Effect of milk supplementation on growth and viability of starter and probiotic bacteria in yogurt during refrigerated storage

In the present study, the effects of milk supplementation on growth and viability of yogurt (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) and probiotic bacteria (Lactobacillus acidophilus and bifidobacteria) were studied during yogurt production and 33 days of refrigerated storage. The incubation time to reach pH = 4.5 was greatly affected by the addition of milk powder (MP), tryptone (TRY) and sucrose (SUC). Also, the increase in titrable acidity depended on added supplement. Viable counts of L. delbrueckii subsp. bulgaricus were significantly (P<0.05) increased in yogurt supplemented with whey powder (WP), TRY and milk powder plus five fold starter culture (MP-SC). However, milk supplementation did not affect the counts of S. thermophilus in probiotic yogurt until the end of storage. Supplementation with TRY and MP-SC promoted the growth and viability of L. acidophilus, whereas milk supplementation with whey protein concentrate (WPC), yeast extract (YE), SUC and Cysteine, adversely affected the viability of L. acidophilus in probiotic yogurt. Finally, using a high level of inoculums (MP-SC) improved the viability of bifidobacteria during storage for 33 days. In conclusion, tryptone and milk powder plus five fold starter culture were found the most effective supplements to improve growth and viability of starter and probiotic (L. acidophilus and bifidobacteria) bacteria in probiotic yogurt during refrigerated storage for a five week period. These findings would be applicable in industrial production of probiotic yogurt.