Acid-Induced Cold Gelation of Globular Proteins:  Effects of Protein Aggregate Characteristics and Disulfide Bonding on Rheological Properties

Abstract

The process of cold gelation of ovalbumin and the properties of the resulting cold-set gels were compared to those of whey protein isolate. Under the chosen heating conditions, most protein was organized in aggregates. For both protein preparations, the aggregates consisted of covalently linked monomers. Both types of protein aggregates had comparable numbers of thiol groups exposed at their surfaces but had clearly different shapes. During acid-induced gelation, the characteristic ordering caused by the repulsive character disappeared and was replaced by a random distribution. This process did not depend on aggregate characteristics and probably applies to any type of protein aggregate. Covalent bonds are the main determinants of the gel hardness. The formation of additional disulfide bonds during gelation depended on the number and accessibility of thiol groups and disulfide bonds in the molecule and was found to clearly differ between the proteins studied. However, upon blocking of the thiol groups, long fibrillar structures of ovalbumin contribute significantly to gel hardness, demonstrating the importance of aggregate shape.

Impact of protein pre-treatment conditions on the iron encapsulation efficiency of whey protein cold-set gel particles

“…For the mild heat treatment (WPI-m), WPI was dissolved in MilliQ water at 9 % (w/w) and heated for 30 min at 80°C in a water bath. After heating, the WPI solution was cooled to room temperature and diluted with MilliQ water to 6 % (w/w) [1]. The pH was set to pH 7 and the WPI solutions were stored on ice until further use.…”

“…Depending on the processing conditions (salt, pH, heating time/temperature) whey protein can form diVerent structure elements of varying dimensions, e.g. aggregates [1,2], Wbrils [3,4], semi-Wbrils, microgels [5,6], nanoparticles [7] and microparticles [8]. In this way, whey protein may serve diVerent functional purposes such as a texturant, thickening agent [5], carrier for encapsulation or fat replacer [9].…”

“…As the entrapment (or binding) of iron occurs through cold-set gelation of pre-heat-treated whey protein, the pre-treatment conditions of the whey protein are very important with regard to gel properties and thus protein gel particle properties. By varying the pH, temperature-time, and protein concentration during the pre-heating step, it has been shown that for -lactoglobulin, and also for whey protein, diVerent protein structures, ranging from random aggregates to strands can be formed [1,[4][5][6]. It appeared that whey protein isolate (WPI) behaved similar to its main constituent -lactoglobulin with regard to the formation of diVerent structures.…”

Elastic Networks of Protein Particles

“…A 9% (w/w) whey protein stock solution was prepared by stirring for 2 h at 25°C followed by heating the solution at 68°C for 2.5 h. Heating the whey protein samples resulted in the formation of small protein aggregates of 40-100 nm 35 , without forming a gel. A 1% (w/w) locust bean gum stock solution was prepared by stirring at 80°C for 1 h. The whey protein and locust bean gum stock solutions were cooled to 25°C before mixing.…”

Water-binding of protein particles

“…[5] [6] Repulsive forces between the aggregates prevent further aggregation and gelation, yielding a stable solution of these aggregates. The second step in this process is therefore to induce gelation of the protein aggregate suspension, which can be accomplished by reducing the repulsive forces between the aggregates through reducing the pH.…”