THE EFFECT OF FREEZE-DEHYDRATION ON THE SURVIVAL OF CERTAIN PSYCHROTROPHIC BACTERIA IN SKIMMILK, ICE CREAM MIX SUBSTITUTE AND COTTAGE CHEESE

Abstract

Summary Studies on the effect of freeze-dehydration on the survival of psychrotropic bacteria in milk and ice cream mix substitute showed that a species of Achromobacter was relatively resistant as compared to five cultures of Pseudomonas. The effect of freezing on the viable population varied greatly not only between cultures but also within cultures from one sampling period to another. In some instances increases in viable count were observed after freezing. The effect of freeze-dehydration on the cultures in ice cream mix substitute was in many aspects similar to that observed in skimmilk. Increases in viable count took place when freeze-dehydrated milk cultures of F11 and 54 were rehydrated and stored at 5.5 C. However, a further reduction in viable count took place in samples stored in the dehydrated form. Freeze-dehydration of milk cultures with different concentrations of bacteria did not show a definite pattern in change of viable population with cell concentration. Freeze-dehydration of cottage cheese contaminated with cultures F11 and 54 caused an extensive reduction of the number of viable bacteria. When dehydrated samples were rehydrated and stored at 5.5 C no extensive changes in viable count occurred. In samples stored dry, however, there were further reductions in viable count. The effect of freeze-dehydration on culture 54 in cottage cheese was different from that observed in milk or ice cream mix substitute. Preliminary experiments suggest that the method used to enumerate this organism may be responsible for this phenomenon.