determined. Incubation time and inoculation rate showed interactive effects on biochemical parameters and incubation time, as well as on viability of both probiotic bacteria. The greatest and the lowest pH drop, acidity increase and redox potential increase rates were observed in the treatments 8S-44 (eight-fold inoculation and fermentation at 44 °C) and S-40 (standard inoculation and fermentation at 40 °C), respectively. The concentration of acetic acid was significantly greater (p<0.05) in the treatments incubated at 40 ºC compared with those incubated at 44 ºC. The longest and the shortest incubation times were observed in the treatments S-40 and 8S-44, respectively. The greatest viability for each probiotic bacteria as well as total probiotic bacteria was observed in the treatment 8S-40. The lowest viabilities of L. acidophilus and bifidobacteria were seen in the treatments 8S-44 and S-44, respectively.