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 treatment of concentrated milk systems for preservation and long shelf life or at least a sufficient removal of food pathogens prior to spray drying is a crucial step due to the decreasing stability of these systems toward heat compared to unconcentrated milk. Heat-induced coagulation is observed when temperature-time combinations for the achievement of certain microbial inactivation effects are higher than the heat and colloidal stability of the concentrated milk system allows. In this work, the effects of direct steam injection on the stability of casein micelles in concentrated skim milk (CSM) of 18, 23, and 27% total non-fat solids, heat-treated by direct steam injection (DSI), were investigated. Quantitative differential centrifugation for the separation of aggregates, casein micelles, dissociated submicellar particles, and soluble proteins and subsequent analysis of caseins and whey proteins within these fractions by RP-HPLC were applied. Quantitative separation was monitored by particle size measurements. The dissociation of κ-casein as well as an increase in casein micelle hydrodynamic radius were observed to increase with increasing total solid content of CSM, heat treatment time, and temperature. Heat-induced dissociation of β-Lg-κ-casein complexes at a critical level of 30–35% was found to induce severe coagulation of κ-casein-depleted calcium-sensitive casein micelles in CSM heated by DSI. Dissociated and aggregated proteins were found to be present as distinct colloidal particle classes differing in size from casein micelles.

Protein fractionation by means of microfiltration (MF) is significantly affected by fouling, especially when spiral-wound membranes (SWMs) are used. We investigated the influence of the mode of transmembrane pressure (Δp_TM) increase to target level and the deposit layer pressure history on the filtration performance during skim milk MF at temperatures of 10 °C and 50 °C. Two filtration protocols were established: No. 1: Δp_TM was set directly to various target values. No. 2: Starting from a low Δp_TM, we increased and subsequently decreased Δp_TM stepwise. The comparison of both protocols tested the effect of the mode of Δp_TM increase to target level. The latter protocol alone tested the effect of the deposit layer history with regard to the Δp_TM. As expected, flux and protein permeation were both found to be functions of the Δp_TM. Further, both measures were independent of the filtration protocol as long as Δp_TM was held at a constant level or, as part of protocol No. 2, Δp_TM was increased. Thus, we can state that the mode of Δp_TM increase to target level does not affect filtration performance in SWM. We found that after completion of a full cycle of stepping Δp_TM up from 0.5 bar to 3.0 bar and back down, flux and deposit layer resistance were not affected by the deposit layer history at 10 °C, but they were at 50 °C. Protein permeation, however, was lower for both 10 °C and 50 °C, when the Δp_TM cycle was completed. The processing history had a significant impact on filtration performance due to remaining structural compression effects in the deposited layer, which occur most notably at higher temperatures. Furthermore, temperatures of 50 °C lead to deposit layer aging, which is probably due to an enhanced crosslinking of particles in the deposit layer. Apart from that, we could show that fouling resistance does not directly correlate with protein permeation during skim milk MF using SWM.