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This paper is devoted to the application of front-surface fluorescence to the study of aging and oxidation of oil-in-water emulsions. Emulsions with two oil droplet sizes were stabilized with bovine serum albumin (BSA) and stored at 37 or 47 degrees C. Lipid oxidation was demonstrated by measurement of hydroperoxides and headspace pentane. Front-surface fluorescence spectra (excitation wavelength = 355 nm) revealed gradual formation of oxidized lipid-protein adducts during the 4 weeks of storage. Fluorescence (excitation = 290 nm) of BSA tryptophanyl residues (Trp) declined during the first day of aging and then decreased slightly and linearly. Fourth-derivative Trp spectra exhibited peaks at 316 and 332 nm. Their evolution indicated that the ratio of Trp in hydrophobic environments to total Trp increased in small droplet emulsions. This suggests that, during lipid oxidation, the adsorbed and nonadsorbed protein underwent various degrees of Trp degradations, polymerization, and aggregation. Thus, front-surface fluorescence makes it possible to evaluate, noninvasively, protein modification and lipid oxidation end-products during processing and storage of food emulsions.

The displacement of a globular protein (bovine serum albumin, BSA) from the surface of oil droplets in concentrated oil-in-water emulsions by a nonionic surfactant (polyoxyethylene sorbitan monolauarate, Tween 20) was studied using front-face fluorescence spectroscopy (FFFS). This method relies on measurement of the change in intensity (I(MAX)) and wavelength (lambda(MAX)) of the maximum in the tryptophan emission spectrum. A series of oil-in-water emulsions (21 wt % n-hexadecane, 0.22 wt % BSA, pH 7.0) containing different molar ratios of Tween 20 to BSA (R = 0-131) were prepared. As the surfactant concentration was increased, the protein was progressively displaced from the droplet surfaces. At R > or = 66, the protein was completely displaced from the droplet surfaces. There was an increase in both I(MAX) and lambda(MAX) with increasing Tween 20 concentration up to R = 66, which correlated with the increase in the ratio of nonadsorbed to adsorbed protein. In contrast, there was a decrease in I(MAX) and lambda(MAX) with Tween 20 concentration in protein solutions and for R > or = 66 in the emulsions, which was attributed to binding of the surfactant to the protein. This study shows that FFFS is a powerful technique for nondestructively providing information about the interfacial composition of droplets in concentrated protein-stabilized emulsions in situ. Nevertheless, in general the suitability of the technique may also depend on protein type and the nature of the physicochemical matrix surrounding the proteins.

The structural modification of globular proteins (bovine serum albumin, BSA) in the aqueous phase of emulsions produced by homogenization was studied using front-face fluorescence spectroscopy (FFFS). A series of hydrocarbon oil-in-water emulsions (30 wt % n-hexadecane, 0.35 wt % BSA, pH 7.0) were homogenized to differing degrees with a high-speed blender and a high-pressure valve homogenizer. The wavelength of the maximum in the tryptophan emission spectrum (lambda(max)) of serum phases collected from the emulsions by centrifugation was measured and compared to lambda(max) values of BSA solutions subjected to the same homogenization conditions. There was no significant (p < 0.05) change in lambda(max) with homogenization conditions for BSA solutions. In contrast, lambda(max) of serum phases from emulsions blended for 2 min in a high-speed blender was significantly smaller (p < 0.05) than nontreated BSA solutions (Deltalambda(max) = 2 nm). In addition, there was a further significant decrease in lambda(max) of the serum phases with an increasing number of passes of the emulsion through the high-pressure valve homogenizer (e.g., Deltalambda(max) = 4 nm for 12 passes). This study shows that globular proteins present in the aqueous phase of a hexadecane-in-water emulsion after homogenization could be altered, which is probably caused by surface modification of the protein structure during temporary adsorption to emulsion droplet surfaces during homogenization.

homogenized milks (Dufour and Riaublanc, 1997). MIR with an ATR cell was successfully applied to the determination of polyethylene glycol during in-vitro digestion of bread (Belleville et al., 1995) and to the study of conformational changes of ␤-lactoglobulin under pH and ethanol effects (Dufour et al., 1994).We formed films using the wet spinning process that is used in the food sector to make textured products. The spinning process has the advantage of being operated in a continuous mode and is applicable on an industrial scale. Using this technique, Dumont (1997, 1998) evaluated the effects of plasticizers such as polyols and urea on mechanical properties of films and their chemical composition. They also studied the effects of washing and tanning. The addition of plasticizers enhanced mechanical characteristics. Washing eliminated the plasticizers and made films brittle. More resistant films were obtained by tanning the proteins with formaldehyde, well known as a cross-linking agent.The spinning process was historically applied to the production of textile fibers and several studies have been reported on the molecular structure of such fibers. It was long been known that X-ray diffraction and mechanical measurements are in agreement with a uniform alignment of the protein chains in the direction of the fiber axis. The orientation is induced by a stretching of the fibers at the spinneret exit. The orientation developed along the spinning path is mainly the result of competition between the orienting effect of the velocity field and the disorienting effect of Brownian motions (Ziabicki,1967). The structure of silk fibers has been investigated using Raman spectroscopy (Magoshi et al., 1985). At extension rates of 100-450 mm/min, the fibroin molecules are oriented in the direction of extension and present a random coil conformation. By increasing the extension rates over 500 mm/min, the secondary structure of the protein mostly becomes organized in ␤ sheet form. The oriented model observed in the case of textile fibers has been thereafter considered for edible fibers made of plant proteins (Hartman, 1978;Culioli et al., 1996). No evidence has confirmed the hypothesis concerning a structural arrangement of the molecules in edible fibers.Our objective was to clarify whether the spinning process induces an organization of the proteins in films. Changes in the secondary and tertiary structures of proteins during film processing, as well as the network orientation of films, were investigated by spectroscopic and rheological methods.