Heat-Induced, pH-Dependent Dissociation of Casein Micelles on Heating Reconstituted Skim Milk at Temperatures below 100 °C

Abstract

The effect of pH (6.3−7.1) and temperature (20−90 °C) on the dissociation of casein from the micelles in reconstituted skim milk was investigated. These pH conditions encompass those naturally found, whereas heat treatments below 100 °C are commonly encountered during the processing of milk and milk products. Low levels of casein were rendered soluble at pH below 6.7 regardless of heating temperature, whereas increasing levels of casein were solubilized as the pH was increased from 6.7 to 7.1. This pH-dependent dissociation of the casein micelles showed an unusual dependence on temperature. Low levels of casein were dissociated at 20 °C at all pH values. The quantity of casein solubilized increased with temperature to a maximum dissociation at about 70 °C and then decreased at higher temperatures. The dissociation behavior of αs-casein and β-casein at pH ≥ 6.7 showed dependence on temperature similar to that of the total casein. In contrast, above pH 6.5, the dissociation of κ-casein increased essentially linearly with increasing temperature over the entire temperature range studied. The proportion of β-casein in the soluble casein was essentially constant regardless of the temperature and pH, whereas the proportions of αs-casein and κ-casein varied with both temperature and pH. The results of this study have indicated that, at certain pH, a marked dissociation of protein from the casein micelles occurs on heating at temperatures below 100 °C; this phenomenom has not previously been reported to occur under such mild heating conditions. Keywords: Casein micelles; dissociation; heat treatment; reconstituted skim milk

Fermentation by lactic acid bacteria at two temperatures of pre-heated reconstituted milk. I - Behaviour of proteins and minerals

“…The ratio of unpelleted k-CN to total k-CN was 6-7% in unheated milk and increased to 30-40% in heated milk, as previously reported [2,18,34].…”

“…The behaviour of a-LA and b-LG are dependent upon the time of heating, the temperature, the pH and the amount of proteins [5,25]. After heating at 85°C for 10 min, a marked increase in the concentration of k-casein (k-CN) in the supernatant has been reported (from 15 in unheated to 23% of total k-CN in heated milk) [18], but it should be noted that increasing the pH value to 7.1 leads to a greater concentration of supernatant casein in the milk heated to 70-80°C [2]. Above 80°C, at pH 7.1, the proportion of k-CN in the supernatant increases, while that of a S -CN (a S1 + a S2 ) decreases [2].…”

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

“…The heat-induced dissociation rates were conversely much lower in the case of β-and α s -caseins than for κ-casein (not shown). This indicated a significantly higher sensitivity of this casein to heat-induced dissociation from the micelles, in agreement with previous reports [1,17,30].…”

“…The increased proportion of heat-induced aggregates in the serum phase of milk, as pH increases, appears to be related to the increased dissociation rate of the casein molecules, and especially, of κ-casein, in alkaline pH and/or high temperature conditions [1]. 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].…”

Skim milk powders with enhanced foaming and steam-frothing properties

“…The increase in foaming properties of milk as pH is increased above the natural pH of milk may also be attributed to an increased availability of caseins as pH is increased. Increasing levels of casein were found to solubilize as the pH was increased from 6.7 to 7.1 [2]. At high pH, it is probable that the viscosity effects have a major contributory effect on enhancing foam stability.…”