The Microbiology of Sweet Water and Glycol Cooling Systems Used in HTST Pasteurizers in Fluid Milk Processing Plants in the United States

Abstract

Refrigerated water and water/glycol mixtures used in high temperature short time (HTST) pasteurizers have been implicated as potential sources of spoilage and pathogenic bacteria in milk. This study was carried out to determine the incidence of bacteria in these cooling systems. Sweet water and glycol coolants from 68 fluid milk plants were sampled to determine the incidence of psychrotrophs, mesophiles, coliforms, and salmonellae. A modified most probable number (MPN) technique was performed using 333 ml of coolant in lactose broth (LB, Difco). Samples were incubated for 7 d at 10°C or for 48 h at 37°C. MPN vessels positive for growth at 37°C were used to inoculate selective enrichment, differential and biochemical confirmation media for salmonellae, and brilliant green bile broth (BGB, Difco) for enumeration of coliforms. In addition, 5 L of each sample were passed through 0.45 μm filters (Millipore). The filters were incubated in LB. The previously described procedure for the detection of salmonellae was used. The procedure of Lovett et al. (17) was used to examine each sample for the presence of listeria. The population of organisms that grew at 10 and 37°C varied greatly from plant to plant. MPN values ranged from <0.21 to >240/100 ml of coolant. When present, coliforms were usually at low levels. Eight samples had coliform MPN values >2.2/100 ml of coolant. Salmonella typhimurium was isolated from one sweet water sample using the filtration procedure. No Listeria were isolated. The use of sanitizer in dairy coolant was not associated with low microbial populations in the coolant samples. This study suggests that coolants used in HTST pasteurizers may serve as a reservoir for bacterial contamination of pasteurized milk.