Effect of cysteine on the viability of yoghurt and probiotic bacteria in yoghurts made with commercial starter cultures

Viability of yoghurt bacteria (Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus) and probiotic bacteria (Lactobacillus acidophilus and bifidobacteria) was assessed during manufacture and 35d storage in yoghurts made with four commercial starter cultures supplemented with 0, 50, 250 and 500mg L−1 cysteine. Incubation time to reach a pH of 4.5 was greatly affected by the addition of cysteine. During refrigerated storage, the relative drop in pH was higher in yoghurts containing 250 and 500mg L−1 cysteine, but similar in yoghurts prepared with 0 and 50mg L−1 cysteine. The redox potential remained negative for 25–30d in yoghurt supplemented with 500mg L−1 cysteine, but remained positive throughout storage in yoghurt made without cysteine. The redox potential gradually increased to positive values in all yoghurt samples; however, the rate of increase was affected by the level of cysteine and the type of starter culture. Supplementation with 50mg L−1 of cysteine promoted the growth of yoghurt bacteria in yoghurts made with all the four commercial starter cultures. The growth of S. thermophilus was adversely affected with cysteine above 50mg L−1 and electron micrographs showed cellular damage at 500 mg L−1 cysteine. Counts of L. acidophilus during manufacture and storage were higher in yoghurt containing 250 mg L−1 cysteine. Addition of cysteine adversely affected the viability of bifidobacteria in yoghurts made with starter cultures that contained both yoghurt bacteria, whereas the viability was improved in yoghurts made with starter cultures that contained S. thermophilus as yoghurt bacteria.