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

Abstract

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.

Conjugation of milk proteins and reducing sugars and its potential application in the improvement of the heat stability of (recombined) evaporated milk

“…Mild heat treatments, such as pasteurization, commonly result in little or no noticeable instability of milk, whereas physical instability of products, especially evaporated milk, may occur when subjected to intense sterilization treatments. The heat-induced instability can be exhibited as a form of coagulation or gelation, or invisible aggregates that precipitate during storage (Dumpler, Huppertz, & Kulozik, 2020;Huppertz, 2016). Meanwhile, an alteration of particle size and rheological properties may occur in milk.…”

Impact of thawing methods on physico‐chemical properties and microstructural characteristics of concentrated milk

“…The casein micelles in milk were in balance with calcium ions, phosphate ions, magnesium ions, and citrate ions. When the milk was concentrated, the increase in the concentration of each component led to the breakdown of the balance, resulting in a decrease in the thermal stability of the milk (Dumpler et al, 2020).…”

Raw Cow Milk Protein Stability under Natural and Technological Conditions of Environment by Analysis of Variance

“…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 [ 5 ]. Milk stability is considered the total time for visual coagulation to occur at a given pH and temperature, and it is directly related to the ability of milk to resist coagulation at certain temperatures [ 6 ].…”

“…It means that it is ideal for the heat treatment processes to produce dairy products with a more extended shelf-life. The above mentioned is why it is crucial to study the sources of HS variability in practical conditions, although milk hygiene has improved during the last couple of decades, rendering milk less sensitive to coagulation [ 5 ]. Scientific and professional technological interest in the analysis of environmental (farm technology) and biochemical effects, including effects of food technology affecting the HS of lactoproteins also in the milk of small ruminants, can be found according to the other papers [ 13 , 14 ].…”

“…The HS is, therefore, substantial property of raw milk. This represents the resistance of lactoproteins against thermal coagulation [ 5 ]. In other words, it is the resistance of the milk to heat denaturation.…”