Effects of Surfactant Volume Fraction on the Antioxidant Efficiency and on The Interfacial Concentrations of Octyl and Tetradecyl p-Coumarates in Corn Oil-in-Water Emulsions.

Surfactants have been used for decades in the food industry for the preparation of lipid-based emulsified food stuffs. They play two main roles in the emulsification processes: first they decrease the interfacial tension between the oil and water, facilitating droplet deformation and rupture; second, they reduce droplet coalescence by forming steric barriers. However, addition of surfactants to binary oil-water mixtures also brings up the formation of three-dimensional interfacial layers, surrounding each emulsion droplet, that significantly alter chemical reactivity. This is the case, for instance, in the inhibition reaction between antioxidants and the lipid radicals formed in the course of the spontaneous oxidation reaction of unsaturated lipids, which are commonly employed in the preparation of food-grade emulsions. The rate of the inhibition reaction depends on the effective concentrations of antioxidants, which are mostly controlled by the amount of surfactant employed in the preparation of the emulsion. In this work, we analyze the effects of the surfactant Tween 20 on the oxidative stability and on the effective concentrations of two model antioxidants derived from cinnamic acid, determining their interfacial concentrations in the intact emulsions to avoid disrupting the existing equilibria and biasing results. For this purpose, a recently developed methodology was employed, and experimental results were interpreted on the grounds of a pseudophase kinetic model.

Exploring the Use of Bryophyllum as Natural Source of Bioactive Compounds with Antioxidant Activity to Prevent Lipid Oxidation of Fish Oil-In-Water Emulsions.

The current industrial requirements for food naturalness are forcing the development of new strategies to achieve the production of healthier foods by replacing the use of synthetic additives with bioactive compounds from natural sources. Here, we investigate the use of plant tissue culture as a biotechnological solution to produce plant-derived bioactive compounds with antioxidant activity and their application to protect fish oil-in-water emulsions against lipid peroxidation. The total phenolic content of Bryophyllum plant extracts ranges from 3.4 to 5.9 mM, expressed as gallic acid equivalents (GAE). The addition of Bryophyllum extracts to 4:6 fish oil-in-water emulsions results in a sharp (eight-fold) increase in the antioxidant efficiency due to the incorporation of polyphenols to the interfacial region. In the emulsions, the antioxidant efficiency of extracts increased linearly with concentration and levelled off at 500 µM GAE, reaching a plateau region. The antioxidant efficiency increases modestly (12%) upon increasing the pH from 3.0 to 5.0, while an increase in temperature from 10 to 30 °C causes a six-fold decrease in the antioxidant efficiency. Overall, results show that Bryophyllum plant-derived extracts are promising sources of bioactive compounds with antioxidant activity that can be eventually be used to control lipid oxidation in food emulsions containing (poly)unsaturated fatty acids.

Adsorption of gallic acid, propyl gallate and polyphenols from Bryophyllum extracts on activated carbon.

The adsorption of gallic acid (GA) and propyl gallate (PG) on activated carbon (AC) was studied as a function of the AC mass and temperature. Clean first order behavior was obtained for at least three half-lives and the equilibrium was reached after ∼4 h contact time. An increase in the temperature (T = 20-40 °C) increases their adsorption rate constant values (k1) by 2.5 fold but has a negligible effect on the amount of antioxidant adsorbed per mass of AC at equilibrium. We also analyzed the adsorption process of polyphenols from Bryophyllum extracts and ca 100% of the total amount of the polyphenols in the extract were adsorbed when using 7 mg of AC. Results can be explained on the basis of the Freundlich isotherm but do not fit the Langmuir model. Results suggest that the combination of emerging in vitro plant culture technologies with adsorption on activated carbon can be successfully employed to remove important amounts of bioactive compounds from plant extracts by employing effective, sustainable and environmental friendly procedures.

Targeting Antioxidants to Interfaces: Control of the Oxidative Stability of Lipid-Based Emulsions.

The oxidation of lipid-based emulsions and nanoemulsions strongly affects their overall quality and safety. Moreover, introduction of oxidatively unstable emulsions into biological systems either as an energy source in parenteral nutrition or as delivery systems of bioactives may promote oxidation "in situ" leading to the overproduction of reactive oxygen species, initiating new harmful oxidative reactions and increasing the oxidative damage. Addition of antioxidants, AOs, may help to prevent the oxidative degradation of unsaturated lipids. Nevertheless, prediction of the optimal antioxidant or set of antioxidants and their efficiency is still far from being completely understood because the site of reaction is often uncertain and because the effective concentrations of reactants in the different regions of the emulsion have been frequently overlooked. Furthermore, the absence of quantitative relationships between the hydrophobicity of the antioxidants and their partitioning among the oil, water, and interfacial regions hampers their optimal use. Here we investigated the effects of gallic acid and some of its alkyl derivatives on the oxidative stability of soybean oil-in-water emulsions and determined their effective concentrations in the different regions of the emulsion (aqueous, oil, and interface). The results provide physical evidence for the crucial role played by the interfacial region in the reaction between antioxidants and lipid radicals: a direct relationship between interfacial concentrations and the oxidative stability could be established. The results indicate that AOs accumulate in the interfacial region, where the effective concentration is 20-180 times higher than the stoichiometric concentrations. Control of the hydrophobicity of the AOs and of the surfactant concentration allows control of interfacial concentrations: the lower the concentration of surfactant employed, the higher the effective interfacial concentration.

Physical evidence that the variations in the efficiency of homologous series of antioxidants in emulsions are a result of differences in their distribution.

BACKGROUND: The relationships between the hydrophilic-lipophilic balance (HLB) of antioxidants (AOs) and their distributions and efficiencies in emulsions are not fully understood. Recent reports indicate that, for series of homologous antioxidants of different hydrophobicity, the variation of their efficiency with the HLB of the AO increases with the alkyl chain length up to a maximum (C3 -C8 ester) followed by a decrease (cut-off effect). RESULTS: We determined the distributions of a series of caffeic acid derivatives in intact soybean emulsions by employing a specifically designed chemical probe located in the interfacial region of the emulsion. We also determined the AO efficiencies in the very same emulsions. We demonstrate that the variation of the percentage of AO in the interfacial region of soybean oil-in-water emulsions with the AO HLB parallels that of their antioxidant efficiency. CONCLUSION: The results provide physical evidence that the variations in the efficiency of homologous series of antioxidants in emulsions are the result of differences in their distribution. The results confirm that, with other things being equal, there is a direct relationship between the percentage of AO in the interfacial region of the emulsions and their efficiency, providing a natural explanation, based on molecular properties, of the cut-off effect. © 2016 Society of Chemical Industry.

A direct correlation between the antioxidant efficiencies of caffeic acid and its alkyl esters and their concentrations in the interfacial region of olive oil emulsions. The pseudophase model interpretation of the "cut-off" effect.

Recently published results for a series of homologous antioxidants, AOs, of increasing alkyl chain length show a maximum in AO efficiency followed by a significant decrease for the more hydrophobic AOs, typically called the "cut-off" effect. Here we demonstrate that in olive oil emulsions both antioxidant efficiencies and partition constants for distributions of AOs between the oil and interfacial regions, PO(I), show a maximum at the C8 ester. A reaction between caffeic acid, CA, and its specially synthesised C1-C16 alkyl esters, and a chemical probe is used to estimate partition constants for AO distributions and interfacial rate constants, kI, in intact emulsions based on the pseudophase kinetic model. The model provides a natural interpretation for both the maximum and the "cut-off" effect. More than 70% of the CA esters are in the interfacial region even at low surfactant volume fraction, ΦI=0.005.

Transfer of antioxidants at the interfaces of model food emulsions: distributions and thermodynamic parameters.

Knowledge on the driving force for the hydrophobic effect that partitions antioxidants (AOs) between the oil (O), aqueous (W) and interfacial (I) regions of food emulsions is crucial to predict their efficiency in inhibiting lipid oxidation and to preserve the organoleptic properties of lipid-based foods. Here, we have investigated the effects of temperature and surfactant volume fraction (ΦI) on the distribution of two representative AOs, the water insoluble α-tocopherol (TOC) and the oil insoluble caffeic acid (CA), in a model food emulsion composed of stripped corn oil, acidic water and the nonionic surfactant Tween 20. The distribution of the AOs is assessed in the intact emulsions by employing a well-established kinetic method based on the reaction between a hydrophobic arenediazonium ion and the AOs. The variations of the observed rate constant, kobs, with ΦI are interpreted on the grounds of the pseudophase kinetic model, which provides values for the interfacial rate constant kI and the partition constants between the aqueous-interfacial (P) and oil-interfacial (P) regions of the emulsions. From the variations of P, P and kI at a series of temperatures, we determined the Gibbs free energy, enthalpy and entropy values for the transfer of CA from the water to the interfacial (W → I) region and of TOC from the oil to the interfacial (O → I) regions of the emulsions, and the activation parameters for the reaction in the interfacial region. Activation energy values are in line with those expected for a bimolecular reaction. Results show that the W → I and O → I transfer processes are spontaneous and entropy driven.

Maxima in antioxidant distributions and efficiencies with increasing hydrophobicity of gallic acid and its alkyl esters. The pseudophase model interpretation of the "cutoff effect".

Antioxidant (AO) efficiencies are reported to go through maxima with increasing chain length (hydrophobicity) in emulsions. The so-called "cutoff" after the maxima, indicating a decrease in efficiency, remains unexplained. This paper shows, for gallic acid (GA) and propyl, octyl, and lauryl gallates (PG, OG, and LG, respectively), that at any given volume fraction of emulsifier, the concentrations of antioxidants in the interfacial region of stripped corn oil emulsions and their efficiency order follow PG > GA > OG > LG. These results provide clear evidence that an AO's efficiency correlates with its fraction in the interfacial region. AO distributions were obtained in intact emulsions by using the pseudophase kinetic model to interpret changes in observed rate constants of the AOs with a chemical probe, and their efficiencies were measured by employing the Schaal oven test. The model provides a natural explanation for the maxima with increasing AO hydrophobicity.

Effects of emulsifier hydrophile-lipophile balance and emulsifier concentration on the distributions of gallic acid, propyl gallate, and α-tocopherol in corn oil emulsions.

We evaluated the effects of the hydrophile-lipophile balance (HLB) and emulsifier concentration on the distribution of the antioxidants gallic acid (GA), propyl gallate (PG), and α-tocopherol (TOC) between the aqueous, interfacial, and oil regions of food-grade emulsions composed of stripped corn oil, acidic water, and a mixture of the non-ionic surfactants Tween 20, 40, 80, and Span 20. The distribution of the antioxidants (AOs) is described by two partition constants, that between the oil-interfacial region, P(O)(I), and that between the aqueous and interfacial region, P(W)(I), of the emulsions. The partition constants were determined from the kinetic analyses of the variation in the observed rate constant, k(obs), for the reaction between the AOs and the hydrophobic 4-hexadecylbenzenediazonium ions, 16-ArN(2)(+), with the emulsifier volume fraction. The effects of emulsifier HLB on the second-order rate constants in the interfacial region k(I) were also evaluated for each antioxidant. Results show that an increase in emulsifier concentration promotes the incorporation of AOs to the interfacial region of the emulsions, so that at surfactant volume fractions of 0.04, more than 90% of GA and PG and more than 50% of TOC are located in that region. A decrease in the HLB favors the incorporation of PG and TOC to the interfacial region of the emulsions but has a negligible effect on the fraction of GA in that region. The %AOs in the interfacial region of the emulsions does not correlate with the polarity of the antioxidant, so that GA and PG are predominantly located in the aqueous-interfacial regions of the emulsion rather that in the oil droplet interior; meanwhile, TOC is mostly located in the oil-interfacial regions. Results should aid to understand how antioxidants are distributed in food-grade emulsions and their relative efficiency in inhibiting lipid oxidation.

Distribution of hydroxytyrosol and hydroxytyrosol acetate in olive oil emulsions and their antioxidant efficiency.

We employed a kinetic method to determine the distributions of the antioxidants hydroxytyrosol (HT) and hydroxytyrosol acetate (HTA) between the oil, aqueous, and interfacial regions of a model food emulsion composed of stripped olive oil, acidic water, and a blend of Tween 80 and Span 80 [hydrophilic­lipophilic balance (HLB) = 8.05] as an emulsifier. HT is oil-insoluble, but HTA is both oil- and water-soluble (partition constant P(O)(W) = 0.61). Results indicate that, at a given emulsifier volume fraction Φ(I), the fraction of HTA in the interfacial region is higher than that of HT. The percentage of both antioxidants increases with an increasing Φ(I), so that % HT > 40% at Φ(I) = 0.005 and % HT > 80% at Φ(I) = 0.04. HTA appears to be a better antioxidant than HT, as shown by an accelerated oxidative test (Schaal oven method). A correlation between their distribution in the emulsion and their efficiency was established.

Temperature and emulsifier concentration effects on gallic acid distribution in a model food emulsion.

We determined the effects of emulsifier concentration and temperature on the distribution of gallic acid (GA) in a food-grade emulsion composed of 1:9 vol:vol stripped corn oil, acidic water and Tween 20. The distribution of GA can be defined by the partition constant between the aqueous and the interfacial regions, P(W)(I), which was determined by using a kinetic method and the pseudophase kinetic model. Once P(W)(I) is known, determining the distribution of GA is straightforward. Our results show that at least 40% of the total GA is located in the interfacial region of the emulsion at 0.005 volume fraction of Tween 20, and this percentage increases to ca. 85% of the total GA at 0.04 volume fraction of Tween 20. The variation of P(W)(I) with the temperature was used to estimate the thermodynamic parameters for the GA transfer from the aqueous to the interfacial region of the emulsion and the activation parameters for the reaction between 16-ArN(2)(+) and GA in the interfacial region. The free energy of transfer from the aqueous to the interfacial region, ΔG(T)(0,W→I), is negative, the enthalpy of transfer is small and negative, but the entropy of transfer is large and positive. Our results demonstrate that the partitioning of GA in acidic emulsions between aqueous and interfacial regions depends primarily on droplet concentration and is only slightly dependent on temperature.