Changes in pH of Milk During Freezing and Frozen Storage

Open Access
Publisher Website
Google Scholar
Abstract
SU~/(~(ARY Measurement of pH during freezing and frozen storage of pasteurized and raw milk and of raw milk with added calcium, phosphate, and citrate showed that marked pH changes occurred in frozen milk. During slow freezing, the pH of milk decreased to values as low as 5.8, whereas little change in pH occurred during fast freezing. During storage at --7 ° C. as well as at --12 ° C. the pH decreased to a minimum dur- ing the first 2 to 3 wk. of storage, and increased gradually thereafter. The pH mini- mum of pasteurized milk and raw milk with added salt (5.8-6.0) was generally lower than that of raw milk (6.1-6.2). The increase in pH after the pH minimmn was reached was more gradual in pasteurized milk and raw milk with added chemicals than in raw milk. Although the amount of precipitated protein was measured simultane- ously with pit, the data did not indicate a definite relation between pH changes and protein precipitation. Changes in the pH of millk during freezing and frozen storage have been suggested as a causative factor in protein destabilization in frozen milk (4, 5). Information on such pH changes, however, is limited, presumably because of the difficulties involved in measuring pH at temperatures below freezing. It would appear that the only pertinent work is that of Tessier and Rose (5) who showed that significant changes in pH do occur. Using the technique of centrifugation to separate ice and liquid, they measured pH values as low as 5.8 at -7 ° c. Recently developed electrodes which will allow pH measurements at below-freezing temperatures without separating liquid and ice phases (9) offered a new approach to the problem. This paper presents the results of a study of pH changes in milk during freezing and subsequent storage using these electrodes. EXPERIMENTAL PROCEDURE Milk obtained from morning milkings at the Central Experimental Farm, Ottawa, was separated, pasteurized (when required) at 65 ° C. for 30 mill., cooled to about 2 ° C., and then frozen immediately in metal cans, using seeding with ice to reduce supercooling. Samples in which the pH was measured during freezing were frozen by cooling in agitated brine from -1 ° C. initial temperature at rates of 1 ° C/hour or more (fast), 0.25 ° C/hour (moderately slow), and 0.1 ° C/hour or less (slow freezing). Several samples of the same milk were frozen simultaneously and samples were removed at each desired temperature and analyzed. Most freez- ing tests were made with raw milk; but pasteurized milk and milk ultrafiltrates also were used.