Encapsulation of oil in the high moisture extrusion of wheat gluten: Interrelation between process parameters, matrix viscosity and oil droplet size

In plant protein-based meat substitutes, such as wheat gluten, oil is often added with the aim of improving sensory properties, like juiciness. When oil is added directly in the high moisture extrusion process, it must be homogeneously distributed and finely dispersed in the protein matrix to ensure both a stable process and a high quality of the final product. To gain a better understanding of droplet formation during the extrusion process, we investigate the relationship between process parameters, material properties and oil droplet size. We showed that at all oil contents, the use of reverse transport elements (RTE) prior to oil addition results in smaller oil droplets compared to forward transport elements. We attributed this to the fact that RTE increases the material temperature and residence time in the extruder and hence the viscosity of the protein matrix, which results from increased polymerization of the wheat gluten. The increased viscosity of the matrix can cause both an enhanced droplet breakup and a reduced coalescence of the droplets. However, with increasing the oil content from 1% to 4%, we noticed a remarkable increase in droplet size, independent of the matrix viscosity, which indicates the occurrence of coalescence.