The Spoilage of UHT-Treated Chocolate Milk by Thermoduric Bacteria

The sporadic occurrence of swollen packs of chocolate milk at a local milk processing plant is the main reason for this investigation and to identify and characterize the spoilage agents. Of the 62 packs of chocolate milk collected, 15 became swollen within 24 h. All of these had high viable bacterial counts on plate count agar, whereas those that did not swell were free from bacterial contamination. There were two bacterial types found either individually or together in the swollen packs. Both isolates were confirmed as spoilers because when they were inoculated into intact packs, spoilage occurred within 48 h and the inoculant organisms were re-isolated from these packs. In addition, the onset of swelling was more rapid with increasing inoculum rate. Both isolates showed versatility in their ability to grow over fairly wide pH and temperature ranges both aerobically and anaerobically. Although they showed no evidence of sporulating, both isolates identified as Enterobacter sp. and Micrococcus sp. showed fairly high temperature tolerance but low H₂O₂ tolerance. The temperature-tolerant gram-negative rod which was identified as an Enterobacter sp. could have implications for the acceptable levels of heat treatment to be used in milk preservation in Trinidad.