Purpose: The purpose of this research was to evaluate the efficacy of commercially produced protective cultures of lactic acid bacteria (LAB) in controlling L. monocytogenes in broth, milk, and high-moisture cheese.
Methods: Co-culture assays containing each of eight LAB cultures and L. monocytogenes (100:1 ratio, respectively) were conducted in Brain Heart Infusion (BHI) and de Man Rogosa Sharpe (MRS) broths incubated at 35°C for 48 h. Similar assays were conducted in UHT milk at ratios of 100:1; 1000:1; and 1,000,000:1 incubated at 35°C for 48 h or 7°C for 7 days. High-risk cheeses (pH >6, moisture >50%) surface inoculated with L. monocytogenes (4 log CFU/g) were subsequently inoculated with each LAB strain (6 log CFU/g) and stored at 7°C for 36 days.
Results: Co-culture treatments identified seven strains capable of reducing L. monocytogenes to below detectable levels within 24-48 h in MRS with and without added rennet. Only three strains inhibited L. monocytogenes growth in BHI over 48 h. Strain BS-10 at a ratio of 1,000,000:1 was the only treatment that inhibited growth in milk at 35°C. All strain treatments at this ratio inhibited growth in milk at 7°C. Four LAB strain treatments produced slight reductions in L. monocytogenes counts on cheese and inhibited growth over 36 days at 7°C when compared to controls.
Significance: This study demonstrates the efficacy of commercially produced protective cultures for use in controlling L. monocytogenes under varying conditions. These results serve as the basis for the identification of cultures and their combinations to control L. monocytogenes throughout the manufacture and storage of high-risk cheeses.