The pH of peas and of poultry meat frozen pre- and post-rigor was measured during frozen storage at −10°C for up to 6 months. In peas it decreased sharply from 6.7 to as low as 6.0 during the first 3 days of storage, increased to 7.0 during the next 2-3 weeks, decreased to 6.4 in another 3 weeks, and remained there with only small fluctuations during the rest of the storage time. Breast and leg meat of poultry resembled each other in pH changes after freezing: increases and decreases of about 0.2–0.3 unit occurred in all samples at about the same time. Meat frozen pre-rigor differed from meat frozen post-rigor, however, the latter increasing 0.2–0.3 pH unit during freezing, and the former changing little or decreasing slightly under these conditions. Differences in pH between samples at a given time were related to differences in initial pH.

Studies with salt solutions as similar as possible in composition to the foods tested, and with gelatin solutions, showed that pH changes in frozen foods were caused mainly by increasing concentration of food components, including proteins, in the unfrozen phase, by precipitation of salts, by interaction of proteins with ionic substances, and by enzymatic activity (e.g. lactic acid formation) during frozen storage.