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Abstract Esters of L-ascorbic acid with long-chain fatty acids (E-304) are employed as antioxidants in foods rich in lipids. Although their enzymatic synthesis offers some advantages compared with the current chemical processes, most of the reported methods employ the immobilized lipase from Candida antarctica as biocatalyst and free fatty acids or activated esters as acyl donors. In order to diminish the cost of the process, we have investigated the synthesis of ascorbyl oleate and ascorbyl palmitate esters with the immobilized Thermomyces lanuginosus lipase Lipozyme TL IM-which is significantly less expensive than Novozym 435-and triglycerides as source of fatty acids. Lipozyme TL IM gave rise to a lower yield of 6-O-ascorbyl oleate than Novozym 435 when using triolein (64 vs. 84%) and olive oil (27 vs. 33%) as acyl donors. Both 6-O-ascorbyl oleate and 6-O-ascorbyl palmitate displayed excellent surfactant and antioxidant properties. The Trolox Equivalent Antioxidant Capability values for the oleate and palmitate were 71 and 84%, respectively, of those obtained with L-ascorbic acid; however, both derivatives were able to stabilize soybean oil towards peroxide formation.
Keywords Vitamin C * Ascorbyl esters * L-Ascorbyl oleate * Lipase * Thermomyces lanuginosus * Enzymatic transesterification * Alkyl esters * Triglycerides * Surface tension * CMC * Antioxidant activity
Introduction
The modification of natural antioxidants to improve their chemical, oxidative and/or thermal stability, or to alter their hydrophile-lipophile balance (HLB), yields a series of "semisynthetic" antioxidants with a great impact in industry [I]. These derivatives are generally prepared under harsh conditions using strongly corrosive acids such as sulfuric acid or hydrogen fluoride [2, 3]. To overcome these shortcomings, new approaches based on the use of biological catalysts are being evaluated, which are characterized by mild reaction conditions, low energy requirements and a minimization of the isomerization and rearrangement side reactions [1, 4]. In addition, biocatalysts are biodegradable and display chemo-, regio- and/or stereospecificity resulting in decreased by-product formation thus avoiding the need for functional group protection and activation [5, 6].
L-Ascorbic acid (vitamin C) is the major water-soluble natural antioxidant. Acting as a free radical scavenger, L-ascorbic acid and its derivatives react with oxygen, thus removing it in a closed system. The combination of L-ascorbic acid and primary antioxidants like a-tocopherol renders a synergic effect that results...