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Physicochemical and functional properties of chicken breast muscle samples and their salt-extractable proteins were investigated as a function of increasing post-mortem storage time at abovefreezing temperature prior to deboning and further processing. Functional properties, including water binding capacity, emulsifying capacity and thermally-induced gelation, were generally poorer after 2 days of storage, compared to no storage (0 day). After 7 days of storage, functionality improved significantly, and in most cases did not differ (P>0.05) from functionality at 0 day. No significant differences (P>0.10) were observed for pH after different storage times, but salt-extractable protein content of the chicken mince was higher at 7 days than at 0 or 2 days. Salt-extracted proteins from 7 day samples had lower solubility and higher hydrophobicity, while dispersibility and sulfhydryl group content did not differ compared to 0 and 2 day samples. On the other hand, the exposed hydrophobicity of proteins after heat treatment was lower at 7 days than at 0 or 2 days. Sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated some changes in the myosin and troponin components in the extracts from different post-mortem storage times.

Human milk formed fine, dispersible floes at pH 4, in contrast to large clots from bovine milk. In order to simulate human casein composition (primarily β-casein), bovine casein (2%, pH 7, 11 mM CaCl 2 ) was selectively coagulated with rennin (0.1–0.5 mg/g casein) and heating. Electrophoretic analyses indicated an increase in β-:α sl -casein ratio from 0.7 to 3, with protein recoveries of 20–30%. β-Casein rich fractions were obtained from casein or skimmilk modified at 37°C or 4°C, although partial hydrolysis of β-casein occurred at 4°C. At pH 2, pepsin digestibility of the clear or turbid solutions followed the order α sl -casein > β-casein, and bovine control > modified caseins or milks. This trend was reversed at pH 4, the small loose clots of β-casein and modified samples being hydrolyzed more readily than the large hard clots of α sl -casein and bovine control samples. Human casein and milk showed low digestibility at pH 2 and 4, and were shown by electron microscopy to consist of tightly clustered clots. Thus, both casein composition and clot microstructure affect pepsin digestibility. The loose clot and high digestibility at pH 4 of rennin-modified samples may be advantageous for infant feeding.