INFLUENCE OF HEAT TREATMENT OF SKIMMILK UPON GROWTH OF ENTEROPATHOGENIC AND LACTIC BACTERIA1

Abstract

By use of agar plate count techniques, growth patterns of Streptococcus cremoris R-1, Streptococcus lactis C-2, Streptococcus agalactiae OARDC, enteropathogenic Escherichia coli OSU 66, Salmonella typhimurium OSU 471, and Staphylococcus aureus OSU 256 were determined in nonheated skimmilk and in skimmilks heated at 62.8 C – 32 min, 80 C – 12 min, and 121 C – 10 min. Calculations were made of the generation time, number of generations, and rate of multiplication expressed as the velocity coefficient, k, = 0.692/generation time. The k values of the organisms in nonheated skimmilk following 2 hr incubation ranged from 0.38 for S. cremoris to 1.15 for E. coli. Heat treatment of skimmilk stimulated the rate of multiplication of the organisms and in most instances better growth occured in skimmilk heated at 80 or 121 C than in the skimmilk heated at 62.8 C. The average percent stimulation of the growth rate at 2 hr upon heat treatment of the skimmilk ranged from 9% for S. agalactiae to 45.5% for S. cremoris. Streptococcus agalactiae and E. coli were least stimulated by heat treatment of skimmilk and S. aureus and S. cremoris the most. Growth inhibitory properties of nonheated skimmilk were dissipated upon incubation for 4–8 hr.