Effect of interactions between denatured whey proteins and casein micelles on the formation and rheological properties of acid skim milk gels

Abstract

The effect of interactions of denatured whey proteins with casein micelles on the rheological properties of acid milk gels was investigated. Gels were made by acidification of skim milk with glucono-δ-lactone at 30°C using reconstituted skim milk powders (SMP; both low- and ultra-low-heat) and fresh skim milk (FSM). The final pH of the gels was ∼4·6. Milks containing associated or ‘bound’ denatured whey proteins (BDWP) with casein micelles were made by resuspending the ultracentrifugal pellet of heated milk in ultrafiltration permeate. Milks containing ‘soluble’ denatured whey protein (SDWP) aggregates were formed by heat treatment of an ultracentrifugal supernatant which was then resuspended with the pellet. Acid gels made from unheated milks had low storage moduli, G′, of <20 Pa. Heating milks at 80°C for 30 min resulted in acid gels with G′ in the range 390–430 Pa. The loss tangent (tan δ) of gels made from heated milk increased after gelation to attain a maximum at pH ∼5·1, but no maximum was observed in gels made from unheated milk. Acid gels made from milks containing BDWP that were made from low-heat SMP, ultra-low-heat SMP and FSM had G′ of about 250, 270 and 310 Pa respectively. Acid gels made from milks containing SDWP that were made from ultra-low-heat SMP or FSM had G′ values in the range 17–30 Pa, but gels made from low-heat SMP had G′ of ∼140 Pa. It was concluded that BDWP were important for the increased G′ of acid gels made from heated milk. Addition of N-ethylmaleimide (NEM) to low-heat reconstituted milk, to block the —SH groups, resulted in a reduction of the G′ of gels formed from heated milk but did not reduce G′ to the value of unheated milk. Addition of 20 mm-NEM to FSM, prior to heat treatment, resulted in gels with a lower G′ value than gels made from reconstituted low-heat SMP. It was suggested that small amounts of denatured whey proteins associated with casein micelles during low-heat SMP manufacture were probably responsible for the higher G′ of gels made from milk containing SDWP and from milk heated in the presence of 20 mm-NEM, compared with gels made from FSM.

Article

“…Furthermore, micellebound aggregates of whey proteins are necessary to reach a value of G', at acid pH, close to that of heated milk [28]. Anyway, soluble aggregates of denatured whey proteins lead to higher final G' values than in unheated milk [28]. The maximum of tan d was obtained with soluble or with micellebound aggregates of whey proteins, but was more pronounced with micelle-bound aggregates [28].…”

“…A great increase in the final value of G' was observed for a heat treatment ³ 80°C for 30 min, at more than 70% denaturation of whey proteins. The heat treatment of milk at 80°C for 30 min also leads to a higher pH at gelation, 5.25 instead of 4.80 in unheated milk [28]. The increase in the pH at gelation was observed when milk contained soluble and micelle-bound aggregates of denatured whey proteins [28].…”

“…The heat treatment of milk at 80°C for 30 min also leads to a higher pH at gelation, 5.25 instead of 4.80 in unheated milk [28]. The increase in the pH at gelation was observed when milk contained soluble and micelle-bound aggregates of denatured whey proteins [28]. Furthermore, micellebound aggregates of whey proteins are necessary to reach a value of G', at acid pH, close to that of heated milk [28].…”

“…The increase in the pH at gelation was observed when milk contained soluble and micelle-bound aggregates of denatured whey proteins [28]. Furthermore, micellebound aggregates of whey proteins are necessary to reach a value of G', at acid pH, close to that of heated milk [28]. Anyway, soluble aggregates of denatured whey proteins lead to higher final G' values than in unheated milk [28].…”

Binding Interactions Between α-glucans from Lactobacillus reuteri and Milk Proteins Characterised by Surface Plasmon Resonance

“…1 It may interact with the κ-casein, as well as with partly denatured whey protein, of which β-lactoglobulin constitutes the greatest fraction. 12 The formation of the protein network may initiate at pH 5.2-5.3, 14 and hence EPS-protein interactions occurring at pH 5.5 to 4.0 are important for the texture.…”