Impacts of heat-induced changes on milk protein digestibility: A review

Abstract

Milk proteins play important roles in human nutrition because of their high quality and digestibility. During the processing and storage of dairy products, milk proteins undergo various heat treatments that result in modifications at molecular, microstructural and macrostructural levels. The type and the extent of these heat-induced changes are influenced by the matrix and the heating conditions, which lead to different impacts on the digestion behaviour and the digestibility of milk proteins. This review summarises the scientific literature investigating the impact of heat-induced changes on milk protein digestion, with emphasis on how heat treatment can influence the digestibility of a protein differently depending on the product matrix and the heating conditions. The different effects of heat treatment on the digestion of different milk proteins, e.g., caseins, β-lactoglobulin and α-lactalbumin, are highlighted.

Introduction

Milk and dairy products are important sources of protein in the human diet. With a growing world population and an expanding middle class in developing countries, there is an increasing demand for animal proteins (Boland & Hill, 2019). Dairy provides a quality supply of dietary proteins both for their richness in essential amino acids (EAAs) and for their high digestibility and bioavailability. Because of their diversified functionalities, milk proteins are consumed in the form of various dairy products such as liquid milk, yoghurt and cheese. Milk-based infant formulae are designed to suit the needs of infants by simulating the composition of human breastmilk, which differs from bovine milk in its lack of caseins and β-lactoglobulin (β-LG) and its abundance of α-lactalbumin (α-LA) and other whey proteins.

During the manufacture of dairy products, milk proteins undergo different processing treatments that can alter their macro- and microstructures and can cause modifications at the molecular level, all of which may influence their digestion behaviour and their digestibility. In recent years, tremendous research effort has gone into understanding the digestion behaviour and digestibility of the proteins in milk and dairy products (Horstman et al., 2021; Mulet-Cabero, Mackie, Brodkorb, & Wilde, 2020; Mulet-Cabero, Mackie, Wilde, Fenelon, & Brodkorb, 2019; Van Lieshout, Lambers, Bragt, & Hettinga, 2020; Ye, 2021; Ye, Cui, Dalgleish, & Singh, 2016a, 2017). Typical methods used to study milk protein digestion include static and (semi-) dynamic in vitro methods, animal models (rats and pigs) and human studies. In terms of in vitro studies, infant digestion conditions are adopted by some researchers, which differ from the adult digestion conditions, e.g., higher gastric pH and lower activity of the digestive enzymes (Ménard et al., 2018).

The diversified functionalities of milk proteins arise from their compositions and structures. Milk proteins can be distinguished into two major groups, caseins and whey proteins, by their solubility at pH 4.6. Caseins make up 80% of bovine milk proteins and are mostly present in milk in the form of casein micelles that consist of from 10³ to 10⁶ individual casein molecules. The surface of casein micelles is composed of the amphiphilic κ-caseins, which stabilise them against aggregation (McSweeney & Fox, 2013). Coagulation of the casein micelles can be achieved by disrupting the κ-casein surface layer by acidification or enzymatic hydrolysis (chymosin or pepsin). Caseins are heat stable, both in casein micelles and in the form of caseinates, because of their unstructured nature. In contrast, whey proteins are heat labile and are readily denatured under heating at around 70 °C (McSweeney & Fox, 2013). The major whey protein in bovine milk, β-LG, contains a free thiol group that is readily exposed by thermal unfolding. It is responsible for the heat-induced aggregation between different whey proteins and between whey proteins and the casein micelles in milk because of the formation of disulphide bonds.

Heat treatment is widely used in almost all types of dairy product to ensure food safety, extend shelf life and improve the functionality of the proteins. Liquid consumer milk products are pasteurised (typically 72–75 °C for 15–20 s), in-can sterilised (115–120 °C for 20–30 min) or UHT treated (135–140 °C for a few seconds). In addition to whey protein denaturation, heat treatments of liquid milk at temperatures above 100 °C also induce other physicochemical changes, including κ-casein dissociation, micelle aggregation, the Maillard reaction and dephosphorylation of the caseins. During the production of yoghurt, the milk is typically heated at 90–95 °C for about 5 min or at 80 °C for 30 min, with the aim of denaturing most of the whey proteins, which contributes greatly to the textural quality of the yoghurt. Milk for cheese production is normally minimally heat treated, e.g., by thermisation at around 60 °C for 15 s, because denaturation of the whey proteins and their incorporation into the cheese curd is generally undesirable for cheese quality. Milk powders, infant formulae and milk protein ingredients commonly undergo spray drying, during which the liquid droplets are typically heated to 70–80 °C. The drying process has been shown to induce lactosylation of the proteins, the extent of which is influenced by the processing parameters (Guyomarc'h, Warin, Muir, & Leaver, 2000). Prior to the drying process, the protein solution for powder production is preheated to different extents to obtain certain functionalities for different applications. Glycation of the proteins in long-shelf-life dairy products, such as UHT milk and milk powders, could occur during storage, with the rate increasing at higher storage temperatures (Aalaei, Rayner, & Sjöholm, 2018; Guyomarc'h et al., 2000; Li, Ye, & Singh, 2021; Rauh et al., 2015).

One key point to note is that heat treatment could result in different chemical and structural changes of the same protein(s), depending not only on the heating conditions but also on the food matrix (e.g., the presence of other proteins or sugars), the environmental conditions (e.g., pH, ionic strength and water activity) and the other processing treatments (e.g., homogenisation, shear and fermentation), which influence the properties and digestion behaviours of the final product. The interactions between the food matrix, the heat-induced physicochemical changes and the digestion behaviour make elucidating the influence of heat treatment on the digestibility of milk proteins quite complex. This review demonstrates the impact of heat treatments on the digestion behaviour and digestibility of milk proteins. The emphasis is on understanding whether heat treatment affects the digestion of the same protein differently in different contexts, i.e., different food matrices and different heating conditions.