Effect of pH on the Association of Denatured Whey Proteins with Casein Micelles in Heated Reconstituted Skim Milk

Abstract

Skim milk was adjusted to pH values between 6.5 and 6.7 and heated (80, 90, and 100 degrees C) for up to 60 min. Changes in casein micelle size, level of whey protein denaturation, and level of whey protein association with the micelles were monitored for each milk sample. Changes in casein micelle size were markedly affected by the pH at heating. At low pH (6.5-6.55), the casein micelle size increased markedly during the early stages of heating, and the size plateaued on prolonged heating. The maximum increase in size was approximately 30-35 nm. In contrast, at high pH (6.7), much smaller changes in size were observed on heating and the maximum increase in size was only approximately 10 nm. An intermediate behavior was observed at pH values between these two extremes. The rate of denaturation of the major whey proteins, alpha-lactalbumin and beta-lactoglobulin, was essentially unaffected by the pH at heating for the small pH changes involved in this study, and the changes in casein micelle size were poorly related to the level of whey protein denaturation. In contrast, the level of denatured whey proteins associating with the micelles was markedly dependent on the pH at heating, with high levels of association at pH 6.5-6.55 and low levels of association at pH 6.7. Changes in casein micelle size were related to the levels of denatured whey proteins that were associated with the casein micelles, although there was a small deviation from linearity at low levels of association (<15%). Further studies on reconstituted and fresh milk samples at smaller pH steps confirmed that the association of whey proteins with the casein micelles was markedly affected by the pH at heating. These results indicate that the changes in casein micelle size induced by the heat treatment of skim milk were a consequence of the whey proteins associating with the casein micelles and that the level of association was markedly influenced by small pH changes of the milk. It was not possible to determine whether the association itself influenced the casein micelle size or whether parallel reactions involving micellar aggregation caused the increase in micelle size as whey protein association progressed.

Effect of heat treatment at alkaline pH on the rennet coagulation properties of skim milk

“…These results correlated well with the amount of denatured whey protein found to be associated with the casein micelles as a function of heating time, heating temperature and heat-treatment pH [3,6]. The effect of the heat-treatment pH was, however, negligible and the effect of the heat-treatment itself seemed barely significant, at as low a heating regime as 90 °C for 30 s [3,4], as used in the present study.…”

“…The analysis of the micellar phase of the same milk samples did not, however, indicate that less whey protein/κ-casein aggregates could be bound to the casein micelles as heat-treatment pH increased. Earlier studies using the same method, however, reported a clear correlation between a lower heat-treatment pH and a higher proportion of micelle-bound whey protein/κ-casein aggregates [3][4][5]. The results of the present study were accounted for by the mild heating regime.…”

“…The interaction between κ-casein and denatured whey proteins, and formation of aggregates upon heating, further enhance the dissociation rate of κ-casein, but the question of whether this interaction precedes, or follows, the dissociation of κ-casein is still debated [2,4]. The smaller size of the heat-induced (serum) aggregates of whey proteins and κ-casein, as pH increases, may also be related to the higher dissociation rate of κ-casein ( [24], Fig.…”

“…4) and swelling of the casein micelle due to the increase in the electrostatic repulsion between negativelycharged casein molecules. Earlier studies have reported in detail the effect of heattreatment pH values ranging from 6.5 to 6.7 and heating regime [3,4]. In those studies, it was clearly demonstrated that increasing heating time or temperature, as well as decreasing the heat-treatment pH led to an increase in average micelle size.…”

Controlling milk protein interactions to enhance the reconstitution properties of whole milk powders – A minireview

“…For instance, at pH 6.5, at which about 70% of the denatured whey proteins are associated with the micelles, the average micelle size increases by about 35 nm. At pH 6.7, at which the level of whey protein association is about 30%, the micelle size increases only by about 5 nm [1].…”