Heat-Induced Gelation of Pea Legumin: Comparison with Soybean Glycinin FRANCESCA E. O’KANE, †,‡ RANDOLPH P. HAPPE, ‡,§ JOHAN M. VEREIJKEN, # HARRY GRUPPEN, ‡,⊥ AND MARTINUS A. J. S. VAN BOEKEL* ,†,‡ Product Design and Quality Management Group, Centre for Protein Technology TNO-WU, and Laboratory of Food Chemistry, Wageningen University, Wageningen, The Netherlands; Centre for Protein Technology TNO-WU, TNO Nutrition and Food Research, Zeist, The Netherlands; and Agrotechnology and Food Innovations B.V., Wageningen, The Netherlands Gel network formation of pea legumin (8.4% on a protein basis, pH 7.6) was monitored via dynamic rheological measurements. Gelation was performed in the absence and presence of the thiol-blocking reagent N-ethylmaleimide, at different rates of heating and cooling. Overall, it was shown that pea legumin gel formation was not effected by changes in the heating rate, and the two differently heated samples were unaffected by the addition of 20 mM NEM, which indicated that disulfide bonds were not essential within the network strands of these legumin gels. However, slowly cooling the legumin samples caused disulfide bonds to become involved within the network; this was observed by a large increase in gel strength that was then substantially reduced when repeating the sample in the presence of NEM. These experiments were repeated with soybean glycinin in order to determine whether a common model for gel formation of legumin-like proteins could be built, based upon molecular reasoning. The two proteins were affected in the same way by changes in the conditions used, but when applying a procedure of reheating and recooling the gel networks responded differently. Pea legumin gel networks were susceptible to rearrangements that caused the gels to become stronger after reheating/recooling, yet glycinin gel networks were not. It was concluded that the same physical and chemical forces drove the processes of denaturation, aggregation, and network formation. Each process can therefore be readily targeted for modification based upon molecular reasoning. Pea legumin and soybean glycinin gel networks had structurally different building blocks, however. A model of gelation aimed at texture control therefore requires additional information. KEYWORDS: Pisum; legumin; glycinin; gelation; small deformation rheology; texture control INTRODUCTION Globular proteins from various sources (in the form of isolates) play important roles in many foodstuffs, both because of their nutritional value and of their contribution to food texture (1). These texture contributions come from the network structures created by the proteins. Since gelation is one of the most important functional properties of the globular proteins used to modify food texture (2), it should be important to understand which factors determine the gel network and how they are affected by processing parameters. Such an understand- ing would enable better control of food textures. Protein isolates from soybean dominate the market, though presently there is a trend for alternative protein isolates having similar functional and nutritional properties as soya (3). A potential alternative plant protein in Europe is pea (Pisum satiVum L.). As with soybean, it contains two major globulin proteins, namely legumin and vicilin. Pea vicilin functionality has been dealt with in a previous paper (4), so only legumin will be given further consideration in this paper. Legumin is a polypeptide of ∼60 kDa, though this polypeptide is commonly denoted as a legumin subunit that assembles into higher molecular weight oligomers. A feature of legumin subunits is that they split into acidic (40 kDa) and basic (20 kDa) polypeptides via disulfide bond reduction. Similar subunits compose the legumin-like proteins of Glycine max.(5) and Vicia faba (6). In all cases the disulfide-bonded acidic and basic polypeptides are formed when the protein precursor is pro- teolytically processed in the plant (7). In contrast to vicilins, legumins are recognized for their cysteine content: pea and fababean legumin contain approximately 5 residues per 60 kDa subunit, and soybean glycinin approximately 8. * Address correspondence to this author at the Product Design and Quality Management Group, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands (telephone +31-317-484281; fax +31-317-483669; e-mail tiny.vanboekel@wur.nl). † Product Design and Quality Management Group, Wageningen Uni- versity. ‡ Centre for Protein Technology TNO-WU. § TNO Nutrition and Food Research, Zeist, The Netherlands. # Agrotechnology and Food Innovations B.V., Wageningen, The Neth- erlands. ⊥ Laboratory of Food Chemistry, Wageningen University. J. Agric. Food Chem. 2004, 52, 5071-5078 5071 10.1021/jf035215h CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004