Aggregation profile of 11S, 7S and 2S coagulated with GDL Sok Li Tay, Guo Qin Xu, Conrad O. Perera * Department of Chemistry, FST Program, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Received 12 April 2004; received in revised form 17 June 2004; accepted 17 June 2004 Abstract The aggregation process of the proteins coagulated by glucono-d-lactone (GDL) was monitored by using atomic force micros- copy (AFM). Solutions of 11S, 7S and 2S proteins, after heating at 100 °C for 10 min, were mixed with GDL and formed aggregates with different aggregation profiles. When the three protein solutions (11S, 7S and 2S) were mixed with GDL and deposited onto the mica for 1, 2 and 4 min, 11S proteins formed the largest clusters of aggregates, 2S proteins formed smaller clusters of aggregates than 11S but bigger clusters of aggregates than 7S and 7S proteins formed the smallest cluster of aggregates. It was also found, by tur- bidity measurement, that when GDL was added to the three protein fractions, the level of turbidity was in the order of 11S > 2S > 7S. Both these results showed that, when GDL was added to the three protein fractions, the speed of aggregation was in the order of 11S > 2S > 7S. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Soy protein fractions; Turbidity; Aggregation; Gelation; GDL 1. Introduction Gelation and aggregation are closely related. Gela- tion takes place when protein aggregates form a network (Lakemond et al., 2003). Doe (1993) reported that gran- ularity gel transparencies were related to aggregate size and Lakemond et al. (2003) reported that aggregation size was related to the thickness of the strands of the gel. Hence, protein aggregation plays an important part in gelation. Aggregation of proteins can be brought about by heating, or by varying the pH or ionic strength of the protein solution. It has been found that, depending on the conditions, both glycinin (11S) and b-conglycinin (7S) are able to form large aggregates when heated (Mills, Huang, Noel, Gunning, & Morris, 2001). There are various ways to analyze protein aggregates, such as particle size distribution, electron microscopy, dy- namic viscoelasticity measurement and spectrophoto- metric methods (Dybowska & Fujio, 1998). Atomic force microscopy (AFM) is widely used to study the structure of biological macromolecules (McIntire & Brant, 1999). Its high resolution makes it a powerful tool for studying protein aggregation. AFM is able to image the shape of the protein particles and aggregates. It is also able to provide information about the size of particles or even the nature of interactions responsible for gelling (Morris et al., 2001). Analytical ultracentrifugation of soy proteins in phosphate buffer of pH 7.6, with ionic strength of 0.5, showed that soy proteins contain four major globulins, namely, 15S, 11S, 7S, and 2S protein. These protein fractions are characterized by their sedimentation coeffi- cients. The percentage content each of 15S, 11S, 7S and 2S was found to be 9.1%, 41.9%, 34% and 15%, respec- tively (Fukushima, 1991). 7S and 11S were shown to have different gel-forming properties (Kohyama & Nish- inari, 1993; Tay & Perera, 2004), which could be due to differences in their aggregation processes. 0308-8146/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2004.06.027 * Corresponding author. Fax: +65 677 57895. E-mail address: chmpco@nus.edu.sg (C.O. Perera). www.elsevier.com/locate/foodchem Food Chemistry 91 (2005) 457–462 Food Chemistry