Microbiological Analysis and Starter Culture Growth in Retentates

Abstract

Pasteurized skim milk was concentrated by UF to 2-, 4-, and 5-fold. The retentates were evaluated for microbiological quality, heat treatments to inactivate microorganisms, and lactic acid bacterial starter culture activity. Aerobic mesophilic bacterial counts in raw milk decreased from an initial 1.4 × 106 to 3.9 × 102 cfu/ml after pasteurization. During UF, counts increased from 3.9 × 102 cfu/ml in pasteurized milk to 1.4 × 103, 1.4 × 104, and 1.8 × 104 cfu/ml in 2-, 4- and 5-fold retentates, respectively. Psychrotrophic bacterial counts decreased from 9.9 × 105 cfu/ml in raw milk to 3.7 × 101 cfu/ml in pasteurized milk and gradually increased to 1.0 × 102, 2.5 × 102, and 1.4 × 103 cfu/ml in 2-, 4-, and 5-fold retentates, respectively. Thermophilic bacterial counts remained <10 cfu/ml in all samples. Skim milk and retentates inoculated with five starter cultures at 1% failed to decrease the pH below 4.6 in (2-, 4- and 5-fold). The 4- and 5-fold retentates inoculated with Lactococcus lactis spp.cremoris or Lactococcus lactis spp. lactis cultures were partially coagulated with pH >5.6. In general, the pH of retentates remained higher than that of skim milk. Clotting of uninoculated samples was observed, and a spore-forming contaminant, tentatively characterized as Bacillus cereus and capable of clotting milk at a pH >6, was isolated from the clotted samples. Heat treatments of 72, 78, 84, and 90°C each for 15s were not adequate to inactivate the spore former that caused clotting. A heat treatment of 80°C for 15s followed by a 4-h incubation at 32°C and a second heat treatment of 72°C for 15s inactivated the spore former and prevented clotting. Bacteria could not be recovered from the heat-treated milk after 6h of incubation at 32 or 50°C. However, after an 18-h incubation at 32°C, surviving bacteria (<100 cfu/ml) could be isolated from retentates. After extending the heat treatment for activating spores from 15s (at 80°C) to 10min, bacteria could not be recovered. Skim milk and retentates subjected to the double heat treatment had lower pH values after inoculation with five starter cultures compared with samples that did not receive the same heat treatment. However, the pH of retentates were higher than those of skim milk.