Heat Resistance of Three Strains of Psychrophilic Organisms Added to Skimmilk for Cottage Cheese Manufacture

Abstract

The thermal resistance characteristics of Pseudomonas viscosa, Pseudomonas fluorescens, and Pseudomonas fragi were determined in sterile skimmilk at 110, 115, and 120° F. The thermal resistance of each organism was completely characterized by D and z values calculated from survivor curve data. P. fragi was more heat-resistant than the two other organisms. The results obtained provided evidence that these microorganisms would not survive the normal pasteurization of milk or skimmilk. Furthermore, the survival of these microogranisms during the different stages of making Cottage Cheese was determined. The following conclusions can be drawn from this investigation: (a) These microorganisms do not survive a cooking process of 120° F. for 30 min.; (b) generation of these organisms during incubation is not a problem, because the acidity developed by the time of cutting the curd, produces a 90% reduction in count; (c) none of the species of Pseudomonas added to the skimmilk before it was made into cheese could be isolated from the creamed cheese during a storage period of 4 wk. at 38° F.