To evaluate the effect of colloidal parameters on the activity of natural antioxidants, the effect of selected phenolic acids on both bulk and emulsified methyl linoleate oxidation (in the dark at 40 degrees C) was examined. Oxidation was monitored by determining the formation of hydroperoxides; their isomer distribution and ketodiene (oxodiene) products were monitored by using high-performance liquid chromatography. This study showed the system- and concentration-dependent antioxidant activity of phenolic acids. The scavenging of alpha,alpha-diphenyl-beta-picrazylhydrazyl radicals reflected the antioxidant activity in a bulk oil system but not in an emulsion. Specific interactions of the antioxidant with other compounds, for example, the emulsifier, and intramolecular hydrogen bonds may play an important role in reducing the antioxidant activity. Furthermore, these interactions of antioxidants with emulsifier have a strong influence on the partitioning of antioxidants. Thus, the proportion of the antioxidant solubilized in the lipid phase and particularly in the interface did not necessarily reflect the efficiency of the antioxidant.

The partitioning behavior of a series of hydroxybenzoic acids and their derivatives was determined in biphasic water-oil systems, emulsifier solutions, and oil-in-water (O/W) emulsions. The effect of gradually decreasing polarity on partitioning behavior and antioxidant efficiency in O/W emulsions was investigated by using gallic acid and its esters as antioxidants. Sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), polyoxyethylene 20 cetyl ether (Brij 58), and partially hydrolyzed soybean lecithin (PHLC, Emultop ® ) were used to investigate the influence of different classes of emulsifiers on the partitioning behavior. The antioxidant activity of gallic acid and its methyl, ethyl, propyl, butyl, and octyl esters showed markedly different trends in O/W emulsions depending on the emulsifier used. The results are discussed with respect to the properties of the emulsifiers, such as hydrogen bond basicity, hydrophobic interactions, and structural properties.

Antioxidative effect of the main sinapic acid derivatives from rapeseed and mustard oil by-productsAmongst oilseeds, rapeseed and mustard are rich sources of phenolic compounds, which also prominent in the by-products of their respective oil processing or in commercial rapeseed and mustard press cakes. These cakes are rich sources of sinapic acid derivatives, which could be extracted as free sinapic acid or sinapine, the choline ester of sinapic acid. Sinapic acid is a widely investigated antioxidative compound. However, the main compound in the press cakes is present as sinapine. Investigations on the free-radical-scavenging activity of sinapic acid and sinapine indicate that sinapine had a significant but lower activity as compared to sinapic acid. Apart from this, sinapic acid, sinapine and different tocopherols were compared as antioxidants for inhibition of the formation of lipid oxidation products in purified rapeseed oils. The oxidation at 40 7C was monitored by the formation of hydroperoxides and propanal. The experiments indicate that in contrast to tocopherol mixtures addition of sinapic acid causes increasing inhibition of hydroperoxide formation when enhancing the concentration from 50 to 500 mmol/kg oil. Sinapine was not able to inhibit the formation of hydroperoxides, compared to sinapic acid. This indicates that sinapic acid-rich extracts, as compared to sinapine-rich fractions, could better inhibit the lipid oxidation in bulk lipid systems.

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This study examined partitioning of alpha-, beta-, and gamma-tocopherol and six polar antioxidants (Trolox, ferulic acid, caffeic acid, propyl gallate, gallic acid, and catechin) in mayonnaise. Partitioning of antioxidants between different phases was determined after separation of mayonnaise by either (a) centrifugation + ultracentrifugation or (b) centrifugation + dialysis. Antioxidants partitioned in accordance with their chemical structure and polarity: Tocopherols were concentrated in the oil phase (93-96%), while the proportion of polar antioxidants in the oil phase ranged from 0% (gallic acid and catechin) to 83% (Trolox). Accordingly, proportions of 6% (Trolox) to 80% (gallic acid and catechin) were found in the aqueous phase. Similar trends were observed after dialysis. After ultracentrifugation, large proportions of polar antioxidants were found in the "emulsion phase" and the "precipitate" (7-34% and 2-7%, respectively). This indicated entrapment of antioxidants at the oil-water interface in mayonnaise. The results signify that antioxidants partitioning into different phases of real food emulsions may vary widely.