Lipase-catalyzed in situ reactive extraction of oilseeds with short-chained alkyl acetates for fatty acid esters production

Abstract

Substituting short-chained alkyl acetates for short-chained alcohols as acyl acceptors for fatty acid esters production, the negative effects of glycerol and alcohol on lipase can be eliminated. Short-chained alkyl acetates, like other short-chained esters, are also suitable solvents for seed oil extraction. Thus, methyl acetate and ethyl acetate were adopted as extraction solvents and transesterification reagents at the same time for in situ reactive extraction of Pistacia chinensis Bunge seed and Jatropha curcas L seed in this work. Fatty acid methyl esters and ethyl esters were, respectively obtained with higher yields than those achieved by conventional two-step extraction/transesterification. The improvement ranged from 5.3% to 22%. The key parameters such as solvent/seed ratio and water content were further investigated to find their effects on the in situ reactive extraction. The highest P. chinensis Bunge and J. curcas L methyl/ethyl esters could attain 92.8%, 89.5%, 86.1% and 87.2%, respectively under the optimized conditions.

Introduction

Reserves shortage and price increase are causing a growing substitution of fossil fuels with fuels derived from vegetable origin such as fatty acid esters (biodiesel). Different processes are currently available to achieve transesterification of oils for the production of fatty acid esters, which include chemical or enzyme catalysis or supercritical alcohol treatment [1], [2], [3], [4]. Although fatty acid esters can be successfully produced by chemical approach, there are several associated problems, such as recovery of catalyst and glycerol, as well as high energy requirements [5], [6]. Use of biocatalysts (lipases) in transesterification of oils for fatty acid esters production overcomes these problems and offers an environmentally more attractive option to the conventional processes [7], [8], [9]. However, there are two bottlenecks in enzymatic approach for fatty acid esters production. One is the high cost of lipase and its short operational life caused by the negative effects of excessive short-chained alcohol and by-product glycerol [10], [11], [12], [13], [14]. It has been demonstrated that more than 0.5 M equivalent methanol are insoluble in vegetable oils and the immobilized lipases are easily inactivated by contacting with insoluble methanol existing as drops in the oils. By-product glycerol is hydrophilic and insoluble in the oil, so it is easily adsorbed onto the surface of the immobilized lipase also leading to negative effect on lipase activity and operational stability [9], [15], [16]. The other bottleneck for lipase-catalyzed fatty acid esters production is the high cost of the feedstock. Perhaps the largest impediment to wider adoption of fatty acid esters is its cost. When produced from refined oils, feedstock cost contributes more than 70% to the cost of the product.

Recently the use of methyl acetate and ethyl acetate as acyl acceptor for the transesterification of vegetable oils has been reported [17], [18], [19]. This transesterification method eliminates the risk of deactivation of enzyme by short-chained alcohol and glycerol, as short-chained alcohol is substituted with short-chained alkyl acetate and no glycerol is produced in the reaction (Scheme 1). Furthermore, short-chained alkyl acetates such as methyl acetate and ethyl acetate have also been shown to be suitable extraction solvents for oilseeds extraction [20]. Consideration of the current route from oilseeds to fatty acid esters caused us to inquire whether isolation of the oil from the seed, and its refining, were necessary. In contrast, transesterification reagents such as methyl acetate and ethyl acetate might be able to access acylglycerides resident in oilseeds and achieve their transesterification directly, in situ. Such a route to fatty acid esters could eliminate the expense associated with solvent extraction and oil cleanup, and simplify the steps in fatty acid esters production. This could result in a decrease in the cost of the product. Therefore, Haas et al. had tried “in situ transesterification” using a chemical method [21], [22]. However, we investigated the use of methyl acetate and ethyl acetate as extraction solvents and the in situ reactive extraction of oilseeds yielding directly fatty acid esters using a biological method in this work.