PRELIMINARY INCUBATION OF RAW MILK SAMPLES AS AN AID IN EVALUATING BACTERIOLOGICAL QUALITY

Abstract

A total of 194 milk samples were obtained from 35 bulk tank grade-A milk producers in the Manhattan, Kansas, milk-shed during different seasons, and examined for standard, coliform, and psychrophile plate counts both before and after preliminary incubation (P.I.). Samples also were tested for inhibitory substances. Farms were ranked I, II, or III in general sanitation on the basis of visible conditions, with rank I most desirable. Except for one doubtful sample, all tests for inhibitory substances were negative. With standard plate counts, when no consideration was given to initial count level, there was no statistically significant relation between P.I. counts and season or sanitation rank, although summer growth ratios were always lowest. When data were grouped on the basis of initial count range, P.I. counts and growth ratios increased with increased sanitation rank in the lower count ranges. In initial count ranges > 50,000/ml the relationship between growth ratio and sanitation rank declined. With coliform counts, considerable inconsistency occurred in P.I. counts among seasons and sanitation ranks. Initial and P.I. coliforms counts showed little relationship with standard plate counts (except in summer) or with psychrophile counts. With psychrophile counts, growth ratios during P.I. were influenced by season, being highest in winter and lowest in summer. P.I. counts generally increased with increase in sanitation rank. The relation between growth ratios and sanitation rank was statistically significant (P <0.05).