Survival of Listeria monocytogenes During Manufacture and Storage of Cottage Cheese

Abstract

Cottage cheese was made by the short-set procedure in pilot-plant-sized vats from pasteurized skim milk inoculated to contain 10⁴ – 10⁵ Listeria monocytogenes (strains Scott A or V7)/ml. Half the curd from each trial (two trials with each strain of L. monocytogenes) was creamed and half remained uncreamed. Numbers of L. monocytogenes were determined by surface-plating samples diluted in Tryptose Broth (TB) on McBride's Listeria Agar (MLA). Initial TB dilutions were then stored at 3°C and plated on MLA after 2, 4, 6 and 8 weeks or until L. monocytogenes was recovered. Selected L. monocytogenes colonies from each sample were serologically confirmed. Results for both strains indicate that during manufacture, numbers of L. monocytogenes remained relatively constant until after cooking of curd was completed. All samples analyzed after cooking curd 30 min at 57.2°C (135°F) contained fewer viable L. monocytogenes than could be detected by our methods (10 or 100 CFU/g or ml). Both strains were recovered from cold-enrichment samples, indicating that small numbers of the organism survived the cheesemaking process. Using direct plating onto MLA, L. monocytogenes was recovered from 43 of 112 (38.4%) cottage cheese samples during storage at 3°C for up to 28 d. After cold-enrichment in TB for up to 8 weeks, 59 of 112 (52.7%) samples were positive for L. monocytogenes. Greater numbers of L. monocytogenes, particularly strain Scott A, were found in creamed rather than uncreamed cottage cheese; however, numbers seldom exceeded 100/g.