RESUMEN
Milk containing naturally modified fat was obtained by feeding lactating dairy cows a Control diet and two experimental diets containing either extruded soybeans or sunflower seeds. Milk from cows fed the experimental diets contained higher levels of both long chain (C18-C18:2) and unsaturated fatty acids than the milk from cows fed the Control diet. Each milk was pasteurized, standardized to 3.6% milk fat, and inoculated with Listeria monocytogenes (strains Scott A and V7), Salmonella typhimurium and Salmonella senftenberg , before manufacturing into Blue or stirred-curd Cheddar cheeses. Populations of L. monocytogenes and Salmonella spp. were monitored during manufacture and aging using Oxford and Xylose Lysine Desoxycholate agars, respectively. During the manufacture of Blue and Cheddar cheese, and during the aging of Blue cheese, behavior of Salmonella spp. and L. monocytogenes in the experimental cheese was similar to the Control cheese. During aging of Cheddar cheese, the rate and extent of decline of Salmonella spp. and L. monocytogenes varied among the cheeses. Declines correlated with the accumulation of specific fatty acids, namely C12, C14, C18:1 and C18:2. These fatty acids were also found to be inhibitory to S. typhimurium and L. monocytogenes when incorporated into tryptic soy agar plates at 37°C. Therefore, the natural fat modification of Blue and Cheddar cheeses enhanced the safety of these cheeses.
RESUMEN
A direct plating procedure was developed for the enumeration of Listeria spp. and Listeria monocytogenes in foods. Both naturally contaminated foods and foods spiked with L. innocua , L. seeligeri , and L monocytogenes were studied. The enhanced hemolysis agar (EHA) developed by Cox and modified in our study resulted in two types of agar, referred to as listeria enumeration agar (LEA) no. 1 and 2, used for products of lighter and heavier background microbial populations, respectively. On LEA plates, total Listeria spp. counts were determined by fluorescence caused by the breakdown of 4-methylumbelliferyl-ß-d-glucoside contained in EHA. L. monocytogenes counts were determined by picking a representative number of hemolytic colonies and stabbing them into a xylose agar plate to distinguish L. monocytogenes from L seeligeri . Contamination levels of >200 Listeria cells per g of food can be accurately quantified by this procedure with >80% recovery. Counts of <200 Listeria cells per g of food were considered estimates. When the level of contamination was < 100 Listeria cells per g of food, the recovery was <58%. Occasionally, with low-level inocula, Listeria was not detected. Nevertheless, when the procedure was combined with incubation of the enrichment mixture (used for the 1:10 direct plating dilution) and subsequent streaking, Listeria contamination could still be detected and the level, therefore, was determined to be between 1 and 150/g.
RESUMEN
This study was carried out to investigate microbiological safety of reduced-fat Cheddar cheese. This was done by studying the behavior of two strains of Listeria monocytogenes , (Scott A-4b and V7-1a) and two species of the genus Salmonella , ( Salmonella typhimurium and Salmonella senftenberg ) during manufacture and aging of reduced or low-fat stirred curd Cheddar cheese made from milk containing 1.5 to 2.0% fat. The fat content of reduced-fat cheeses was between 20.03 and 21.13% while that of control cheeses was between 28.11 and 30.41%. Listeriae declined slowly in both cheeses and their rate of decline was not affected by fat reduction. During the 20-week aging period, the average (3 trials) log10 colony forming units (CFU)/g decline in Listeria population was 0.84 in control cheese and 0.62 in reduced-fat cheese. During the same period, the average log10 CFU/g decline in Salmonella population was 4.81 in control cheese and 5.16 in reduced-fat cheese. Salmonellae were affected by fat reduction, and during the entire aging period their population was lower in reduced-fat cheese than in control cheese. Thus, reduction of fat in the dry matter of cheese from 48 to 36% had no effect on listeriae but salmonellae declined faster in reduced-fat stirred curd Cheddar cheese.