Association of denatured whey proteins with casein micelles in heated reconstituted skim milk and its effect on casein micelle size

Abstract

When skim milk at pH 6·55 was heated (75 to 100 °C for up to 60 min), the casein micelle size, as monitored by photon correlation spectroscopy, was found to increase during the initial stages of heating and tended to plateau on prolonged heating. At any particular temperature, the casein micelle size increased with longer holding times, and, at any particular holding time, the casein micelle size increased with increasing temperature. The maximum increase in casein micelle size was about 30–35 nm. The changes in casein micelle size were poorly correlated with the level of whey protein denaturation. However, the changes in casein micelle size were highly correlated with the levels of denatured whey proteins that were associated with the casein micelles. The rate of association of the denatured whey proteins with the casein micelles was considerably slower than the rate of denaturation of the whey proteins. Removal of the whey proteins from the skim milk resulted in only small changes in casein micelle size during heating. Re-addition of β-lactoglobulin to the whey-protein-depleted milk caused the casein micelle size to increase markedly on heat treatment. The changes in casein micelle size induced by the heat treatment of skim milk may be a consequence of the whey proteins associating with the casein micelles. However, these associated whey proteins would need to occlude a large amount of serum to account for the particle size changes. Separate experiments showed that the viscosity changes of heated milk and the estimated volume fraction changes were consistent with the particle size changes observed. Further studies are needed to determine whether the changes in size are due to the specific association of whey proteins with the micelles or whether a low level of aggregation of the casein micelles accompanies this association behaviour. Preliminary studies indicated lower levels of denatured whey proteins associated with the casein micelles and smaller changes in casein micelle size occurred as the pH of the milk was increased from pH 6·5 to pH 6·7.

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

“…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. 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.…”

“…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.…”

Role of protein aggregation in heat-induced heat stability during milk powder manufacture

“…Unfortunately many different techniques have been used to separate the micellar and serum phases. Centrifugation is the most common but many different conditions have been used, ranging from 175 000× g/60 min to 19 200× g/240 min [2,4,11,17,18,22,29,32,33,[35][36][37]41]. Other researchers have rejected the use of centrifugation on the basis of being too harsh and likely to cause micellar disintegration and have used treatment with rennet to precipitate the micelle fraction instead [38].…”

Effects of Heating Conditions on Physicochemical Properties of Skim Milk Powder during Production Process

“…Therefore, β-lactoglobulin and α-lactalbumin denaturation in milk and various model systems has been widely studied (Corredig and Dalgleish, 1999;Dannenberg and Kessler, 1988a;Parris et al, 1991). Numerous reports have found that heating milk to temperatures > 60℃ leads to whey protein denaturation, thereby facilitating their interaction with each other or with κ-casein to form aggregates (Anema and Li, 2000, 2003a, 2003bCorredig and Dalgleish, 1999;Dannenberg and Kessler, 1988a;Guyomarc'h et al, 2003;Pearse et al, 1985;Smits and Brouwershaven, 1980;.…”