Ropiness is milk… Psychrophilic bacteria and California milk quality

Although California's milk supplies, with rare exception, are easily kept well within California Agricultural Code specifications, a few dairy plants have occasionally encountered ropiness in pasteurized milk. This ropiness and some other storage defects such as fruity flavor are attributable to psychrophilic bacteria introduced following pasteurization. Psychrophiles are organisms that grow at refrigerated storage temperatures and can cause noticeable symptoms in milk after about a week. Ropiness in pasteurized milk is likely to become more of a problem for processors as total bacteria counts continue to be lowered by refrigeration and improved sanitation and as storage periods become longer. Under these conditions, a larger fraction of the organisms present in milk will undoubtedly be psychrophiles. Since many of these psychrophiles do not grow at the standard plate count temperature of 35° C used for routine laboratory testing, this study suggests modifications in procedure to allow more accurate determination of the quality of pasteurized milk.