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Mixed gels of plant proteins are being investigated for use as meat analogues. Juiciness is an important characteristic for the acceptability of meat analogues. The juiciness is assumed to be governed by the hydration properties, or water holding capacity, of the gel (WHC). We analysed the WHC of single-phase gels of respectively soy protein and gluten by applying Flory-Rehner theory. This enabled us to describe the WHC of more the complex mixed gels. The WHC of mixed soy protein - gluten gels is shown not to be a linear combination of their constituents. At high volume fractions, soy forms a continuous network and swells similarly to pure soy without being hindered by gluten. However, increasing gluten content leads to a gradual decrease in soy swelling. This is due to the mechanical interaction between soy and gluten. We propose that gluten-rich gels have a continuous gluten network that entraps soy and hinders its swelling. The elastic moduli of the gluten network were extracted from WHC data, and are in reasonable agreement with experimentally determined moduli. A better understanding of the effect of mixed gel composition on WHC is valuable for the development of the next generation meat analogues.

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In this paper, we present the rich physics and chemistry of the gastric digestion of protein gels. Knowledge of this matter is important for the development of sustainable protein foods that are based on novel proteins sources like plant proteins or insects. Their digestibility is an important question in the design of these new protein foods. As polyelectrolyte gels, they can undergo volume changes upon shifts in pH or ionic strengths, as protein gels experience when entering the gastric environment. We show that these volume changes can be modelled using the Flory-Rehner theory, combined with Gibbs-Donnan theory accounting for the distribution of electrolytes over gel and bath. In-vitro experiments of soy protein gels in simulated gastric fluid indeed show intricate swelling behaviour, at first the gels show swelling but at longer times they shrink again. Simulations performed with the Flory-Rehner/Gibbs-Donnan theory reproduce qualitatively similar behaviour. In the final part of the paper, we discuss how the model must be extended to model realistic conditions existing in the in-vivo gastric environment.

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