Properties Of Heat-induced Protein Gels Research Articles

Commercial plant protein isolates: The effect of insoluble particles on gelation properties

Commercial plant protein isolates generally contain a substantial fraction of large insoluble protein particles. We study how these affect the gelation properties of heat-induced protein gels (15% w/w protein, pH 7) and used homogenization as a mean to reduce the particle size of the insoluble fraction before gelation. Three different commercial plant proteins were studied as models for isolates differing in dispersibility and size of insoluble protein particles: Fava bean (FBPI), Pea (PPI) and Mung bean (MBPI). As expected, homogenization (50 MPa, 2 cycles) reduced the particle size and improved the dry matter dispersibility of protein dispersions. We found that the (shear) storage modulus G’ and critical shear strain for fracture of gels obtained after heating were not consistently impacted by homogenization. In contrast, fracture properties in uniaxial compression were sensitive to homogenization, with homogenization leading to gels that are more resistant to fracture. Gels obtained from the isolate with the smallest fraction of insoluble particles and still contained native proteins, FBPI, were the most fracture resistant. Fluorescence microscopy indicated that the insoluble fraction remained well-dispersed in the matrix after heat-induced gelation and that homogenization led to a more homogeneous appearance of the protein networks, with smaller particles being homogeneously dispersed in the continuous background. We conclude that mechanically breaking down insoluble particles in commercial plant protein isolates contributes strongly to their functionality by reducing the number of large insoluble particles that act as effective nucleation points for hydrogel fracture.

Transglutaminase treatment of pea proteins: Effect on physicochemical and rheological properties of heat-induced protein gels

Rheological properties of heat-induced pea protein isolate (PPI) gels with added microbial transglutaminase (MTGase) were studied under various reaction conditions. A positive linear relationship was observed between level of MTGase used (0 to 0.7% w/w) and shear stress and shear strain of heat-set commercial pea protein isolate (PPIc) gels at 92 °C following incubation at 50 °C. Use of MTGase allowed for preparation of PPIc gels of similar strength and elasticity as commercial soy protein isolate gels and commercial meat bologna. MTGase treatment did not alter thermal properties of PPI gels. The shear stress and strain of PPIc gels were also improved following low temperature (4 °C) incubation of PPI with MTGase. Enhancement of shear strain or gel elasticity of heat-induced PPI gels with MTGase has not been reported before and provides opportunities for extending the properties of pea proteins when developing new food products.

Scaling and fractal analysis of viscoelastic properties of heat-induced protein gels

The scaling of viscoelastic properties of heat-induced gels prepared from egg white was investigated at five pH values (pH 3–11) and various protein concentrations (5–20%) using small-deformation oscillatory rheological measurements. Protein gels were formed by heating samples at 80 °C for 1 h followed by cooling to 25 °C. Storage modulus, G′ and critical strain, γ 0 of the gels exhibited pH-dependent power-law relationships with protein concentration. Based on the power-law exponent values, fractal dimension, d f of heat-induced protein gels was estimated using scaling models from the literature. Low d f values (1.9–2.1) were obtained for gels prepared at acidic and neutral pHs (3–7) whereas higher d f values (2.2–2.4) were obtained for gels formed under basic pH conditions. These d f values lied well within the range of fractal dimension values (1.5–2.8) reported for protein gels. However, they slightly differed from d f for diffusion-limited and reaction-limited cluster–cluster aggregation processes, which made it difficult to justify an assumption regarding the nature of the aggregation process of these protein systems.

Rheological Study on the Fractal Nature of the Protein Gel Structure

The effects of ionic strength on fractal structures in heat-induced gels prepared from globular proteins were investigated in the framework of a fractal aggregation of colloidal particles. All gels formed at 90 °C exhibited power law relationships between the storage shear modulus (G‘) and protein concentration. At 25 mM NaCl, the fractal dimension, d (∼2.2), calculated based on the value of the power law exponent agreed with those for reaction-limited cluster−cluster aggregation. Further addition of NaCl (50, 80, 500, 1000 mM) decreased the values of d (∼1.8), which agreed with d for diffusion-limited cluster−cluster aggregation. These results suggest that the predominant effect of an increase in ionic strength on globular protein gelation is ascribed to shielding charges on the surface of the proteins, thereby increasing the reaction probability of protein aggregation. The effective structure-determining rheological properties of heat-induced protein gels are characterized by fractal dimensions deduced ...