Psychrotrophic Bacteria cause Changes in Stability of Milk to Coagulation by Rennet or Heat

Raw and pasteurized milks were inoculated with psychrotrophs, held at 7 C for 8 days, and observed periodically for stability to coagulation by heat and rennet. The longer raw milks with or without added psychrotrophs were incubated, the more rapidly they were coagulated by rennet. Raw milks inoculated with Lactobacillus spp. (No. 29 and 34) and Pseudomonas spp. All pasteurized and inoculated milks clotted in substantially less time than did pasteurized control milk when rennet was added. Clotting times of milks varied when different isolates of the same genus or of different genera served as inoculum. Coagulation of raw milks by rennet took less time for pasteurized inoculated milks. Uninoculated raw milk and raw milks inoculated with Pseudornonas spp. (No. 29) held for 6 and 8 days of refrigerated storage were coagulated by heat at 63, 73, and 85 C for 30 min. Pasteurized milk inoculated with Pseudornonas sp. (No. 31) was unstable to heat after 6 days at 7 C. The pH of raw milks decreased slightly over the 8 days whereas the pH of inoculated milks which were pasteurized generally did not start to decrease until day 4 and then decreased only slightly.