Multilevel structural responses of β-conglycinin and glycinin under acidic or alkaline heat treatment

The structural responses of soy protein isolate under various processing conditions are of practically important for broadening its functionalities. In this work, multilevel structural responses of β-conglycinin and glycinin under acidic or alkaline heat treatment were investigated. Our results suggested that heat treatment under acidic (i.e., pH2.5) or alkaline (i.e., pH8.5) conditions induced multilevel structural responses of β-conglycinin and glycinin: under acidic heat treatment, both β-conglycinin and glycinin underwent hydrolysis and experienced disruption and reorganization in ordered secondary structure. This process was accompanied with changes in tertiary structure where previously buried regions were exposed to the aqueous phase to different extent. Small-angle x-ray scattering (SAXS) results indicated that the protein conformations evolved from globular ones to elongated ellipsoids, with a more remarkable elongation effect in glycinin than β-conglycinin. Upon solvent evaporation, β-conglycinin favored aggregation with a low "lateral to vertical ratio", glycinin tended to aggregate in a high "lateral to vertical ratio" style. In contrast, alkaline heat treatment did not induce hydrolysis but disturbed the secondary structure instead. The protein monomers maintained the globular conformation and assembled into irregular large aggregates during solvent evaporation. Under either treatment, glycinin responded more sensitively than β-conglycinin at all structure levels. The observed multi-level structural responses can be used to guide the rational modification of soy protein isolate with controlled conformation.