Food Research International, Vol. 28, No. 5, pp. 431443, 1995 Elsevier Science Ltd Copyright 0 1995Canadian Institute of Food Science and Technology Printed in Great Britain. All rights reserved 0963-9969195 $9.50 + .OO zyxwvutsr 0963-9969(95)00034-8 Effect of sodium level on the microstructure and texture of whey protein isolate gels zyxwvutsrqponmlkjihgfedcba Shai Barhut Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada, NIG 2 W I The effect of sodium level (25-500 mM, at pH 7.0) on the gelation of whey protein isolate (WPI) was investigated. At low sodium level clear gels were formed. They were composed of fine protein strands, which exhibited good water holding capacity (WHC). As sodium level increased gel clarity gradually decreased. At high sodium levels (>200 mM) aggregated, opaque gels with poor WHC were formed. Scanning and transmission electron micrographs of the gel made with 500 mM NaCl resembled the gel formed in the presence of low cal- cium (25 mM) level. A progressive increase in the size of the protein strands was observed as sodium level was increased up to a point where aggregates were formed. The formation of aggregates was related to a significant decrease in WHC and gel strength, indicating that the fine gel structure can hold moisture much better than the aggregated structure. Keywords: whey proteins, gelation, microstructure, water holding, sodium. INTRODUCTION The ability of various proteins to form different gel structures is very important to the food industry. The textural properties, sensory characteristics and yield of processed foods such as cheese, yoghurt, custards and sausages are directly related to the formation of protein gels during heating. The properties of the gels depend on the type of protein, its concentration, pH, ionic environment, heating schedule and interactions with other ingredients (Clark, 1992; Barbut, 1994). Gels can exhibit diverse microstructures and textural properties, and as such are difficult to define. Clark (1992) in his review, defined a gel as a material containing a contin- uous and well-defined solid network that is assembled from particles or polymers imbedded in an aqueous solvent. Overall, protein gels can be divided into two major categories (Doi, 1993). The first is an aggregated, opaque structure, typically formed by egg white (at pH 7). The second consists of a fine-stranded gel, produced by an association of small diameter molecules to form an ordered network. These fine-stranded gels are usually clear or semi-clear and can be formed by gelatin or whey pro- tein in isolate (WPI) in the presence of low sodium levels. There is a lot of interest, both industrially and aca- demically, in the ability of dairy proteins to form differ- ent structures (Kalab, 1993; Matsudomi et al., 1991; Mulvihill & Kinsella, 1990). In the case of whey protein concentrate (WHC), Schmidt et al. (1978) indicated that the mineral components have a significant effect on gel characteristics. They reported that dialysing the concentrate prior to heating formed stronger and more cohesive gels which were more sensitive to salt addition than gels formed with non-dialysed concentrate. Maxi- mum gel hardness was obtained with 200 mM NaCl or 11.1 mM CaCl, and it decreased with higher salt con- centrations (Schmidt et al., 1978179). Mulvihill and Kinsella (1988) also reported that maximum hardness values of @lactoglobulin gels were obtained with 200 mM NaCl or 10 mM CaCl,. They noted that CaCl, appeared to be more affective than NaCl in increasing gel strength since lower CaCl, levels were required to pro- duce similar hardness. However, they have not closely examined the significant difference in WHC between the two types of gels. Kuhn and Foegeding (1991) indi- cated that increasing NaCl from 25 to 75 mM resulted in a sharp increase in shear stress which later decreased as NaCl was raised to 500 mM in a 10% WPI gel. Shear strength decreased from 25 to 150 mM NaCl and later slightly increased as NaCl level was raised. Harwalkar and Kalab (1985) reported that, within a P-lactoglobulin system, protein solutions (0.25-9.0%) 437