Lipase-catalyzed Esterification between Erythorbic Acid and Lauric Acid for the Production of Erythorbyl Laurate

Abstract

Erythorbic acid, a stereoisomer of L-ascorbic acid, has been extensively been used as an antioxidant, but cannot be applied to lipid-base foods due to poor lipophilicity. For this reason, synthesis of erythorbyl laurate (6-O-lauroyl-erythorbate) was performed in acetonitrile using an immobilized lipase from Candidia antartica as a biocatalyst to increase its lipophilicity. Response surface methodology was used to optimize the erythorbyl laurate synthesis conditions in terms of enzyme content (1,000-5,000 propyl laurate unit, PLU), molar ratio of laurate to erythorbate (5-25), and reaction temperature (25-65°C). The central composite experimental results showed the conditions for maximum molar conversion yield were as follows: enzyme content, 2,994 PLU; 24.33 laurate to erythorbate molar ratio, and reaction temperature, 53.03°C. The maximum molar conversion yield reached 77.81%, which was in agreement with the predicted value (76.92%). The erythorbyl laurate was purified and identified by MALDI-TOF and FT-IR. Erythorbyl laurate was continuously synthesized by esterification in a pack-bed enzyme reactor with immobilized-lipase from Candida antarctica. Response surface methodology (RSM) based on 5-level 3-factor central composite design was adopted to optimize conditions for the enzymatic esterification. The reaction variables, such as reaction temperature (10-70oC), substrate molar ratio ([lauric acid]/[erythorbic acid], 5-15), and residence time (8-40 min) were evaluated and optimum conditions were found to be 56.2oC, 14.3, and 24.2 min, respectively. Under the optimum conditions, the degree of esterification was predicted as the value of 86.3%. In addition, removal of water by adsorption on ion exchange resins in potassium form has been used to improve esterification, and erythorbyl laurate typed powder was stable at a temperature not exceeding 50 oC for 30 days. This research could help to develop an economical method for production of erythorbyl laurate for use as a novel food-grade emulsifier with antioxidant activity.