In emulsions lipid oxidation is mainly influenced by the properties of the interface. The aim of this work was to investigate the effects of droplet size and interfacial area on lipid oxidation in protein-stabilized emulsions. Emulsions, made of stripped sunflower oil (30% vol/vol) and stabilized by BSA were characterized by surface area values equal to 0.7, 5.1, and 16.3 m 2 ·cm −3 oil. The kinetics of O 2 consumption and conjugated diene (CD) formation, performed on emulsions and nonemulsified controls, showed that emulsification prompted oxidation at an early stage. On condition that oxygen concentration was not limiting, the rates of O 2 consumption and CD formation were higher when the interfacial area was larger. Protein adsorbed at the interface probably restrained this pro-oxidant effect. Once most of the O 2 in the system was consumed (6-8 h), CD remained steady at a level depending directly on the ratio between oxidizable substrate and total amount of oxygen. At this stage of aging, the amounts of primary oxidation products were similar whatever the droplet size of the emulsion. Hexanal and pentane could be detected in the headspace of emulsions only at this stage. They were subsequently produced at rates not depending on oil droplet size and interfacial area.Paper no. J10026 in JAOCS 79, 425-430 (May 2002).KEY WORDS: Conjugated dienes, droplet size, interfacial area, lipid oxidation in emulsions, oxygen, protein, volatile compounds.Many foods are oil-in-water emulsions stabilized by protein and lipid surfactants in a protein or polysaccharide matrix. In these systems, oxidation of unsaturated FA leads to the formation of volatile compounds, which are major contributors to the aroma or to undesirable off-odors and off-flavors (1). Oxidation in bulk and emulsified oils is influenced by several common factors, but additional factors are important in emulsions (2). These include the structure of the emulsions and the physicochemical properties of the aqueous phase and of the droplet membrane. The size of the oil droplets is often claimed as an important factor. On one hand, small droplet size signifies a large surface area, implying a high potential of contact between diffusing oxygen, water-soluble free radicals and antioxidants, and the interface. It also implies a high ratio of oxidizable FA located near the interface to FA embedded in the hydrophobic core of the droplets (3). Decreasing the size of the oil droplets is therefore expected to favor development of oxidation. On the other hand, when the droplet size decreases, the number of lipid molecules per droplet diminishes and the amount of surface-active compounds adsorbed at the interface is increased. This could limit initiation and propagation, as surface-active compounds may act as a barrier to the penetration and diffusion of pro-oxidants or even interfere with oxidation. For instance, homogenization is reported to protect milk fat from oxidation catalyzed by metal complexes (4) because casein, which adsorbs to the droplet surface, is an efficie...
Six model dairy desserts, with three different textures and two sucrose levels, were equally flavored with a blend of four aroma compounds [ethyl pentanoate, amyl acetate, hexanal, and (E)-2-hexenal] and evaluated by a seven person panel in order to study whether the sensory perception of the flavor and the aroma release during eating varied with the textural characteristics or the sweetness intensity of the desserts. The sensory perception was recorded by the time intensity (TI) method, while the in vivo aroma release was simultaneously measured by the MS-nose. Considering the panel as a whole, averaged flavor intensity increased with sucrose level and varied with the texture of the desserts. Depending on the aroma compound, the averaged profile of in vivo aroma release varied, but for each aroma compound, averaged aroma release showed no difference with the sucrose level and little difference with the texture of the desserts. Perceptual sweetness-aroma interactions were the main factors influencing perception whatever the texture of the desserts.
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 present study deals with the physicochemical interactions between aroma compounds and various dairy media used as models of complex food matrices, as well as the consequences of the interactions on aroma partitioning between the air and matrix. Five aroma compounds were investigated: amyl and isoamyl acetate, ethyl pentanoate, hexanal and t-2-hexenal. Skim milk, anhydrous milk fat and full-fat cream were chosen as dairy media, while water was used as a reference medium. Apparent partition coefficients of the five aromas were determined between the air and media between 30 and 80 °C by static headspace-gas chromatography. Partition coefficients over full-fat cream were also calculated from partition coefficients over skim milk and anhydrous milk fat. Compared with water, a significant retention of t-2-hexenal was observed in skim milk (nearly 90% whatever the temperature), whereas the retention of the other aromas varied from 6% for isoamyl acetate to 40% for hexanal in skim milk. Hydrophobic interactions were responsible for the retention of esters, whereas covalent binding of t-2-hexenal by dairy proteins was probably involved. The volatility of the 5 aromas was drastically reduced over anhydrous milk fat, because of their hydrophobic nature (log P > 1). There were discrepancies between calculated and measured partition coefficients over full-fat cream, which depended on aroma compounds and temperature. Measured and calculated ethyl pentanoate retentions were similar whatever the temperature. For isoamyl acetate, hexanal and, to a lesser extent, t-2-hexenal, aroma retention was greater than expected. The reverse phenomenon was observed with amyl acetate. The so-called "enthalpy of affinity" was calculated from the variation of the partition coefficient with temperature. This parameter allowed an overview of the relative importance of aroma compounds-matrices interactions.
Anhydrous milk fat
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