Aging of soybean globulins: effect on their solubility in buffer at pH 7.6

Isoelectric forms of freeze-dried soybean protein isolates were stored in the dark at room temperature for more than 5 yr. During this time, protein solubilities decreased 1-4%/yr in 0.5 ionic strength buffer at pH 7.6 when the buffer contained 0.01 M 2-mercaptoethanol and 4-6% when the reducing agent was absent. When measurements were made in buffer only, ultracentrifugal analyses of the soluble proteins indicated decreases in solubility of all fractions with time, with the exception of the > 15S fraction. In buffer plus 2-mercaptoethanol, aging resulted in decreases in the solubilities of 2S, 7S and 11S fractions, whereas the 15S and > 15S fractions increased in concentration. Disulfide-crosslinked polymers were observed throughout the study, but loss in protein solubility appeared to be caused by formation of other kinds of polymers involving mainly the 7S and 11S proteins.