foods Article WPI Gel Microstructure and Mechanical Behaviour and Their Influence on the Rate of In Vitro Digestion Stephen Homer * , Roderick Williams, Allison Williams and Amy Logan   Citation: Homer, S.; Williams, R.; Williams, A.; Logan, A. WPI Gel Microstructure and Mechanical Behaviour and Their Influence on the Rate of In Vitro Digestion. Foods 2021, 10, 1066. https://doi.org/10.3390/ foods10051066 Academic Editor: Didier Dupont Received: 29 March 2021 Accepted: 4 May 2021 Published: 12 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). CSIRO Agriculture & Food, 671 Sneydes Road, Werribee, VIC 3030, Australia; roderick.williams@csiro.au (R.W.); allison.williams@csiro.au (A.W.); amy.logan@csiro.au (A.L.) * Correspondence: stephen.homer@csiro.au Abstract: The influence of microstructure and mechanical properties on the in vitro digestibility of 15% whey protein isolate (WPI) gels was investigated. Gels were prepared via heat set gelation at three pH values (pH 3, 5 and 7), which produced gels with distinct microstructures and mechanical properties. The gels were minced to simulate an oral/chewing phase, which led to the formation particles of various sizes and textures. The minced gels were passed through either an Infogest (pre-set pH of 3) or Glass stomach (dynamic pH) protocol. Gels were digested in the gastric phase for up to 120 min, at which point the extent of digestion was measured by the amount of filterable nitrogen passing through a sieve. The digesta from both gastric methods were passed through an in vitro simulated intestinal phase. A strong link was found between the elasticity of the initial gel and the gel particle size following simulated oral processing, which significantly (p < 0.01) affected the rate of digestion in the gastric phase. A weaker correlation was also found between the pH of the gels and the extent of gastric digestion. This work highlights the differences in the rate of gastric digestion, arising from oral processing, which can be attributed to the material properties of the substrate. Keywords: whey protein; gels; microstructure; particle size; digestion; pH 1. Introduction Commercial whey protein isolate (WPI) is a highly refined product derived from the bovine serum (whey) proteins of milk. The behaviour of WPI in water is strongly gov- erned by the principle constituent protein β-lactoglobulin (β-lg), which usually comprises more than 50% of the total protein, with the remainder consisting of the other serum pro- teins found in milk, which includes α-lactalbumin, immunoglobulins, proteose peptones, serum albumin and lactoferrin [1,2]. Whey protein structure and functionality has been reviewed earlier by Guyomarc’h et al. [3] and Nicolai et al. [4]. This behaviour is heavily influenced by electrostatics, with the iso-electric point (IEP) of β-lg generally quoted as pH 5.2 [5]. At pH values close to the IEP, β-lg and WPI solutions form a white suspension of spherical aggregates without the need for heating, which can be redispersed back into solution by adjusting the pH away from the IEP [6,7]. Heating native WPI solutions or suspensions at temperatures above the denatura- tion temperature of β-lg leads to the formation of irreversible protein aggregates [8,9]. The morphology of the aggregates following heating was shown to be related to that of the suspension/solution prior to heating [6]. When heated at pH values away from the IEP, fine-stranded protein aggregates are formed [10,11]. At pH values close to the IEP, spherical aggregates become irreversibly fixed [12,13]. The types of aggregates have been discussed in detail in the literature [3,14,15]. If heating is performed at concentrations above the critical gelation concentration (C g ), the aggregates form a percolating network and gels are formed. The value of C g is linked with pH and ionic strength and the trends in behaviour of WPI as a function of pH Foods 2021, 10, 1066. https://doi.org/10.3390/foods10051066 https://www.mdpi.com/journal/foods