Electrostatic Effects on the Physical Properties of Particulate Whey Protein Isolate Gels

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Title: Electrostatic Effects on the Physical Properties of Particulate Whey Protein Isolate Gels
Author: McGuffey, Matthew Kenneth
Advisors: E. Allen Foegeding, Chair
Harold Swaisgood, Member
Mitzi Montoya-Weiss, Member
Abstract: Whey protein isolate (WPI) forms particulate, heat-induced gels under conditions of low electrostatic repulsion among proteins. Particulate gels appear opaque, release water when deformed, and can be formed by 1) adjusting the pH to near the isoelectric point (pI) of beta-lactoglobulin ("pH treatment"), and 2) addition of >200 mM NaCl at pH 7.0 ("NaCl treatment"). The objective of our research was to understand the electrostatic contribution to particulate gel formation, using large strain rheology and microstructural techniques to characterize gel properties.Gels of WPI (10% protein w/v) were formed by heating at 80 degrees C for 30 minutes. The large strain fracture properties of shear strain at fracture, shear stress at fracture, and slope ratio (R0.3) (indication of non-linear stress/strain behavior) were measured with a torsion gelometer. Gel microstructure was observed using scanning electron microscopy (SEM). Gel permeability (Bgel) was evaluated by measuring the flow of solvent through a fixed gel matrix and calculating the permeability coefficient based on Darcy's Law. Fracture properties for both treatments were determined from pH 5.2 to 5.8 and NaCl concentration from 0.2 to 0.6 M (pH 7). When fracture stress and fracture strain curves for pH and NaCl treatments were overlaid, there was a distinct overlap. Based on this, treatment pairs were formulated to match rheological properties. The "high stress" pair was matched at pH 5.47 and 0.25 M NaCl, pH 7.0 (fracture stress=23 kPa and fracture strain=1.86) and the "low stress" pair at pH 5.68 and 0.6 M NaCl, pH 7.0 (fracture stress=13 kPa and fracture strain=1.90). Each pair was not significantly different for fracture stress(p>0.05), and there was no significant difference among all treatment pair values for fracture strain (p>0.05). However, the R0.3 was treatment, not pair specific. The NaCl treatments (0.25 M NaCl, pH 7 and 0.6 M, pH 7) had R0.3 values of 1.23 and 1.26, respectively, which were not significantly different (p>0.05). The gels formed at pH 5.47 and 5.68 had R0.3 values of 1.05 and 1.11, respectively, which were significantly different (p1) have been related to the presence of more than one structural component in the gel network (Bot et al., 1996). The increased strain hardening for NaCl treatments may be largely due to intermolecular disulfide bond formation, which is preferred at pH 7 relative to isoelectric pH due to the pKa of the thiol group. Random coil structure may be the other structural component that contributes to strain hardening in WPI gels, but more research is needed in this area.With the fracture properties normalized, no relationships were observed with the permeability or the microstructure between treatments. Lower permeability values were found for the treatment values (0.25 M NaCl, pH 7 and pH 5.68) near the fine-stranded transition regions (~pH 6 and 0.2 M NaCl, pH 7). These networks had more of a stranded-type structure with smaller, less defined aggregates as observed in SEM. This further demonstrates that there is little relationship between fracture rheology and microstructure of particulate gels.
Date: 2000-09-21
Degree: MS
Discipline: Food Science
URI: http://www.lib.ncsu.edu/resolver/1840.16/2457


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