Short communication Lipase-catalysed ester synthesis in solvent-free oil system: Is it esterification or transesterification? Jingcan Sun a , Bin Yu c , Philip Curran c , Shao-Quan Liu a,b,⇑ a Food Science and Technology Programme, Department of Chemistry, National University of Singapore, Science Drive 3, Singapore 117543, Singapore b NUSRI, Suzhou, China c Firmenich Asia Pte. Ltd., Tuas, Singapore article info Article history: Received 1 March 2013 Received in revised form 29 April 2013 Accepted 22 May 2013 Available online 1 June 2013 Keywords: Transesterification Esterification Lipase Mechanism Solvent-free Coconut oil abstract Ester synthesis was carried out in a solvent-free system of lipase, coconut oil and ethanol or fusel alcohols to ascertain the reaction mechanism. During ester formation, octanoic and decanoic acids increased ini- tially and then decreased gradually, indicating that ester production was a two-step reaction consisting of hydrolysis and esterification, rather than alcoholysis. With ethanol as the alcohol substrate, added butyric acid inhibited ester synthesis. However, when fusel alcohols were used as the alcohol substrate, no sig- nificant inhibitory effect by butyric acid was observed. Added octanoic acid did not show any adverse effect on the synthesis of corresponding esters. The results suggest that polarity of the reactants deter- mines lipase activity. This study provides the first evidence on the mechanism of immobilised lipase-cat- alysed ester synthesis in a solvent-free system involving both hydrolysis and esterification. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Lipases (triacylglycerol hydrolase, E.C. 3.1.1.3) are versatile en- zymes that are able to hydrolyse triacylglycerols, and synthesise esters via esterification and transesterification (Reetz, 2002). As popular industrial biocatalysts, lipases have been frequently ap- plied to the production of fatty acids, modified fats and lipids, fatty acid esters (Fernandez-Lafuente, 2010; Rodrigues & Fernandez-Laf- uente, 2010; Wu, Jääskeläinen, & Linko, 1996). Among these prod- ucts, fatty acid esters are one group of important flavour compounds that can be synthesised by lipases through transesteri- fication of simple esters or triacylglycerols with alcohols (Lubary, ter Horst, Hofland, & Jansens, 2008; Mahapatra, Kumari, Kumar Garlapati, Banerjee, & Nag, 2009). In particular, immobilised li- pase-catalysed transesterification of triacylglycerols with alcohols becomes a popular method for the synthesis of flavour fatty acid esters since it allows the effective utilisation of natural and low- cost vegetable oils (Belafi-Bako, Kovacs, Gubicza, & Hancsok, 2002; Köse, Tüter, & Aksoy, 2002). Ester synthesising reactions catalysed by immobilised lipases can be carried out in solvent-free systems which are more advan- tageous than organic solvent systems, considering safety issues in scale-up production (Szczesna Antczak, Kubiak, Antczak, & Bie- lecki, 2009). Our previous studies showed that coconut oil and fu- sel alcohols could be effectively converted into octanoic acid esters in a solvent-free system (Sun et al., 2012; Sun, Yu, Curran, & Liu, 2012). Ester synthesis from lipids in solvent-free systems is generally assumed to occur through transesterification. There are two differ- ent viewpoints on the mechanism of transesterification in the liter- ature. The first one is that transesterification is direct alcoholysis of triacylglycerols (one-step reaction). The second one is that transe- sterification is a two-step process consisting of hydrolysis of tria- cylglycerols and esterification of liberated fatty acids (Cheirsilp, H-Kittikun, & Limkatanyu, 2008). Based on these two assumptions, kinetic modelling of the lipase-mediated transesterification of veg- etable oil with alcohols has been done (Al-Zuhair, 2005; Al-Zuhair, Ling, & Jun, 2007; Cheirsilp et al., 2008). However, data that can convincingly demonstrate whether transesterification is one-step or two-step reaction are still rather limited. Besides the reaction mechanism, substrate polarity also plays important role in ester synthesis. In a solvent-free system, the lipase activity could be ad- versely affected by polar substrates such as short-chain alcohols and fatty acids (Bezbradica, Mijin, Šiler-Marinkovic ´, & Knez ˇevic ´, 2007; Hernandez-Martin & Otero, 2008). To determine the optimal reaction conditions and obtain high yields of esters, it is important to understand the mechanism of lipase-catalysed reactions and the effects of substrate polarity on lipase activity. 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.05.109 ⇑ Corresponding author at: Food Science and Technology Programme, Depart- ment of Chemistry, National University of Singapore, Science Drive 3, Singapore 117543, Singapore. Tel.: +65 6516 2687; fax: +65 6775 7895. E-mail address: chmlsq@nus.edu.sg (S.-Q. Liu). Food Chemistry 141 (2013) 2828–2832 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem