Comparative study of gelation and cross-link formation during enzymatic texturisation of leguminous proteins

Isopeptide bonds that resulted from protein cross-linking, catalysed by a microbial transglutaminase (MTG), substantially contributed to the physicochemical modification of leguminous proteins. For the development of texturised vegetable protein (TVP)-based foodstuffs using MTG, valid methods for an efficient control of the gelation process are a prerequisite. Formation of ε-(γ-glutamyl)lysine cross-links in a simple food model system, containing proteins from soy [Glycine max (L.) Merr.] or pea (Pisum sativum L.), was monitored by quantification of the ε-(γ-glutamyl)lysine bond via HPLC-MS and by determining the depletion of free amino groups during cross-linking spectrophotometrically after derivatisation with o-phthaldialdehyde (OPA). Increasing gel strengths during incubation with MTG were measured via texture analysis. The OPA method proved too unspecific for controlling the enzymatic gelation process of leguminous proteins. Specifically for each substrate, the levels of isopeptide cross-links, detected via HPLC-MS analysis, correlated well with the gel strength of the texturised proteins (R² = 0.961-0.994). Rapidly measurable, gel strength was shown to be a reliable command variable for managing MTG-induced gelation. Its use also allowed indirect estimation of the degree of feasible cross-linking. Industrial relevance: Leguminous proteins represent a valuable alternative to animal proteins for the manufacture of texturised foodstuffs. However, due to the poor gelling properties of the native proteins, their potential is still unexploited. For the development of TVP using MTG, simple gel strength measurements were shown to be a valid tool for the control of the gelation process. For this purpose, unlike OPA determination of free amino groups and LC-MS analysis of isopeptide bonds, tedious sample preparation is not required.