FACTORS AFFECTING RESISTANCE TO HEAT AND RECOVERY OF HEAT-INJURED BACTERIA

An unidentified Pseudomonas sp. isolated from pasteurized milk stored at 4 C appeared killed by a heat treatment of 55 C for 30 min, since no colonies formed on trypticase soy agar plates during incubation for 48 to 72 hours. However, when the heated bacteria were held in trypticase soy broth for 48 to 72 hours at 20 C or two to three weeks at 4 C, some cells recovered ability to grow normally. Heat resistance and recovery were affected by the physiological state of the bacteria and the nature of the heating and the recovery medium. More complex heating media, including milk whey, stabilized the bacteria to heat, and favored recovery. Recovery was also found with five species of Salmonella and is probably a general phenomenon after heating at time and temperature combinations just above those which are apparently lethal. © 1969, American Dairy Science Association. All rights reserved.