EFFECTS OF PROCESSING CONDITIONS AND INGREDIENTS ON THE PHYSICOCHEMICAL AND RHEOLOGICAL PROPERTIES OF PEA PROTEIN ISOLATE GELS
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The main objectives of this project were to study the rheological and textural properties of heat-induced pea protein isolate (PPI) gels and the effects of processing conditions and microbial transglutaminase (MTGase)on the gels, using chemical and instrumental methods. Physicochemical and gelation properties of commercial pea protein isolate were compared to laboratory prepared PPI with minimal denaturation (designated as native) and to homologoussoy protein isolates (SPI).Four studies were carried out in this research. In the first study, solubility and thermal properties of native and commercial PPI were investigated. Native PPI exhibited higher nitrogen solubility index (NSI) values compared to commercial PPI at pHs higher and lower than the isoelectric point. Differential scanning calorimetry (DSC) of native PPI slurries (10% protein concentration)revealed a single endothermic peak for the globulin fraction. SPI showed separate peaks for conglycinin (7S) and glycinin (11 S). By increasing the NaCI concentration from 0% to 2% (w/w),the denaturation temperature (Td) of the globulin fraction of native PPI was shifted from 85 to 93°C and the Td of the 7S and 11 S fractions of native SPI were increased from 75 to 82°C and from 93 to 100°C, respectively. The globulin protein fractions in commercial PPI and SPI were probably denatured, as DSC of commercial PPI and SPI showed no endothermic peak for the globulin protein fraction and no response to presence of NaCl. When the heating temperature was increased from 82 to 92°C (i.e. higher than the Td of native PPI globulin with 1 % NaCI, w/w, 90°C),a considerable increase in the shear stress of native PPI gels (19.6% protein, 1% NaCI concentration, w/w)was observed. Native PPI gels exhibited lower shear stress and higher shear strain than commercial PPI gels. Slurries of native PPI and SPI (10% protein, w/w) showedno structure development during small oscillation dynamic rheometry when heated from 35 to 75°C. Native PPI showed lower storage modulus (G')values (i.e. less solid-like)after 75°C than did native SPI. Commercial SPI slurries exhibited higher G' values during the complete heating process with a higher degree of initial structure development, than was observed for commercial PPI slurries. In the second study, the effects of process conditions (heating temperature, pH and NaCl level) on the gelation properties of commercial PPI were evaluated using a rotatable response surface design. As heating temperature increased from 79 to 95°C, there was a significant increase in gel shear stress and hardness. Shear strain and cohesiveness of gels were considerably increased (P<O.05) by increasing pH from 6.1 to 8.1, but were not significantly affected by heating temperature. NaCI concentration (0% to 2%) did not exhibit significant effects on any rheological and textural characteristics of commercial PPI gels, possibly due to the lack of sensitivity of the denatured protein to NaCI concentration changes. In the third study, at the optimum incubation temperature (50°C), both M'I'Gase product concentration (0%to 0.7%, w/w)and incubation time (20to 80 min) showed a positive linear relationship with shear stress and strain of the resulting commercial PPI gels, and the maximum values were obtained at the highest MTGase concentration (0.7%). The shear stress and strain decreased when the incubation temperature was higher or lower than 50°C. The torsion rheometry map indicated that the commercial PPI gel prepared by addition of 0.7% MTGase had shear stress and strain similar to those of the commercial SPI gel without MTGase addition and meat bologna. In the fourth study, SDS-PAGE showed that the major subunits of the globulin protein of native and commercial PPI were cross-linked by MTGase catalysis to form larger molecular weight polymers. There was no significant effect of MTGase addition on DSC thermal transition temperatures or enthalpy change for either native or commercial PPI. Low temperature incubation (4°C for up to 18 h) with MTGase increased the strength and elasticity of commercial PPI gels (19.6% protein). In addition, commercial SPI gels showed an earlier increase of both shear stress and strain with MTGase than did the PPI gels, indicating better MTGase accessibility for SPI. The shear stress and strain of heat-induced commercial PPI gels were modified by adjustment of processing conditions and MTGase addition. The cross-linking catalyzed by MTGase enhanced the strength and elasticity of commercial PPI gels, resulting in shear stress and strain similar to that of homologous SPI gels. Thus,an improvement in the gelation properties of commercial PPI was achieved.