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

Abstract

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.

Keywords: antioxidant efficiency; food-grade nanoemulsions; hydroxytyrosol; lipid oxidation; partition constants.

Figure 1

Hydroxytyrosol derivatives employed in this work.

Figure 2

Synthetic routes used to prepare HT esters of different hydrophobicities.

Figure 3

(A) Changes in the absorbance of DPPH^● with time at six C3/DPPH^● mole ratios-: 0.047, 0.094, 0.141, 0.188, 0.235, 0.282, T = 25 °C. Solid lines were drawn to aid the eye. (B) Variation in the percentage of DPPH^● as a function of the AO/DPPH^● ratio at different reaction times and the corresponding linear fits (solid lines).

Figure 4

Illustrative kinetic plots showing the determination of the observed rate constant for the reaction of 16-ArN₂⁺ with the C₁ (A) and C₁₆ (B) HT derivatives in intact 1:9 fish oil-in-water emulsions. Experimental conditions: Surfactant (Tween 80) volume fraction Φ_I = 0.0284), [16-ArN₂⁺] ≈ 2.9 × 10⁻⁴ M, [AO_T] = 2.5 × 10⁻³ M, T = 25 °C.

Figure 5

(A) Distribution size (D3,2) for a 1:9 (o/w) fish oil nanoemulision at Φ_I = 0.038 (inset: distribution size for a fish oil coarse emulsion prepared by employing the same emulsifier volume fraction). (B) Variation of the mean droplet size (D3,2), (C) polydispersity index (PDI), and (D) ζ-potential with the emulsifier volume fraction (Φ_I) in 1:9 (o/w) fish oil nanoemulsions.

Figure 6

Variation of the mean droplet size (D3,2, (A)) and of the polydispersity index (B) of 1:9 (o/w) fish oil nanoemulsions stabilized with different amounts of Tween 80 with time.

Figure 7

Illustrative examples of the variations of k_obs and 1/k_obs with Φ_I for the reaction of 16-ArN₂⁺ with HT (A), C3 (B), C8 (●), C10 (▲) (C), C12 (●), and C16 (▼) (D) in intact 1:9 fish oil nanoemulsions as determined by the azo-dye derivatization method (see Section 2.2.5). Experimental conditions: [16-ArN₂⁺] = 2.8 × 10⁻⁴ M, [AO] = 2.5 × 10⁻³ M, [NED] = 0.02 M, T = 25 °C.

Figure 8

A linear relationship showing the increasing contribution of methylene groups in the HT alkyl chains to the value of the partition constant according to Equation (4).

Figure 9

Changes in percentages of HT and HT esters as a function of Φ_I in the (A) aqueous (%AO_W), (B) interfacial (%AO_I), and (C) organic regions (%AO_O). Distribution data were calculated from the partition constants in Table 3, determined at T = 25 °C.

Figure 10

Changes in the fraction of the AOs located in the different regions ((A) aqueous, (B) interfacial, and (C) oil) of the 1:9 fish oil-in-water nanoemulsions with the interfacial volume fraction Φ_I.

Figure 11

(A) Variation in the interfacial antioxidant fraction with the hydrophobicity of AO in fish oil-in-water nanoemulsions prepared with (A) three different emulsifier volume fractions (Φ_I) and at a constant o/w ratio and (B) different o/w ratios and at constant Φ_I = 0.005.

Figure 12

(A) Kinetic profiles for the variation of the conjugated dienes formed with the time in 1:9 fish oil-in-water nanoemulsions stored in the absence and in the presence of HT and its derivatives at T = 25 °C (Φ_I = 0.005, [AO_T] = 0.1 mM in the total volume of the nanoemulsion). (B) Effect of the emulsifier volume fraction Φ_I on the time needed to reach 0.5% of conjugated dienes in the absence and in the presence of the different AOs in nanoemulsions prepared with Φ_I = 0.5%, 1%, and 3%.

Figure 13

Variation of the percentage of AOs in the interfacial region (%AO_I) of the effective interfacial concentration and antioxidant efficiency, expressed in terms of the time required to reach 0.5% CD with the hydrophobicity of the AO in fish oil-in-water nanoemulsions prepared by employing two different emulsifier volume fractions (red symbols ●, ■, ▲), Φ_I = 0.005 (blue symbols, ●, ■, ▲) Φ_I = 0.01. [AO_T] = 0.1 mM.