Effect of acid casein hydrolysate and cysteine on the viability of yogurt and probiotic bacteria in fermented frozen dairy desserts

The effect of acid casein hydrolysate and cysteine on the viability of yogurt bacteria (Streptococcus salivarius ssp. thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) and probiotic bacteria (Lactobacillus acidophilus and bifidobacteria) during a storage period of 12 weeks was assessed in fermented frozen dairy desserts. Three batches of fermented frozen dairy desserts were made with milk supplemented with acid casein hydrolysate or cysteine. The control batch was made with milk supplemented with 2% skim milk powder. Viable counts of yogurt bacteria and probiotic bacteria were enumerated and pH, titratable acidity and β-D-galactosidase activity measured at fortnightly intervals for 12 weeks. S. salivarius ssp. thermophilus was most stable in all the samples of fermented frozen dairy desserts with counts ≥10 7 cfu/g throughout the storage period. The counts of L. delbrueckii ssp. bulgaricus was reduced by 2 log cycles in the control sample, whereas in those supplemented with acid casein hydrolysate and cysteine, the counts decreased by 1 and 2 log cycles respectively. The counts of L. acidophilus and bifidobacteria decreased to 10 5 cfu/g in the samples supplemented with acid casein hydrolysate or cysteine. The β-D-galactosidase activity decreased rapidly in the control sample during 12-weeks storage at -18°C, as compared with those prepared with acid casein hydrolysate and cysteine.