Influence of Transglutaminase on Glucono-δ-lactone-Induced Soy Protein Gels

Enzymatic cross-linking of proteins can be used to create a variety of foods with appealing textures and facilitate further processing such as slicing or packaging. Transglutaminase (TG), a protein-glutamine-γ-glutamyltransferase, has been used in the production of processed meat and fish products. However, it has a reaction optimum at pH 7 and is therefore typically unsuitable for application in up-and-coming acidic food products such as fermented vegan meat product alternatives. To determine whether a simultaneous addition of slowly acidifying glucono-δ-lactone (GDL) and TG can facilitate protein cross-linking prior to the pH becoming too low, TG and GDL were added separately or in combination to soy protein suspensions composed of 10–15% soy protein concentrates or isolates (SPI). Texture analysis showed that SPI gels at pH 5–6, induced by GDL and TG combined, were harder than only GDL-induced gels and as hard as TG-induced gels at the optimum pH of 7. Decreased tan δ values and confocal laser scanning microscopy images indicated that protein cross-linking had taken place in combined gels. The results suggest an initial network formation induced by TG, which later in the acidification is being supplemented by an agglomeration of proteins due to weakened electrostatic repulsion. Taken together, the combination of slow acidification by GDL and TG addition leads to acidic gels with enhanced textural properties and shelf life.