Ability of surfactant micelles to alter the partitioning of phenolic antioxidants in oil-in-water emulsions

In oil-in-water emulsions, the physical location of antioxidants can be an important determinant in their activity. Surfactants can potentially influence the physical location of antioxidants in oil-in-water emulsions by causing solubilization of lipid-soluble antioxidants into the aqueous phase. Excess Brij micelles in an oil-in-water emulsion were found to increase the partitioning of phenolics into the continuous phase with polar antioxidants (propyl gallate) partitioning more than nonpolar antioxidants (butylated hydroxyltoluene). Solubilization of propyl gallate was rapid coming to equilibrium in less than 5 min. Increasing surfactant micelle concentrations from 0.3 to 2.8% increased the solubilization of propyl gallate by 2.3-fold. Solubilization of phenolic antioxidants into the aqueous phase by Brij micelles did not alter the oxidative stability of salmon oil-in-water emulsions, suggesting that surfactant micelles influenced oxidation rates by mechanisms other than antioxidant solubilization.     

Ability of surface-active antioxidants to inhibit lipid oxidation in oil-in-water emulsion

Lipid oxidation in dispersed lipids is prevalent at the oil-water interface where lipid hydroperoxides are decomposed into free radicals by transition metals. Free radical scavenging antioxidants are believed to be most effective in lipid dispersions when they accumulate at the oil-water interface. The surface activity of antioxidants could be increased by their conjugation to hydrocarbon chains. In this study, p-hydroxyphenylacetic acid (HPA) was conjugated with either a butyl or dodecyl group. The HPA conjugates were more effective at decreasing interfacial tension than unconjugated HPA, indicating that they were able to adsorb at lipid-water interfaces. However, free HPA was a more effective antioxidant than butyl and dodecyl conjugates in Menhaden oil-in-water emulsions as determined by both lipid hydroperoxides and thiobarbituric acid reactive substances. The increased antioxidant activity of free HPA could be due to its more effective free radical scavenging activity and its higher concentration in the lipid phase of oil-in-water emulsions in the presence of surfactant micelles where it can act as a chain-breaking antioxidant.     

Impact of tween 20 hydroperoxides and iron on the oxidation of methyl linoleate and salmon oil dispersions

To determine the role of surfactant hydroperoxides on the oxidative stability of fatty acids, the oxidation of methyl linoleate micelles and salmon oil-in-water emulsions was measured as a function of varying Tween 20 hydroperoxide concentrations. Increasing Tween 20 hydroperoxide concentrations from 3.5 to 14.7 micromol hydroperoxide/g Tween 20 decreased the lag phase of headspace hexanal formation but did not increase the total amount of hexanal formed in methyl linoleate/Tween 20 micelles. In the micelle system, Fe(2+) decreased the lag phase of hexanal formation but increased total hexanal concentrations only in micelles with the highest Tween 20 hydroperoxide concentrations (14.7 micromol hydroperoxide/g surfactant). Increasing Tween 20 surfactant hydroperoxide concentrations also increased the oxidation of salmon oil-in-water emulsions as determined by lipid hydroperoxides and headspace propanal. In both the micelle and emulsion systems, the prooxidant effect of Fe(2+) decreased with increasing Tween 20 hydroperoxide concentrations. These data show that surfactant hydroperoxides such as those in Tween 20 could decrease the oxidative stability of lipids in food emulsions.     

Ability of surfactant micelles to alter the physical location and reactivity of iron in oil-in-water emulsion

The purpose of this research was to determine how surfactant micelles influence iron partitioning and iron-promoted lipid oxidation in oil-in-water emulsions. Lipids containing ferric ions were used to produce oil-in-water emulsions, and continuous-phase iron concentrations in emulsions were measured as a function of varying continuous-phase polyoxyethylene 10-lauryl ether (Brij) concentrations. Continuous-phase iron concentrations increased with increasing surfactant micelle concentrations (0.1-2.0%) and storage time (1-7 days). At pH 3.0, the concentration of continuous-phase iron was higher than at pH 7.0. Similar trends in iron solubilization by Brij micelles were observed when either hexadecane or corn oil was used as the lipid phase. Lipid oxidation rates, as determined by the formation of lipid hydroperoxides and headspace hexanal, in corn oil-in-water emulsions containing iron decreased with increasing surfactant concentrations (0.5-2.0%). These results indicate that surfactant micelles could alter the physical location and prooxidant activity of iron in oil-in-water emulsions.     

Ability of lipid hydroperoxides to partition into surfactant micelles and alter lipid oxidation rates in emulsions

Lipid hydroperoxides are important factors in lipid oxidation due to their ability to decompose into free radicals. In oil-in-water emulsions, the physical location of lipid hydroperoxides could impact their ability to interact with prooxidants such as iron. Interfacial tension measurements show that linoleic acid, methyl linoleate, and trilinolein hydroperoxides are more surface-active than their non-peroxidized counterparts. In oil-in-water emulsion containing surfactant (Brij 76) micelles in the continuous phase, linoleic acid, methyl linoleate, and trilinolein hydroperoxides were solubilized out of the lipid droplets into the aqueous phase. Brij 76 solubilization of the different hydroperoxides was in the order of linoleic acid > trilinolein > or = methyl linoleate. Brij 76 micelles inhibited lipid oxidation of corn oil-in-water emulsions with greater inhibition of oxidation occurring in emulsions containing linoleic acid hydroperoxides. Surfactant solubilization of lipid hydroperoxides could be responsible for the ability of surfactant micelles to inhibit lipid oxidation in oil-in-water emulsions.     

Ability of surfactant hydrophobic tail group size to alter lipid oxidation in oil-in-water emulsions

Oxidation of oil-in-water emulsion droplets is influenced by the properties of the interfacial membrane surrounding the lipid core. Previous work has shown that an important factor in the oxidation of oil-in-water emulsions is surfactant properties that impact interactions between water-soluble prooxidants and lipids in the emulsion droplet. The purpose of this research was to study the impact of surfactant hydrophobic tail group size on lipid oxidation in oil-in-water emulsions stabilized by polyoxyethylene 10 lauryl ether (Brij-lauryl) or polyoxyethylene 10 stearyl ether (Brij-stearyl). The ability of iron to decompose cumene peroxide was similar in hexadecane emulsions stabilized by Brij-stearyl and Brij-lauryl. Oxidation of methyl linoleate in hexadecane emulsions containing cumene peroxide was greater in droplets stabilized by Brij-lauryl than in those stabilized by Brij-stearyl at pH 3 with no differences observed at pH 7.0. Oxidation of salmon oil was greater in emulsions stabilized by Brij-lauryl than in those stabilized by Brij-stearyl as determined by both lipid peroxides and headspace propanal. These results suggest that surfactant hydrophobic tail group size may play a minor role in lipid oxidation in oil-in-water emulsions.     

Lipid oxidation in a menhaden oil-in-water emulsion stabilized by sodium caseinate cross-linked with transglutaminase

Transglutaminase-catalyzed cross-linking of interfacial proteins in oil-in-water has been shown to influence physical stability, but little is known about how this reaction impacts lipid oxidation. Therefore, this study evaluated the influence of transglutaminase-induced interfacial protein cross-linking on the oxidative stability of casein-stabilized menhaden oil-in-water emulsions. Interfacial casein in menhaden oil-in-water emulsions cross-linked by transglutaminase (pH 7.0) produced a cohesive interfacial protein layer that could not be removed from the emulsion droplet by Tween 20. Although transglutaminase cross-linked the interfacial casein, these emulsions did not show increased oxidative stability when compared to untreated emulsions as determined by measurement of lipid hydroperoxides and thiobarbituric acid reactive substances. These results indicate that increasing the cohesiveness of proteins at the interface of oil-in-water emulsions does not inhibit lipid oxidation. This could be due to the ability of prooxidative species such as iron to diffuse through the cross-linked protein layer where it could promote the decomposition of lipid hydroperoxides into free radicals that could oxidize unsaturated fatty acids in the emulsion droplet core.     

Revisiting the polar paradox theory: a critical overview

The polar paradox is a theory that illustrates the paradoxical behavior of antioxidants in different media and rationalizes the fact that polar antioxidants are more effective in less polar media, such as bulk oils, while nonpolar antioxidants are more effective in relatively more polar media, such as oil-in-water emulsions or liposomes. For 2 decades since it was proposed, the theory has been used to interpret results in antioxidant efficiency studies. However, more recently, new evidence from more comprehensive assessments has emerged that contradicts the polar paradox theory, hence necessitating its re-evaluation. More complex factors in addition to polarity must be taken into account to explain antioxidant efficacy.     

Ability of surfactant headgroup size to alter lipid and antioxidant oxidation in oil-in-water emulsions

Oxidation of oil-in-water emulsion droplets is influenced by the properties of the interfacial membrane surrounding the lipid core. To evaluate how surfactant headgroup size influences lipid oxidation rates, emulsions were prepared with polyoxyethylene 10 stearyl ether (Brij 76) or polyoxyethylene 100 stearyl ether (Brij 700), which are structurally identical except for their hydrophilic headgroups, with Brij 700 containing 10 times more polyoxyethylene groups than Brij 76. Fe(2+)-promoted decomposition of cumene hydroperoxide was lower in Brij 700-stabilized than in Brij 76-stabilized hexadecane emulsions. Fe(2+)-promoted alpha-tocopherol oxidation rates were similar in hexadecane emulsion regardless of surfactant type. Brij 700 decreased production of hexanal from methyl linoleate and the formation of lipid peroxides and propanal from salmon oil compared to emulsions stabilized by Brij 76. These results indicate that emulsion droplet interfacial thickness could be an important determinant in the oxidative stability of food emulsions.     

Antioxidant activity of a proanthocyanidin-rich extract from grape seed in whey protein isolate stabilized algae oil-in-water emulsions

Algae oil-in-water emulsions stabilized with 0.2% whey protein isolate (WPI) at pH 3.0 and 7.0 were chosen to evaluate antioxidant activity of a proanthocyanidin-rich extract from grape seed. In this emulsion system, (+)-catechin and ascorbic acid (620 microM) were found to be prooxidative at pH 3.0 and ineffective at pH 7.0. Grape seed extract was not able to effectively inhibit both lipid hydroperoxides and propanal formation when added to the emulsion at 124 microM. However, increasing the concentration of the grape seed extract to 620 microM resulted in inhibition of both lipid hydroperoxide and propanal formation at pH 3.0 and 7.0. None of the antioxidants tested had any effect on the physical stability of the WPI-stabilized emulsion. The superior antioxidant activity of the grape seed extract is likely due to the presence of oligomeric procyanidins which are better antioxidants compared to their monomeric counterparts.     

Antioxidant properties of Teaw (Cratoxylum formosum Dyer) extract in soybean oil and emulsions

The antioxidant activity of an extract from Teaw (Cratoxylum formosum Dyer) leaves was studied in soybean oil and soybean oil-in-water emulsions. Samples containing the extract or reference antioxidants including chlorogenic acid, which comprises 60% of the Teaw extract, were stored at 60 degrees C and analyzed periodically for peroxide value (PV) and thiobarbituric acid reactive substances (TBARS) to allow both hydroperoxides and hydroperoxide degradation products to be monitored. Chlorogenic acid and the Teaw extract were more effective than alpha-tocopherol in inhibiting lipid oxidation in bulk oil but were less effective in an oil-in-water emulsion in accordance with the polar paradox. The PV/TBARS ratio for oil samples containing chlorogenic acid was higher than for alpha-tocopherol and BHT because chlorogenic acid inhibits both hydroperoxide formation by radical scavenging and hydroperoxide decomposition by metal chelation. The importance of the metal-chelating activity in retarding hydroperoxide decomposition was confirmed by studying the decomposition of oil samples containing added ferric ions. The PV/TBARS ratio was higher for citric acid than for alpha-tocopherol in the presence of added ferric chloride, but the order was reversed in samples lacking ferric chloride. Samples containing added chlorogenic acid gave the highest PV/TBARS ratios both in the presence and absence of ferric ions. The PV/TBARS ratios for the samples containing antioxidants fell rapidly to lower values in a soybean oil-in-water emulsion than in the soybean oil. This was due to increased hydroperoxide decomposition in the emulsion at the same PV. The Teaw extract contained 12% oil-soluble components, which contributed to a slightly higher oil-water partition coefficient than that of chlorogenic acid. The antioxidant activity of the aqueous phase of the Teaw extract was reduced more than that of chlorogenic acid by partitioning of the oil-soluble components into oil, which showed that the less-polar components contributed to the antioxidant activity of the Teaw extract in aqueous media.     

Activities of antioxidants are affected by colloidal properties of oil-in-water and water-in-oil emulsions and bulk oils

The activity of alpha-tocopherol, Trolox, propyl gallate, gallic acid, methyl carnosoate, and carnosic acid was studied in two oil-in-water (o/w) emulsions, in two water-in-oil (w/o) emulsions, and in bulk oil with and without added emulsifiers. All antioxidants had either moderate or higher activity in bulk oil than in the emulsions. In most emulsions, the most polar antioxidants, propyl gallate and gallic acid, exhibited either prooxidant activity or no antioxidant activity. Methyl carnosoate was the most active antioxidant in w/o emulsions but was less active than Trolox in o/w emulsions. alpha-Tocopherol was less active in bulk oil than in emulsions, but its activity in bulk oil was markedly enhanced by the addition of o/w emulsifiers. Partitioning of antioxidants, hydrogen bonding, interphase transport, surface accessibility, and interaction of emulsifier with antioxidants are considered to be important parameters that determine antioxidant activity in lipid-containing systems.     

Increasing the oxidative stability of liquid and dried tuna oil-in-water emulsions with electrostatic layer-by-layer deposition technology

Omega-3 Fatty acids have numerous health benefits, but their addition to foods is limited by oxidative rancidity. Engineering the interfacial membrane of oil-in-water emulsion droplets to produce a cationic and/or thick interface is an effective method to control lipid oxidation. Cationic and thick emulsion droplet interfacial membranes can be produced by an electrostatic layer-by-layer deposition technique resulting in droplets that are coated by multiple layers of emulsifiers. Tuna oil-in-water emulsion droplets coated by lecithin and chitosan produce cationic emulsion droplets that are more oxidatively stable than emulsions coated by lecithin alone. Ethylenediaminetetraacetic acid (EDTA) was able to increase the oxidative stability of emulsions stabilized with lecithin and chitosan more effectively than mixed tocopherols. The combination of EDTA and mixed tocopherols was not more effective than EDTA alone suggesting that control of prooxidant metals was the most important antioxidant technology. The production of emulsion droplets coated with lecithin and chitosan could be an excellent technology for stabilization of oxidatively unstable lipids for use in a variety of food products.     

Characterization of the antioxidant activity of sugars and polyhydric alcohols in fish oil emulsions

Polyols have been incorporated into fish oil emulsions as a means for the inhibition of lipid oxidation and suppression of fishy flavor. However, the role of sugars and polyhydric alcohols as antioxidants has not been clearly established. Selected polyols were evaluated for their performance as antioxidants and modifiers of oxidation pathways in a model system. Oil/water (O/W) emulsions were prepared with freshly steam-deodorized menhaden oil. A layer of emulsion in aluminum pans held at 5 degrees C was exposed to 2550 lx fluorescent lights for 24 h before peroxide values and volatile flavor compounds were analyzed by GC headspace entrainment procedure. Antioxidant activity was confirmed for fructose, sucrose, raffinose, sorbitol, or mannitol when incorporated at 16% of the aqueous phase into model fish oil-in-water emulsions. Peroxide values were suppressed 10-18% in treated samples compared to control samples. Viscosity data did not exclude possible contributions from a restricted oxygen diffusion mechanism in the antioxidant activity, but revealed that emulsion viscosity did not govern fish oil oxidation rates. Combining polyols with phenolic antioxidants (alpha-tocopherol, BHT, or TBHQ) frequently diminished the antioxidant activity compared to that for individual phenolic antioxidants, which was interpreted as indicating that the H-donating activity of phenolic antioxidants was hindered by the H-bonding activity of polyols. A viscosity-based inhibition of the retroaldol conversion of (E,Z)-2,6-nonadienal to (Z)-4-heptenal with a high fructose concentration (67%) was attributed to a restriction of molecular mobility of reactants, but the conversion was only slightly inhibited by the concentration of fructose (16%) used in experimental emulsions. The data supported a hypothesis that either or both free radical scavenging and transition state metal chelation activities were provided by polyols in fish oil emulsions. Also, polyols retarded the water-requiring retroaldol decomposition of (E,Z)-2,6-nonadienal to (Z)-4-heptenal in the model systems and the reaction may be involved in some suppression of fishy flavors in emulsions.     

Impact of whey protein emulsifiers on the oxidative stability of salmon oil-in-water emulsions

To obtain a better understanding of how the interfacial region of emulsion droplets influences lipid oxidation, the oxidative stability of salmon oil-in-water emulsions stabilized by whey protein isolate (WPI), sweet whey (SW), beta-lactoglobulin (beta-Lg), or alpha-lactalbumin (alpha-La) was evaluated. Studies on the influence of pH on lipid oxidation in WPI-stabilized emulsions showed that formation of lipid hydroperoxides and headspace propanal was much lower at pH values below the protein's isoelectric point (pI), at which the emulsion droplets were positively charged, compared to that at pH values above the pI, at which the emulsion droplets were negatively charged. This effect was likely due to the ability of positively charged emulsion droplets to repel cationic iron. In a comparison of lipid oxidation rates of WPI-, SW-, beta-Lg-, and alpha-La-stabilized emulsions at pH 3, the oxidative stability was in the order of beta-Lg > or = SW > alpha-La > or = WPI. The result indicated that it was possible to engineer emulsions with greater oxidative stability by using proteins as emulsifier, thereby reducing or eliminating the need for exogenous food antioxidants.     

Antioxidant activity of cysteine, tryptophan, and methionine residues in continuous phase beta-lactoglobulin in oil-in-water emulsions

Proteins dispersed in the continuous phase of oil-in-water emulsions are capable of inhibiting lipid oxidation reactions. The antioxidant activity of these proteins is thought to encompass both free radical scavenging by amino acid residues and chelation of prooxidative transition metals; however, the precise mechanism by which this occurs remains unclear. In this study, the oxidative stability of cysteine, tryptophan, and methionine residues in continuous phase beta-lactoglobulin (beta-Lg) in a Brij-stabilized menhaden oil-in-water emulsion was determined. The presence of low concentrations of continuous phase beta-Lg (250 and 750 microg/mL) significantly inhibited lipid oxidation as determined by lipid hydroperoxides and thiobarbituric acid reactive substances analysis. It was observed that cysteine oxidized before tryptophan in beta-Lg, and both residues oxidized before lipid oxidation could be detected. No oxidation of the methionine residues of beta-Lg was observed despite its reported high oxidative susceptibility. It is conceivable that surface exposure of amino acid residues greatly affects their oxidation kinetics, which may explain why some residues are preferentially oxidized relative to others. Further elucidation of the mechanisms governing free radical scavenging of amino acids could lead to more effective applications of proteins as antioxidants within oil-in-water food emulsions.     

The effects of surfactant type, pH, and chelators on the oxidation of salmon oil-in-water emulsions.

Lipid oxidation in emulsions is influenced by the ability of transition metals to associate with emulsion droplets. The oxidative stability of 5% salmon oil-in-water emulsion was influenced by surfactant type, with oxidation rates being greatest in emulsions stabilized by anionic sodium dodecyl sulfate (SDS) followed by nonionic Tween 20 and cationic dodecyltrimethylammonium bromide (DTAB). EDTA inhibited lipid oxidation in all the emulsions, and apo-transferrin inhibited oxidation in the Tween 20-stabilized emulsions at pH 7.0, suggesting that continuous-phase iron was an active prooxidant. Iron associated with Tween-20 stabilized hexadecane emulsion droplets could be partitioned into the continuous phase by lowering the pH to 相似文献   

The relationship between the physicochemical properties of antioxidants and their ability to inhibit lipid oxidation in bulk oil and oil-in-water emulsions

Chain-breaking antioxidants differ in their effectiveness at inhibiting lipid oxidation because of their chemical properties and physical location within a food. Our objective was how the physicochemical properties of four structurally related lipid-soluble antioxidants were related to their antioxidant activity. Antioxidants differed in the number of methyl (alpha-tocopherol and delta-tocopherol) or hydroxyl (butylated hydroxytoluene (BHT) and 4-hydroxymethyl-2,6-ditertiarybutylphenol) groups. Surface activity of the antioxidants was in the order of delta-tocopherol > alpha-tocopherol approximately 4-hydroxymethyl-2,6-ditertiarybutylphenol > BHT. Free-radical scavenging activity was similar between alpha-tocopherol and delta-tocopherol as well as BHT and 4-hydroxymethyl-2,6-ditertiarybutylphenol. In bulk menhaden oil, BHT was a more effective antioxidant than 4-hydroxymethyl-2,6-ditertiarybutylphenol while alpha-tocopherol was more effective than delta-tocopherol. In menhaden oil-in-water emulsions, BHT was a more effective antioxidant than 4-hydroxymethyl-2,6-ditertiarybutylphenol while delta-tocopherol was more effective than alpha-tocopherol. These results indicate that the surface activity and polarity of lipid-soluble antioxidants were not the only determinants of their antioxidant effectiveness in food lipids.     

Front-face fluorescence spectroscopy study of globular proteins in emulsions: displacement of BSA by a nonionic surfactant

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.     

Antioxidant activity of hydroxytyrosol acetate compared with that of other olive oil polyphenols

Hydroxytyrosol acetate was synthesized, and the antioxidant activity of this olive oil component was assessed in comparison with that of other olive oil components, namely hydroxytyrosol, oleuropein, 3,4-DHPEA-EA, and alpha-tocopherol in bulk oil and oil-in-water emulsions. The activity of the compounds was also assessed by scavenging of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. Hydroxytyrosol acetate had a weaker DPPH radical scavenging activity than hydroxytyrosol, oleuropein, or 3,4-DHPEA-EA but it had a radical scavenging activity similar to that of alpha-tocopherol. In oil, the antioxidant activity of hydroxytyrosol acetate was much higher than that of alpha-tocopherol or oleuropein, but in an emulsion 3,4-DHPEA-EA and alpha-tocopherol were more effective as antioxidants than hydroxytyrosol acetate. The antioxidant activity of hydroxytyrosol acetate was rather similar to that of hydroxytyrosol in oil and emulsions despite the difference in DPPH radical scavenging activity.