Air-water interfacial properties of enzymatically hydrolyzed wheat (T. aestivum L.) gluten

Promovendus/a: Arno Wouters

Promotor(en): Prof. dr. ir. Jan Delcour, Ine Rombouts

In many foods and beverages, such as meringues, chocolate mousses or beers, foams play important structural and textural roles. Animal proteins, such as those of egg white, are often used in this context because of their excellent ability to stabilize foams. However, their production is expensive and has a substantial environmental impact. Plant proteins can be a cheaper and more sustainable alternative, but they often lack functionality or have low solubility in aqueous media. A notable example are wheat gluten proteins, a co-product of the industrial wheat starch isolation process. Controlled enzymatic hydrolysis increases the solubility of gluten proteins in aqueous media but also enhances their foaming properties. In current literature, a thorough evaluation of the air-water (A-W) interfacial and foaming characteristics of such hydrolysates is only rarely considered. Furthermore, as protein hydrolysate functionality is usually assessed in relatively simple aqueous solutions, the complexity of food products is often underestimated. Against this background, this dissertation aimed to provide insights in the structure-function relationship of enzymatically hydrolyzed wheat gluten in conditions relevant to food systems.

In a first part, the relationship between the structural, A-W interfacial and foaming characteristics of gluten hydrolysates were studied in depth. Foaming capacity, i.e. the initial amount of foam formed, was related to the rates of diffusion to and adsorption of gluten hydrolysate constituents at the A-W interface. Foam stability, i.e. the amount of foam remaining after one hour, was related to the strength of adsorbed gluten hydrolysates protein films at the A-W interface. The structural features of the gluten hydrolysates responsible for their interfacial behavior were studied by performing a foam fractionation process.

In a second part, the A-W interfacial and foaming properties of gluten hydrolysates were assessed in gradually more complex systems to obtain an image of how such hydrolysates would behave in food relevant media. Firstly, the impact of a varying pH was investigated. Different gluten hydrolysates were more suited as foaming agents than others depending on the pH value. These differences were probably related to pH-induced conformational and aggregation changes in the gluten hydrolysate constituents. Secondly, the impact of the presence of sucrose and ethanol, which commonly occur in food foams, on the A-W interfacial characteristics of gluten hydrolysates was studied. In general, the former led to an increase in the affinity that the gluten hydrolysates constituents had for the interface, while the latter had the opposite effect. Finally, the impact of egg white proteins, which can stabilize foams by themselves, on the interfacial behavior of gluten hydrolysates was investigated. By themselves, egg white proteins usually provide good foam stability while gluten hydrolysates provide good foaming capacity. In a mixed system, foams with high foaming capacity and stability could be produced.

All these steps have yielded fundamental knowledge on the interfacial behavior of wheat gluten hydrolysates in more complex systems, and contribute to their possible use as foaming agents in food. In a final part, as a proof-of-concept, gluten hydrolysates were incorporated in a classical meringue recipe, which contains water, egg white protein and sugar. Meringues containing gluten hydrolysates had better batter and final product properties than those only containing egg white protein.