Effectiveness of ascorbic acid as an oxygen scavenger in improving viability of probiotic bacteria in yoghurts made with commercial starter cultures

The viability of yoghurt and probiotic bacteria was assessed during manufacture and 35 days storage of yoghurt supplemented with four levels of ascorbic acid using four commercial starter cultures. There were no marked differences in titratable acidity and pH of the yoghurt, whereas oxygen content and redox potential varied in yoghurts with various levels of ascorbic acid. Hydrogen peroxide content was initially higher in the products prepared with starter cultures that contained Lactobacillus delbrueckii ssp. bulgaricus. During storage of yoghurt, increase in oxygen content and redox potential and percentage retention of ascorbic acid were identical for all four starter cultures, but different at various levels of ascorbic acid. The oxygen content and redox potential gradually increased during storage in plastic cups and remained lower with higher levels of ascorbic acid. Loss of ascorbic acid occurred during manufacture and storage, with an average retention of only 15-20% after about 35 days storage of yoghurt at 4 degrees C. The viable counts of Streptococcus thermophilus were lower, whereas those of L. delbrueckii ssp. bulgaricus were higher, with increasing concentration of ascorbic acid. The counts of Lactobacillus acidophilus during storage decreased less rapidly with increasing concentration of ascorbic acid for all four starter cultures, whereas the counts of bifidobacteria remained unchanged. (C) 1997 Elsevier Science Ltd. All rights reserved.