Exploring the Use of Hydroxytyrosol and Some of Its Esters in Food-Grade Nanoemulsions: Establishing Connection between Structure and Efficiency.

The efficiency of HT and that of some of its hydrophobic derivatives and their distribution and effective concentrations were investigated in fish oil-in-water nanoemulsions. For this purpose, we carried out two sets of independent, but complementary, kinetic experiments in the same intact fish nanoemulsions. In one of them, we monitored the progress of lipid oxidation in intact nanoemulsions by monitoring the formation of conjugated dienes with time. In the second set of experiments, we determined the distributions and effective concentrations of HT and its derivatives in the same intact nanoemulsions as those employed in the oxidation experiments. Results show that the antioxidant efficiency is consistent with the "cut-off" effect-the efficiency of HT derivatives increases upon increasing their hydrophobicity up to the octyl derivative after which a further increase in the hydrophobicity decreases their efficiency. Results indicate that the effective interfacial concentration is the main factor controlling the efficiency of the antioxidants and that such efficiency strongly depends on the surfactant concentration and on the oil-to-water (o/w) ratio employed to prepare the nanoemulsions.

Interfacial kinetics in olive oil-in-water nanoemulsions: Relationships between rates of initiation of lipid peroxidation, induction times and effective interfacial antioxidant concentrations.

HYPOTHESIS: A detailed quantitative description of the effects of antioxidants in inhibiting lipid peroxidation in oil-in-water emulsions can be achieved by determining the relationships between the rates of initiation of the lipid peroxidation reaction, the length of the induction period preceding the propagation step of the radical oxidation process and the effective antioxidant interfacial concentrations. EXPERIMENTS: We successfully prepared and characterized a series of olive oil-in-water nanoemulsions and allowed them to spontaneously oxidize. Their oxidative stability was evaluated by carrying out in the presence, and absence, of antioxidants derived from gallic acid, by monitoring the formation of primary oxidation products with time, by determining the corresponding induction periods, and by determining the effective interfacial concentrations of the antioxidants in the intact emulsions. FINDINGS: Results show that both, the length of the induction periods and the antioxidant interfacial concentrations change concomitantly, increasing with the hydrophobicity of the antioxidant up to a maximum at the octyl derivative; longer aliphatic chains decrease their efficiency. The ratio between the interfacial antioxidant concentration and the induction period remains constant independently of the antioxidant, demonstrating that the effective concentrations of antioxidant at the interface control their efficiencies in emulsions.

Effects of droplet size on the interfacial concentrations of antioxidants in fish and olive oil-in-water emulsions and nanoemulsions and on their oxidative stability.

HYPOTHESIS: One fundamental and unsolved question in colloid chemistry, and also in the food industry, is whether molecular distributions, specifically the interfacial concentrations of antioxidants (AOI), are independent of the droplet sizes. Pseudophase kinetic models, widely employed to interpret chemical reactivity in colloidal systems and to determine antioxidant distributions, assume that they are independent. EXPERIMENTS: To prove, or discard, the above hypothesis, we prepared and characterized a series of olive and fish oil-in-water nanoemulsions with different droplet sizes, carried out a kinetic study to evaluate their oxidative stability, both in the presence and absence of gallic acid (GA), and determined its interfacial concentrations. FINDINGS: Results indicate that a change in the droplet size (80-1300 nm) does not alter the oxidative stability of the nanoemulsions in the absence of GA. Addition of GA increases their oxidative shelf-life and, at constant surfactant volume fraction, ΦI, the oxidative stability and the antioxidant distribution do not depend on the droplet size. Overall, results suggest that the droplet size does not affect the ratio between the rates of radical production and of inhibition by antioxidants, ratio that defines an "efficient" (or inefficient) antioxidant, providing experimental evidence supporting the operative assumption of pseudophase kinetic models.