Moisture sorption isotherms at +4 °C and +22.5 °C were obtained for β-casein after isolation and after 9 months of storage at -29 °C and +22.5 °C. Glass transition state diagrams (Tg vs. moisture) were determined for β-casein after storage. The results showed that effects of storage temperature on moisture sorption isotherms were varied; however, at any aw differences in moisture content were small (<0.03g H2O/g solids at high aw). β-Casein stored at -29°C had lower mo and Tg values than that of β-casein stored at +22.5°C. The glass transition temperatures for β-casein were above room temperature, even at aw = 0.76. Onset of stickiness occurred above aw = 0.76.
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In the ongoing effort to improve and expand the supply of nutritional and functional food ingredients, the unique properties of casein and individual casein proteins are attractive for many applications. The majority of caseins exist in milk as micelles. Individual caseins are relatively small (20,000-24,000 Daltons), amphipathic, conjugated, randomly coiled, hydrophobic, open molecules that are insoluble around their isoelectric points (pH 4.5-4.9) and have an uneven distribution of polar and hydrophobic residues (Fox, 1989). Caseinates, water soluble derivatives of casein, are 1 Published as paper number 99-1-18-0017 of the contribution series of the Minnesota Agricultural Experiment Station based on research conducted under project 18-24 and supported by the National Dairy and Promotion Board and the USDA National Needs Fellowship Program.