Abstract #T100

Section: Food Safety
Session: Food Safety
Format: Poster
Day/Time: Tuesday 7:30 AM–9:30 AM
Location: Exhibit Hall B
# T100
Survival and growth of Listeria monocytogenes in a model cheese based on pH, moisture, and acid type.
S. K. Engstrom*1, K. A. Glass1, 1University of Wisconsin-Madison, Madison, WI.

High-moisture, low-acid cheeses, e.g., soft, Hispanic-style cheese, have been shown to support growth of Listeria monocytogenes during refrigerated storage. Previous studies have suggested that acetic acid has greater antilisterial activity than lactic acid, and that cheeses of lower pH values (e.g., 5.2) delay growth longer than cheeses of higher pH values (e.g., 5.8); however, no standard pH value for Listeria control has been identified. The objective of this research was to determine the effect of pH, acid type, and moisture on the growth of L. monocytogenes in a model cheese system stored at 4°C for up to 8 weeks. Cream, micellar casein, water, salt, lactose, and acid were combined in 16 formulations targeting 4 pH values (5.25, 5.50, 5.75, or 6.00), 2 moisture levels (50 or 56%) and using 2 acid types (lactic or acetic acid). Each formulation was inoculated with 3-log cfu/g L. monocytogenes (5-strain mixture). 25-g samples were vacuum-sealed and stored at 4°C for up to 8 weeks with triplicate samples enumerated on Modified Oxford agar weekly for L. monocytogenes. All formulations were tested in duplicate trials. Model cheeses formulated with lactic acid supported L. monocytogenes growth at pH ≥ 5.50 for both 50 and 56% moisture levels. Decreasing moisture from 56 to 50% in pH 5.50 model cheeses formulated with lactic acid delayed L. monocytogenes growth approximately 2 weeks, while the same decrease in moisture at pH values of 5.75 and 6.00 did not affect growth. Model cheeses formulated with lactic acid at pH 5.25 did not support L. monocytogenes growth at either moisture level. In contrast, acetic acid delayed growth compared with lactic acid at all pH values. For example, only pH 6.00 acetic acid treatments supported growth in 2 weeks at 4°C, whereas all cheeses with pH adjusted to ≥5.75 with lactic acid supported >2 log increase at the same sampling interval. These data confirm that acetic acid has greater inhibitory properties than lactic acid in high-moisture cheeses, and that modifying pH and/or moisture level will significantly influence L. monocytogenes growth.

Key Words: Listeria monocytogenes, cheese, model cheese