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.

Commercial rapeseed press cakes are rich sources of phenolic compounds, namely, sinapic acid derivatives, which can be extracted as free sinapic acid and its bound forms (such as sinapine, the choline ester of sinapic acid). Fractionated rapeseed extracts rich in sinapic acid and sinapine were compared for their capacity to inhibit the formation of lipid oxidation products. Oxidation at 40°C was monitored by the formation of hydroperoxides (indicating primary oxidation products) and propanal (secondary oxidation products). The 70% methanolic extract of rapeseed meal, added as an equivalent of 500 µmol/kg oil (based on sinapic acid equivalent for sinapic acid-rich extracts or sinapine equivalent for sinapinerich extracts) showed good antioxidative activity compared with the addition of 500 µmol/kg oil sinapic acid. Apart from this, the interaction between a mixture of α-/γ-tocopherol and sinapic acid was investigated using response surface methodology for the experimental design. The experiments indicated that the addition of sinapic acid (concentration dependent) caused inhibition of peroxide formation, complementing further lower endogenous tocopherol concentration in oils.

To characterize parameters influencing the antioxidant activity at interfaces a novel ESR approach was developed, which facilitates the investigation of the reaction stoichiometry of antioxidants towards stable radicals. To relate the activity of antioxidants towards the location of radicals at interfaces NMR experiments were conducted. Micellar solutions of SDS, Brij and CTAB were used to model interfaces of different chemical nature. The hydrophilic Fremy's radical was found to be solubilized exclusively in the aqueous phase of SDS micellar solution but partitioned partly into the hydrophilic headgroup area of Brij micelles. In contrast the hydrophobic galvinoxyl was exclusively located in the micellar phase with the increasing depth of intercalation in the order SDS < Brij < CTAB. Gallates revealed a higher stoichiometric factor towards galvinoxyl in CTAB systems, which is accounted to a concentration effect of antioxidant and radical being both solubilized in the palisade layer. In contrast, in SDS solutions hardly any reaction between galvinoxyl and gallates was found. SDS acted as a physical barrier between radical (palisade layer) and antioxidant (stern layer). The influence of the hydrophobic properties of the antioxidant was clearly seen in Brij micelles. Elongation of the alkyl chain in gallate molecule resulted in increasing stoichiometric factors in the presence of galvinoxyl being located in the deeper region of the bulky headgroup area. The reverse trend was found in the presence of Fremy's radical being located in the hydrated area of the micelles.

The partition behavior of ethyl gallate (EG), 3,4-dihydroxybenzoic acid ethyl ester (DHBEE), and p-hydroxybenzoic acid ethyl ester (PHBEE) in different systems was investigated using ultrafiltration and dialysis techniques. The proportion of solute solubilized by micelles or oil in surfactant solution and biphasic water−oil systems, respectively, was determined directly. To study the partition in oil-in-water (O/W) emulsions, a mathematical model is presented which differentiates between the solubilization capacity of oil, surfactant, and aqueous phase. In contrast to previous studies, this model focuses on the equilibrium between oil and surfactant in the lipid phase of emulsions. The mutual influence of oil and surfactant determining the solubilization equilibrium is described by nonlinear functions derived from the experimental data. The solubilization capacities of oil and surfactant in O/W emulsions differed markedly from the surfactant solutions and biphasic water−oil systems. According to their polarity EG, DHBEE, and PHBEE showed significant differences in their partitioning behavior in O/W emulsions composed of oil, buffer, and different surfactants (sodium dodecyl sulfate, SDS; cetyltrimethylammonium bromide, CTAB; polyoxyethylene 20 cetyl ether, Brij 58; Emultop, a partially hydrolyzed soybean lecithin, PHLC). For instance, the proportion solubilized by oil in SDS containing emulsions increased in the order EG, DHBEE, and PHBEE, but the proportion solubilized by the surfactant showed parabolic behavior. All experimental data were in good agreement with the data computed on the basis of the mathematical model described in this study.