Influence of Dissolved Oxygen on Acid Production in Buffalo Milk by Lactic Cultures

Rates of acid production in buffalo milk by lactic cultures increased with the decrease of oxygen content to 2.90 ppm. Rates of production in milk samples containing less than 2.90 ppm dissolved oxygen did not vary. Incorporating oxygen into milk to raise the initial oxygen content from 5.50 to 9.00 ppm strongly inhibited acid production by all lactic cultures. Development of acidity by lactic cultures was more rapid in milk subjected to prior heat treatment and was maximum in milk heated at 90°C for 10 min. This was attributed to a slight descrease in oxygen content and increase in free sulfhydryls in heated milk. The free sulfhydryls and dissolved oxygen content decreased considerably in milk samples inoculated with lactic cultures but not in uninoculated control samples incubated for up to 2 h. The decrease in free sulfhydryls corresponded with the decrease in dissolved oxygen in all inoculated samples. Acidification of milk with lactic acid to the same extent as that found in inoculated milk samples showed no decrease in either free sulfhydryls or dissolved oxygen content, suggesting that lactic cultures catalyze the oxidation of free sulfhydryls with dissolved oxygen thereby effecting a more rapid rate of acid production during microbial fermentation of heated milk. The rate of decrease in free sulfhydryls and dissolved oxygen content and increase in acidity corresponded well with growth of lactics, being greatest in Streptococcus lactis -and least in Streptococcus thermophilus -inoculated milk.