The impact of transglutaminase on soy proteins and tofu texture S. Bin Md Yasir a,b , K.H. Sutton b , M.P. Newberry b , N.R. Andrews a , J.A. Gerrard a, * a School of Biological Sciences, University of Canterbury, Christchurch, New Zealand b NZ Institute for Crop and Food Research Ltd., Private Bag 4704, Christchurch, New Zealand Received 9 June 2006; received in revised form 16 December 2006; accepted 23 February 2007 Abstract The enzyme transglutaminase was investigated for its cross-linking effect on the soy proteins of tofu. In vitro incubations confirmed that soy proteins are excellent substrates for transglutaminase, especially when denatured. The macroscopic effects resulting from the addition of transglutaminase were compared to changes at the microstructural and molecular level. Treatment produced a firmer tofu, with a significantly increased fracture force. Examination by SEM showed a change in the matrix structure, with transglutaminase result- ing in a finer-stranded, uniform network that accounted for the increase in fracture force. At the molecular level, little, if any, cross-link- ing occurred within the tofu matrix in situ. This suggests that the change in functional properties afforded by addition of transglutaminase to tofu is due to a side reaction of the enzyme, for example hydrolysis of glutamine residues, rather than its cross-link- ing activity. These ideas are further explored in the accompanying paper. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Soy protein; Tofu; Texture; Transglutaminase; Scanning electron microscopy 1. Introduction In many foods, proteins play a major role in product quality and determine many of the functional properties of these systems (Gerrard, 2002). Our research has previ- ously demonstrated that cross-linking of wheat proteins by the enzyme transglutaminase can have a dramatic influ- ence on the properties of bread (Gerrard et al., 1998) and croissants (Gerrard et al., 2000), which could be attributed to cross-linking of specific proteins (Gerrard et al., 2001; Gerrard & Sutton, 2005). In this research, we sought to establish whether similar changes of functional properties in other food products could be correlated with specific changes in cross-linking patterns. Tofu was selected as our model because its functional properties are determined by denaturation of soy proteins to form a gel. Soy proteins, in their native state, do not form a gel; they must be heat-denatured and then coagu- lated to form the tofu (Liu, 1997). Furthermore, the sub- unit composition of tofu has been shown to correlate with tofu quality (Poysa, Woodrow, & Yu, 2006); thus, we were optimistic that manipulation of specific proteins within the tofu matrix may lead to new methods to improve product quality. By adding a cross-linking reagent, in this case an enzyme, transglutaminase (and, in the accompanying paper (Yasir, Sutton, Newberrry, Andrews, & Gerrard, in press), Maillard cross-linking agents, glutaraldehyde, formaldehyde and glyceraldehyde), before and after the soy proteins are denatured during tofu manufacture, it was anticipated that the textural properties of the tofu would change to various degrees. Thus, the rela- tionship between the degree of cross-linking of particular sub-units and tofu texture could be established. The consumption of soy foods in the human diet has increased because of their beneficial effects on nutrition and health (Friedman & Brandon, 2001). This is due to the presence of a near perfect balance of all the essential amino acids, making soy a valuable protein source. Recently, the US Food and Drug Administration (FDA) approved a health claim that soy protein reduces the risk of coronary heart disease (Stewart, 2005). From a product 0308-8146/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2007.02.026 * Corresponding author. Tel.: +64 3 3667001; fax: +64 3 3642590. E-mail address: juliet.gerrard@canterbury.ac.nz (J.A. Gerrard). www.elsevier.com/locate/foodchem Food Chemistry 104 (2007) 1491–1501 Food Chemistry