Formation of disulfide bonds in acid-induced gels of preheated whey protein isolate

Cold gelation of whey proteins is a two-step process. First, protein aggreg ates are prepared by a heat treatment of a solution of native proteins in t he absence of salt. Second, after cooling of the solution, gelation is indu ced by lowering the pH at ambient temperature. To demonstrate the additiona l formation of disulfide bonds during this second step, gelation of whey pr otein aggregates with and without a thiol-blocking treatment was studied. M odification of reactive thiols on the surface of the aggregates was carried out after the heat-treatment step. To exclude specific effects of the agen t itself, different thiol-blocking agents were used. Dynamic light scatteri ng and SDS-agarose gel electrophoresis were used to show that the size of t he aggregates was not changed by this modification. The kinetics of gelatio n as determined by the development of pH and turbidity within the first 8 h of acidification were not affected by blocking thiol groups. During gelati on, formation of large, covalently linked, aggregates occurred only in the case of unblocked WPI aggregates, which demonstrates that additional disulf ide bonds were formed. Results of permeability and confocal scanning laser microscope measurements did not reveal any differences in the microstructur e of networks prepared from treated or untreated whey protein aggregates. H owever, gel hardness was decreased 10-fold in gels prepared from blocked ag gregates. Mixing different amounts of blocked and unblocked aggregates allo wed gel hardness to be controlled. It is proposed that the initial microstr ucture of the gels is primarily determined by the acid-induced noncovalent interactions. The additional covalent disulfide bonds formed during gelatio n are involved in stabilizing the network and increase gel strength.