A Direct and Practical Approach to Assessing the Impact of Emulsion Composition on Vitamin A Stability

Vitamin A is an essential micronutrient involved in vision, immunity, and growth. Despite its widespread use in food, cosmetic, and pharmaceutical products, vitamin A is highly prone to oxidation due to its conjugated double bonds, leading to reduced biological activity and efficacy. While various formulation strategies have been explored to enhance its stability, there is a notable lack of stability data and understanding of vitamin A oxidation, particularly in dispersed systems. This study aimed to evaluate the oxidative stability of vitamin A in model emulsions and identify how emulsion composition affects its degradation. Studying the influence of emulsion composition provides a better understanding of the possible oxidation pathways, including a nonradical pathway. An innovative method combining gentle emulsification via solvent displacement with real-time degradation monitoring was used. Retinyl palmitate (RP) demonstrated the highest stability compared to retinol (RO) and retinyl acetate (RA), due to structural and electronic factors. Among emulsifiers, the cationic type slightly improved stability by repelling positively charged pro-oxidant molecules. Three phenolic antioxidants, α-tocopherol (TOH), butylated hydroxytoluene (BHT), and carnosic acid (CA), improved stability, with TOH being the most effective. However, early-stage degradation could not be completely prevented, suggesting the existence of a predominant nonradical degradation pathway. The impact of iron (Fe²⁺) was minimal and attributed to the low hydroperoxide production, reinforcing the hypothesis of a nonradical initiation. Additionally, electrostatic repulsion in positively charged emulsions further limited iron's pro-oxidant effect. These findings enhance our understanding of vitamin A oxidation mechanisms and highlight potential stabilization strategies for its formulation in emulsified systems.