Improved Methods for Quality Assessment of Raw Milk

Raw milk samples (203) were obtained from locations throughout Virginia for the purpose of evaluating the presently available techniques, used as is or modified, for enumeration of psychrotrophic bacteria as indicators of the actual quality of milk. Each of the samples was split into two sub-samples, one being tested by various bacterial enumeration methods on the fresh sample, and the other subjected to a preliminary incubation of 13°C or 21°C for 18 h prior to bacterial enumeration. Agar pour plates and Petrifilm dry medium culture plates were both used. Results indicated that the Petrifilm technique was not significantly different from agar pour plate methods as evidenced by correlation values, mean log differences, slopes, and Y-intercepts. The Petrifilm method for the modified (rapid) Psychrotrophic Bacteria Count (mPBC) produced similar overall results (0.96, -0.283, 1.016, -0.340, respectively) to the pour plate method (0.94, 0.272, 0.865, 0.684, respectively) when compared to the standard PBC, and enumeration of colonies on the Petrifilm plates was easier due to the incorporated TTC. A modified Preliminary Incubation Count (mPIC) yielded results not significantly different from those of the PIC (0.94, 0.044, 0.991, 0.169, respectively), and was conducted in less time - 2 d vs. 3 d.