Theory for the heat-induced coagulation of a type A milk

Invited review: Heat stability of milk and concentrated milk: Past, present, and future research objectives

The ability of milk and concentrated milk to withstand a defined heat treatment without noticeable changes such as flocculation of protein is commonly denoted as heat stability. A heat treatment that exceeds the heat stability limit of milk or concentrated milk, which has a much lower heat stability, may result in undesired changes, such as separation of milk fat, grittiness, sediment formation, and phase separation. Most laboratory-scale batch heating methods were developed in the early 20th century to simulate commercial sterilization, and these methods have since been standardized. Heat stability studies have been motivated by different objectives during that time, addressing different processing issues and targets in the framework of available technology, legislation, and consumer demand. Although milk hygiene has improved during the last couple of decades, rendering milk less sensitive to coagulation, different standard methods suffered from poor comparability of results, even when comparing results for the same milk sample, indicating that unknown procedural steps affect heat stability. The prediction of heat stability of concentrated milk from the heat stability results of the corresponding unconcentrated milk for rapid quality testing purposes has been difficult, mainly due to different experimental conditions. The objective of this study is to review literature on heat stability, starting from studies in the early 20th century, to summarize the vast number of studies on compositional aspects of milk affecting heat stability, and to lead the way to the most recent work related to compositional changes in concentrates produced by membrane concentration and fractionation, respectively. Particular attention is paid to early and most recent developments and findings, such as the application of kinetic models to predict and limit protein aggregation to assess and describe heat stability as a temperature-time-total milk solids continuum.

Heat stability of milk: further studies on the pH-dependent dissociation of micellar κ-casein

Whey protein complexed and became co-sedimentable with casein micelles after heating milk at ≥ 90°C for 10 min at pH ≤ 6·9 while at higher pH values (7·3) whey proteins and κ-casein-rich protein dissociated from the micelles on heating. κ-Casein-deficient micelles were more sensitive to heat, Ca2+ or ethanol than whey protein-coated or native micelles and were readily coagulable by rennet. Isolated κ-casein added to skim milk before preheating (90°C for 10 min) did not associate with the micelles at pH ≥ 6·9. Sodium dodecyl sulphate increased the level of both non-sedimentable N (NSN) and N-acetylneuraminic acid (NANA) and shifted the NSN-pH and NANA-pH curves to more acidic values while cetyltrimethylammonium bromide had the opposite effect. It is suggested that the pH-dependent dissociation in micellar κ-casein, which appears to be reversible, depends on the surface charge on the micelles; at a certain negative charge, disruption of hydrophobic and electrostatic forces could result in the dissociation of κ-casein from the casein micelles.

Factors affecting the ethanol stability of bovine milk

Changes in the ethanol (EtOH) stability/pH profile of skim-milk induced by forewarming have been investigated as a function of temperature and duration of forewarming and of milk pH prior to heating. Forewarming increased the EtOH stability of a milk in the mid-pH range, shifting the profile laterally to more acid pH values, with little effect being observed on the maximum and minimum EtOH stabilities. No significant changes in forewarming effects were obtained on adding the sulphydryl-blocking agent,N-ethylmaleimide, indicating the lack of involvement of serum protein interactions in the stabilizing effects of forewarming. It is suggested that the observed effects of forewarming are due to a heat-induced precipitation of colloidal calcium phosphate. This suggestion is supported by measurements of Ca2+levels in forewarmed and untreated skim-milk as a function of pH and is in agreement with the previously suggested mechanism for the EtOH-induced coagulation of skim-milk. The effects of permitted additives (citrate, phosphate and Ca) on the EtOH stability of forewarmed milk paralleled their effect on untreated milk. A comparison is made of the effects of forewarming on the heat and EtOH stabilities of skim-milk.

The two-stage coagulation of milk proteins in the minimum of the heat coagulation time-pH profile of milk: effect of casein micelle size

Milks with casein micelles larger or smaller than control milk were prepared by differential centrifugation. The heat stability of these modified milks increased markedly throughout the pH range 6.4 to 7.1 with decreasing casein micelle size. Within the region of the minimum in the heat coagulation time-pH profile, the control milk coagulated by a two-stage process, but the modified milks, because of their narrower casein micelle size distribution, coagulated by a single-stage process at the pH of minimum stability. The content of kappa-CN and protein hydration increased as the size of the casein micelles decreased, and the level of glycosylation of kappa-CN and protein surface hydrophobicity increased as a function of micelle size. The effect of casein micelle size on the heat stability of milk is likely to be related to changes in the above physico-chemical properties.

Heat stability of milk: interrelationship between assay temperature, pH and agitation

As determined by the standard subjective assay procedure, the minimum in the heat stability--pH curve of milk persisted down to at least 116 degrees C. However when samples were not agitated during heating the minimum became progressively less pronounced as the assay temperature was lowered and it disappeared at approximately 116 degrees C. Activation energies (Ea) for unagitated samples were approximately 30 Kcal/mole at pH 6.87 (maximum) and at pH 7.28, throughout the temperature range 116--145 degrees C and for the pH 6.95 (minimum) sample at 116--125 degrees C; however Ea for the pH 6.95 sample increased to approximately 100 Kcal/mole in the range 127--135 degrees C suggesting that some highly temperature-dependent reaction had occurred and caused premature coagulation at certain pH values, i.e. to a heat stability minimum. The stability of concentrated milk (20% total solids) was very low at pH values above 6.9, regardless of whether the samples were agitated or not during heating and the maximum/minimum in the heat stability--pH curve persisted down to at least 90 degrees C in both agitated and quiescent samples.