Shear-induced fat particle structure variation and the stability of foodemulsions:I.Effectsofshearhistory,shearrateandtemperature Wen Xu, Alex Nikolov, Darsh T. Wasan * Department of Chemical Engineering, Illinois Institute of Technology, Perlstein Hall, Room 228, 10 West 33RD Street, Chicago, IL 60616, USA Received 16 March 2001; received in revised form 23 October 2002; accepted 24 February 2004 Abstract The shear-induced fat particle structure variation and the stability of food emulsions were investigated by the back-light scat- teringtechnique.Theeffectsoftemperature,shearrate,andshearhistoryonthefatparticlestructureandstabilityoffoodemulsions were studied. Increasing the shear time initially improves the fat particle structure; afterwards, the fat particle packing structure becomes less ordered. The effect of temperature on the fat particle structure is complicated. It was also found that the optimal temperature,shearrateandshearhistoryexistinordertoobtainawell-developedfatparticlestructure.Aqualitativeexplanationof the experimental results is provided. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Shear stress; Fat particles; Aggregation; Stability; Food emulsions 1. Introduction The development of a stable structure and a suitable product texture in food emulsions and foams depends on the interactions between fat globules, and between fat globules and air bubbles (Buchheim, Barlod, & Krog, 1985). The interactions between fat globules and air bubbles at high fat concentrations (i.e., higher than 20 wt%) have been studied by many researchers (Brooker, 1985; Brooker, 1990; Brooker, Anderson, & Andrew, 1986; Buchheim et al., 1985; Courthandon, Dickinson, & Dalgleish, 1991; Needs & Huitson, 1991) and the stabilization mechanism of fat globules existing at the air/aqueous interface is well understood; the sta- bilization of the air bubbles is a two-step process involving the selective adsorption of proteins to the bubble surface, followed by the attachment of fat globules at the air/aqueous interface under the shear stress. The attached fat globules stay at the air/water interface and protrude slightly (Brooker, 1990). The attached fat globules around the air bubbles thereby mechanically stabilize foams. Another important factor influencing the stability of food emulsions and foams is the developed fat particle structure. The fat particle structure variation and destabilization during the shearing process are very complex. Many factors such as shear rate, shear time, shear temperature, the composition of the surfactant mixture,andtheratioofliquidtosolidcontentinsidea fatglobulecangreatlyaffectthefatparticlestructureof thefinalproduct.Researchershaveinvestigatedsomeof these factors in their relationship to emulsion stability. Darling (1982) investigated the destabilization of dairy emulsion droplets described in terms of flocculation, coalescenceandthecreamingprocess.Hefoundthatthe presence of crystalline fat within emulsion droplets was essential for many destabilization processes because fat crystals could pierce the film between two approaching fat particles, thus leading to droplet aggregation or coalescence. Shear-induced destabilization of food emulsions was also investigated by measuring the vis- cosity variation under the shear stress, and a kinetic model in terms of reversible flocculation followed by partial coalescence was developed to explain the exper- imental results. Boode, Bisperink, and Walstra (1991) investigated the destabilization of the oil-in-water emulsions con- taining fat crystals by temperature cycling. They found Journal of Food Engineering 66 (2005) 97–105 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +1-312-567-3001; fax: +1-312-567- 3003. E-mail address: wasan@iit.edu (D.T. Wasan). 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.02.041