Volume 63, No. 2, 1998—JOURNAL OF FOOD SCIENCE 183 Fat Particle Structure and Stability of Food Emulsions WEN XU, ALEX NIKOLOV, DARSH T. WASAN, ALEX GONSALVES, and RAJENDRA P. BORWANKAR ABSTRACT We used a nondestructive Kossel diffraction technique based on the principle of backlight scattering to characterize the structure formation in concentrated model food emulsions. We also measured the effects of temperature, shear, casein micelles and gum on the fat particle packing structure in such systems. Through this technique, we demonstrated the effects of casein micelles on the pair potential of fat particle interactions and shelf life of such systems. We carried out theoretical calculations using a statistical mechanics ap- proach by numerically solving the Ornstein-Zernike equa- tion with Percus-Yevick closure for a bidisperse system con- sisting of fat particles and micelles, and showed that the experimental results were consistent with theoretical simu- lations. Key Words: particle structure, emulsion stability, light scattering, caseinate, gum, shear rate INTRODUCTION AS WITH MOST COLLOIDAL SYSTEMS, THE INTERACTIONS BE- tween fat particles inside food emulsions are very important in the stability and texture of products. They may determine the rheologi- cal properties and the appearance of the product, as well as its phys- ical instability, as reflected in changes of consistency or loss of ho- mogeneity (Walstra, 1993). Many factors, such as protein concentration, gum, temperature, shear rate, fat particle concentration and polydispersity, and non- adsorbed particles, may affect fat particle interactions and the stabil- ity of food emulsions. Koczo et al. (1996) reported a stability mech- anism for food emulsions due to the microlayering of sodium casein- ate submicelles in the thin liquid films between fat particles. The existence of the submicelle layers in the thin liquid films between fat particles prevented fat particles from approaching each other, thus stabilizing the food emulsions. Creaming (flocculation) of a model food emulsion in the presence of xanthan and guar gums was then studied (Koczo et al., 1997). The mechanism of dispersion separa- tion by xanthan gum was a geometrical incompatibility between the anisotropic, wormlike-chain shaped xanthan molecules and the iso- tropic dispersed fat particles. Particle structure variations under shear stress have been investi- gated by Ackerson and Clark (Ackerson and Clark, 1981). They found that when the dispersions of charged spherical colloidal particles were subjected to an increasing shear rate, the particle structure exhibited a reversible and order-disorder transition and became an amorphous structure at high shear rate. Effects of particle concentration and poly- dispersity on the particle interactions in colloidal dispersions have been studied theoretically (Chu et al., 1996) and experimentally (Xu et al., 1997). In a near hard sphere system, due to the increase of the particle concentration, the particle packing structure was found to become more ordered and both structural energy barrier and deple- tion increased, while the particle packing structure became less or- dered when the particle polydispersity increased. The fat particle packing structure inside whipped cream was stud- ied by Brooker (Brooker et al., 1986; Brooker, 1990; Anderson and Brooker, 1988) using transmission and scanning electron microsco- py. Qualitative information about the particle packing structure in the bulk phase was reported. Much information about the fat particle packing structure has been qualitative due to the lack of measure- ment methods and complexity of food systems. The effects of inter- particle interactions on the stability of food emulsions are not under- stood. Our main objective was to generate a quantitative description of the fat particle structure using backlight scattering technique, and to understand the effects of fat particle-particle interactions in the sta- bility of food emulsions. The effects of protein submicelles, temper- ature, gum (xanthan) and shear rate on the fat particle structuring and the stability of food emulsions were investigated. THEORY Light scattering theory Light scattering is one technique to obtain structural and thermo- dynamic information about dispersions. For a monodisperse system of spherically symmetric particles, when the particle size is slightly smaller than the incident polarized light wavelength, Rayleigh-Gan- Debye theory (Hunter, 1987) can be applied, I(Q) = ANP(Q)S(Q) (1) where I(Q) is the intensity of the scattered light at a scattering vector Q, A is a normalized constant that can be determined by the require- ment (Nieuwenhuis and Vrij, 1979) that  0  Q 2 [S(Q) - 1]dQ = 2 2 N (2) where N is the particle number density, P(Q) is the form factor which is a property of particle shape and size, and can be obtained from light scattering experiment of dilute colloidal dispersions, S(Q)   0 = 1 (3) and P(Q) = [I(Q)   0 ] / (A*N*) (4) where  is the particle volume fraction, A * is an unknown constant, and N * is the particle number density of dilute colloidal dispersions. In this experiment, the fat concentration of food emulsions to infer the form factor was around 0.1 vol%. Then the static structure factor S(Q) could be determined from the following equation, S(Q) = [(A*N*) / AN] [I(Q) / I(Q)   0 ] (5) where S(Q) is the static structure factor which describes the degree of particle structure ordering inside colloidal dispersions. We had found (Xu et al., 1997) that the higher the first peak of the static structure factor, the more ordering there was in the particle packing structure. The typical range of the first peak height for common food colloidal systems is around 1 to 3. [A * N * /AN] is a constant, which can be determined by Eq. (2). Q is the scattering vector that is de- fined by HYPOTHESIS PAPER Authors Xu, Nikolov and Wasan are with the Chemical Engineering Dept., Illinois Institute of Technology, Chicago, IL 60616. Authors Gonsalves and Borwankar are with Kraft Foods, Glenview, IL 60025. Address inquiries to Dr. D.T. Wasan.