Partitioning and antioxidative effect of protocatechuates in soybean oil emulsions: relevance of emulsifier concentration

Distributions of a homologous series of protocatechuic acid (PCA) derivatives of increasing lipophilicity were determined in stripped soybean oil-in-water emulsions and compared with their antioxidant (AO) efficiencies. The distribution was determined by employing a recently developed a kinetic method, based on the reaction between the hydrophobic 4-hexadecylbenzenediazonium ions (16-ArN₂⁺) and the antioxidants, that allows to determine the partition constants between the oil-interfacial, P_O^I, and water-interfacial, P_W^I, regions of the emulsion and their interfacial molarities. Our results show that: 1) At low surfactant volume fractions, Φ_I = 0.005, a significant fraction (>50%) of all protocatechuates is located in the interfacial region of soybean oil emulsions. The percentage increases with increasing Φ_I such that at Φ_I = 0.04, than 90% of the AOs are located in the interfacial region; 2) The variation of the hydrophobicity of the AOs has a modest effect on their concentration in the interfacial region, particularly at Φ_I > 0.01, and an almost negligible effect on their antioxidant efficiency in the same emulsions; 3) A positive correlation between the antioxidant efficiency and the interfacial molarities of the protecatechuates was established.

Practical applications: Results will contribute to a deeper understanding of oxidative stability in polyunsaturated acids enriched emulsions, will enhance current understanding of how AO structure and physical location within the food system affect their efficiency and will provide basic information on the factors controlling AO distributions and efficiencies, allowing a more rational selection of AOs in food stabilization. The outcome of this research will help the development of more efficient antioxidant strategies for food preservation and for the oxidative protection of nutritionally important bioactive lipids.