Heat denaturation of soy glycinin : structural characteristics in relation to aggregation and gel formation

key words: soy protein; glycinin; thermal stability; pH; ionic strength;genetic variant; solubility; gelationThe main aim of this thesis was to study structural changes of soy glycinin at different conditions (pH and ionic strength) during thermal denaturation and their effect on aggregation and gel formation. The results show that, generally, glycinin is predominantly present in the 7S form at pH 3.8, while at pH 7.6 the major component is the 11S form. When, at ambient temperatures, the ionic strength at pH 7.6 is lowered from 0.5 to 0.2 or 0.03 the basic polypeptides within the glycinin molecule shift more to the exterior of the glycinin complex. This structural reorganisation caused the pH of minimal solubility to shift to higher values. The 7S form, which is more unfolded than the 11S form, denatures at a lower temperature than the 11S form. Changes in secondary structure take place simultaneously with denaturation. While at I = 0.03 the gels were found to be fine stranded, gel coarseness increased when the ionic strength was higher. At I = 0.03 finer gel network structures were formed at pH 3.8 than at 7.6, whereas for I = 0.2 and 0.5 the reverse was found. The observed differences in gel stiffness did not correspond to coarseness. The gel structure was clarified into more detail by the use of physico-chemical and spectroscopic techniques. The nature of the primary network particles was different at pH 7.6 (basic polypeptides) than at pH 3.8 (acidic and basic polypeptides). The heat denaturation process develops at 1% protein similarly as the heat denaturation process at 10% protein. However, the final gel structure and strength cannot be predicted from measurements performed at 1% protein. The heat denaturation mechanisms were not solved into more detail due to the fact that the genetic variants of glycinin differ in thermostability. It was found that the denaturation temperatures of these variants increase in the order G1/G2/G3/< A4<G5<G4.