Changes in Characters of Soybean Glycinin Groups I, IIa, and IIb Caused by Heating

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Abstract
Soybean glycinin groups I, IIa, and IIb were purified from soybeans composed of only glycinin groups I, IIa, and IIb, respectively. When these protein solutions were heated, the amount of the particulate protein formed in these solutions was greatest in the order of groups IIa, IIb, and I. The protein solubilities decreased upon the addition of magnesium chloride in the order of groups IIa, IIb, and I. It was determined by differential scanning calorimetry analysis that the denaturation temperatures of groups I, IIa, and IIb were 92.8, 96.0, and 97.9 °C, and that the enthalpies of their transitions were 24.2, 27.4, and 28.1 J g-1, respectively. The α-helix rates of groups I, IIa, and IIb in aqueous solution were analyzed by circular dichroism and were 19, 16, and 15%, respectively. The β-sheet rates of groups I, IIa, and IIb were 44, 38, and 39%, respectively. In all group proteins, the α-helix rates were decreased by heating and the β-sheet rates were increased. The surface hydrophobicity of these group proteins increased as a result of heating, and those of groups IIa and IIb were larger than that of group I. The surface hydrophobicity of these protein groups increased by heating, and those of groups IIa and IIb were larger than that of group I and β-conglycinin. Breaking stress of curds prepared from these group proteins containing more than 1 of β-conglycinin ration showed similar values, but the order of those containing less than 1 in strength was groups I, IIb and IIa. These results suggest that the increase of particulate contents and the curd formation are related to the increase of surface hydrophobicity by heating. Keywords: Soybean; glycinin; surface hydrophobicity; sulfhydryl (SH) group; breaking stress; differential scanning calorimetry; circular dichroism