Changes in milk proteins associated with the growth of psychrophilic bacteria

One of the factors responsible for the loss of initial quality in adequately refrigerated, pasteurized milk and milk products is their widespread contamination with psychrophilic microorganisms. A common characteristic of the members of this taxonomically heterogenous group Is that they grow sufficiently well, though rarely optimally, at refrigeration temperatures to damage severely the organoleptic quality of dairy products in relatively short periods of time. This creates a major problem in modern dairy practice which is magnified as it becomes economically important to lengthen the time interval between production and consumption of dairy foods.

The literature on the relation of psychrophilic microorganisms to dairying is voluminous. The greater part of it treats their distribution and growth in contaminated dairy products with resulting organoleptic quality deterioration while in refrigerated storage. However, comparatively little information deals with the changes in milk constituents in dairy products due to their growth. This may be due to the fact that such changes do not share the commercial significance of organoleptic quality deterioration. Although the organoleptic detection of quality deterioration in dairy products almost invariably precedes any visible changes in milk constituents, that such changes are a part of this deterioration may be shown by other means.

Frequently the bacterial deterioration of milk and dairy products is detected organoleptically as a bitter flavor. Since this bitterness has often been associated with proteolysis, a study was made, using paper electrophoresis, to determine the changes in milk proteins associated with the growth of psychrophilic bacteria.