Antioxidative properties and enzymatic synthesis of ascorbyl FA esters

Abstract

Efficient synthesis of unsaturated FA esters of ascorbic acid is possible with only a small excess of one of the reactants in t-amyl alcohol using Candida antarctica lipase as biocatalyst. Using free acids, we obtained yields that were comparable to yields reached using vinyl-activated acyl donors (71, 80, and 86% yields of esters with FA excesses of 1:1, 1:1.5, and 1:2, respectively). As very low water activity is needed to achieve sufficiently high yields of product, molecular sieves were used to improve the ascorbyl ester yields. Ascorbyl oleate is more amorphous and has a much lower m.p. and lower enthalpy of fusion than ascorbyl palmitate. This leads to a higher solubility of ascorbyl oleate in oil, resulting in an increased antioxidant effect compared to that of the palmitate. In an accelerated storage test using deodorized rapeseed oil, samples incubated with ascorbyl palmitate showed noticeable oxidation after 1 wk of storage, whereas samples incubated with ascorbyl oleate displayed negligible oxidation for 9 and 4 wk at 30 and 40°C, respectively.Paper no. J10512 in JAOCS 80, 795-799 (August 2003).Ascorbyl palmitate is a fat-soluble antioxidant commonly used in foods to prevent oxidation of sensitive fats and oils. Ascorbyl palmitate has a disadvantage, however, in that it has relatively low solubility in oils. The solubility can be improved by the addition of lecithin, although this causes the product to become hygroscopic. Another way to improve the solubility of an ascorbyl FA ester is to modify its structure to make it less crystalline. This can be done, for example, by introducing a double bond in the FA. Oleic acid is unsaturated, readily available, inexpensive, and approved for use in foods. This makes it an attractive candidate to replace the palmitic acid in an ascorbyl ester. The chemical synthesis of saturated ascorbyl FA esters is straightforward (1). A similar procedure for the synthesis of ascorbyl oleate has been described in the literature (2) but has proven hard to repeat. The difficulties are due to the double bond of oleic acid, which is too sensitive for the reaction conditions needed for the chemically catalyzed esterification. Enzymatic catalysis, however, can be performed at lower reaction temperatures and under milder conditions, enabling the enzymatic synthesis of ascorbyl oleate (Scheme 1). Because the reactants differ in polarity, the number of suitable solvents available is limited. We used t-amyl alcohol because it dissolves the reactants at high concentrations and does not deactivate the enzyme.In 1971, Arakawa et al.(3) reported on the first enzymatic synthesis of ascorbyl laurate in aqueous solution. In 1990, Enomoto et al. (4) filed a patent on the esterification in organic solvents, and esterfications in t-amyl alcohol were reported by Humeau et al. (5) in 1995. Since then, not only saturated FA have been used, but also various unsaturated FA (6-8) as well as of phenyl butyric acid (9) and methyl lactate (10). These syntheses have typically reached yields below 50% unles...

Lipase-Catalyzed Synthesis of Ascorbyl Oleate in Acetone: Optimization of Reaction Conditions and Lipase Reusability

“…the highest initial rates were accomplished at 60 , and maximum ascorbyl oleate concentration was achieved at 55 16 . In a two separated investigations of ascorbyl palmitate synthesis using same lipase preparation as in our study high yields were reported at 55 10 and 60 13 . Interestingly, in optimization study using same lipase preparation in synthesis of ascorbyl laurate temperature optimum as 3.2 The effect of molecular sieves addition on specific product yield Since water is product of esterification, the optimization of initial water concentration is not sufficient to provide high reaction yields.…”

“…Generally, lipases capable of ascorbyl ester synthesis are very rare, hence enzyme preparation used in our study Novozyme ® 435 was used in majority of reported studies 2,10,12,13,24 so comparison with results is simple. Applied enzyme concentrations were usually high, up to 2 w/v .…”

“…When it comes to vitamin C concentration, it can be concluded that in researches with aim of molar conversion maximization, lower concentrations were applied usually up to 0.1 M 13,19,24 , while in studies in which output was product yield, higher amounts of L-ascorbic acid were used above 0.1 M 2, 10, 16 . Only a few authors optimized limiting substrate concentration in terms of ester yield 25,32,33 .…”

“…Nowadays enzymatic synthesis of ascorbyl oleate and other ascorbyl esters of unsaturated fatty acids are very attractive, especially since it was reported that ascorbyl oleate has stronger antioxidative activity in comparison with ascorbyl palmitate and vitamin C 13 . In the present, majority of investigations devoted to ascorbyl oleate synthesis, describe reaction in tbutanol or t-amyl alcohol as organic solvents 13,14,16 . As alternative solvents, ionic liquids IL were used, due to the good solubility of reactants and products 17,18 .…”

Successive cycles of utilization of novozym 435 in three different reaction systems

“…Due to the steadily growing demand for natural materials, the synthesis of esters of ascorbic acid by lipasecatalyzed reactions has become of current commercial interest to be used as an antioxidant in the food, cosmetic and pharmaceutical industries (Viklund et al, 2003).…”