CHARACTERIZATION OF A COMMERCIAL SOY ISOLATE BY PHYSICAL TECHNIQUES

The mechanical features of a partially denatured commercial soy isolat e (PP 500E) have been explored for comparison with data available on t he gelation characteristics of native globular proteins and to improve the understanding of its textural properties as a structuring ingredi ent in the production of lowfat products. The say sample was reconstit uted at 30C and networks were developed either during cooling to 5C or on heating to 90C (complete denaturation of the protein) followed by cooling to 5C. Throughout the course of experimentation, dynamic oscil latory (time, temperature, frequency and strain sweeps) and creep test ing (in aqueous or urea solutions) measurements were recorded. Reducti on in the thermal energy of the system causes a monotonic increase in storage modulus (G') whereas the temperature rise results in equilibri um G' values well below the elastic response observed at 30C. The abse nce of a positive thermal transition, observed in the gelation of nati ve globular proteins, indicates a different mechanism for structure fo rmation in commercial say isolates. Application of the cascade treatme nt to the concentration dependence of the storage modulus argues that the heated and cooled networks possess a higher degree of bond permane ncy than their cooled-only counterparts. Mechanical spectra in combina tion with the pattern of network breakage at high deformations suggest s that disulphide bonds participate in the network formed by totally d enatured soy protein (heated and cooled samples). Inclusion of urea in the aqueous preparations destabilises the predominantly physical forc es of attraction in the unheated gels. By contrast, the heated and coo led samples achieve an equilibrium deformation whose storage modulus c an be employed in the constitutive equation of rubber elasticity theor y. On that basis the number of disulphide bridges per molecule was fou nd to vary between 2.0 and 2.03. This result is consistent with the '' string of beads'' model proposed for the three-dimensional structure o f globular protein gels, where a dendric network is built by the occas ional cross-linking of corpuscular strands.