Survival of Listeria monocytogenes in Vanilla-Flavored Soy and Dairy Products Stored at 8°C

The survival of Listeria monocytogenes V37 in vanilla-flavored yogurt (low-fat and nonfat) and soy milk (low-fat and Plus) stored at 8°C for 31 days was investigated. Commercial samples of yogurt and soy milk were used. These samples were inoculated with either 10⁴ or 10⁷ CFU of L. monocytogenes per ml. Sampling was carried out every 3 to 4 days initially and was then carried out weekly, for a total storage time of 31 days. Each time a sample was collected, the pH of the sample was measured. After 31 days, low-fat plain, low-fat vanilla, and nonfat plain yogurt samples inoculated with 10⁴ CFU/ml showed 2.5-log reductions in viable cell populations, and nonfat vanilla yogurt showed a 3.5-log reduction. For yogurt inoculated with 10⁷ CFU/ml, reductions of 2.5 log CFU/ml were observed for plain low-fat and nonfat yogurts, and reductions of 5 log CFU/ ml were observed for vanilla-flavored low-fat and nonfat yogurts. In vanilla-flavored and plain low-fat and Plus soy milk samples, cell counts increased from 10⁴ and 10⁷ CFU/ml to 10⁹ CFU/ ml at 7 and 3 days of storage, respectively, at 8°C. Coagulation in soy milk samples was observed when the cell population reached 10⁹ CFU/ml. In soy milk, the L. monocytogenes population did not change for up to 31 days. Vanillin had an inhibitory effect on L. monocytogenes in yogurt but not in soy milk.