MULTIPLICATION OF SALMONELLA TYPHIMURIUM IN SKIMMILK WITH AND WITHOUT ADDED HYDROCHLORIC, LACTIC, AND CITRIC ACIDS1

Abstract

Cells of Salmonella typhimurium grown in nutrient broth at 37 C were inoculated into samples of sterile skimmilk (approximately 103 cells per ml) which were then acidified with hydrochloric, lactic, or citric acids and incubated at 22 and 37 C. Acids were added at 2-hr intervals in uniform increments sufficient to reduce the pH from 6.70 to 4.0 (hydrochloric acid), 4.25 (lactic acid), or 4.48 (citric acid). Samples taken at 4-hr intervals were plated, and plates incubated at 37 C for 24 hr. At 37 C, maximum numbers (109 cells per ml) were attained after 12 hr, whereas maximum numbers (>108 cells per ml) in acidified milks appeared after 16 hr of incubation. Citric acid was most inhibitory and was followed in order by lactic and hydrochloric acids. At 22 C, highest numbers appeared in all milks after 16 hr of incubation. Again, citric acid proved most inhibitory, and it was followed in order by lactic and hydrochloric acids.