Effect of temperature on lutein esterification and lutein stability in wheat grain Fauziah Tufail Ahmad a, b , Robert E. Asenstorfer a , Imelda R. Soriano a , Daryl J. Mares a, * a The University of Adelaide, School of Agriculture, Food and Wine, PMB1, Glen Osmond, South Australia 5064, Australia b Universiti Malaysia Terengganu, Department of Agrotechnology, Faculty of Agrotechnology and Food Science, Kuala Terengganu 21030, Malaysia article info Article history: Received 25 June 2013 Received in revised form 9 August 2013 Accepted 16 August 2013 Keywords: Lutein Lutein ester Stability Bread wheat abstract The creamy colour of many wheat-based end-products is conferred by endogenous lutein. During post- harvest storage of grain, free lutein may be converted in part to potentially more stable lutein mono- and di-fatty acid esters. This study investigates the synthesis of lutein esters and stability of free lutein and lutein esters over a wide range of temperatures in grain of a high lutein bread wheat, Triticum aestivum L. line DM5685*B12 and a durum, Triticum durum L cv Kamilaroi. Disappearance of free lutein and lutein esterification followed first order reaction rates. The maximum rate of lutein esterification was at z80  C, however, the optimum temperature for maximum synthesis with minimum degradation was between 30  C and 60  C. No ester synthesis was observed at temperatures higher than 120  C. The data are consistent with an enzyme participating in the esterification reaction. Lutein esters were found to be more stable than free lutein with a longer shelf life at 60  C whilst at temperatures 40  C, lutein degradation was minimal. This study provides new information on lutein ester formation and lutein stability that should be useful to grain handlers and food manufacturers seeking to optimise retention of lutein for the benefit of consumers. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Colour is an important quality parameter for most wheat flour- based products. Lutein, an asymmetric dihydroxycarotenoid (xanthophyll), is the major component of the creamy colour of bread wheat flour and many end-products and contributes in part to the bright yellow colour of yellow alkaline noodles (YAN) (Mares and Campbell, 2001). The role of lutein in determining the creamy to yellow colour of end products (Oliver et al., 1993; Humphries and Khachik, 2003; He et al., 2009) and possibly delivering health benefits to the consumer (Bone et al., 1997; Olmedilla et al., 2001; Abdel-Aal et al., 2010) is well documented. However, lutein is subject to direct degradation by heat and light and indirect degradation by free radicals induced by lipid oxidation (Hidalgo and Brandolini, 2008; Wrigley et al., 2009). Esterification of lutein with fatty acids in the wheat grain during storage to produce lutein esters has been reported to increase its stability against heat and UV light (Subagio et al., 1999; Subagio and Morita, 2003; Li et al., 2007). In addition, esterification with fatty acids contributes to a higher fat solubility of lutein and subsequently enhances the effi- ciency of lutein absorption (Bowen et al., 2002; Atienza et al., 2007). The main fatty acids that are substituted at the hydroxyl groups of lutein are palmitic and linoleic acid (Mellado-Ortega and Hornero-Méndez, 2012; Kaneko et al., 1995; MacMurray and Morrison, 1970)(Fig. 1). As lutein has two hydroxyl groups, the esters formed can be either mono- or di-esters. Homogeneous di- esters are produced if similar fatty acids are attached and are het- erogeneous if different fatty acids are involved (Mellado-Ortega and Hornero-Méndez, 2012). Three major di-esters have been re- ported; dilinoleoyllutein, linoleoylpalmitoyllutein and dipalmi- toyllutein (Soriano et al., 2007; Kaneko et al., 1995). In addition, acylation at either end of the lutein molecule leads to the formation of regioisomers (Mellado-Ortega and Hornero-Méndez, 2012). Esterification of lutein is promoted by low relative humidity (Kaneko et al., 1995) which suggests that the mechanism may involve a simple dehydration reaction. The aims of this study were to examine the formation of lutein esters under a range of temperatures during storage; to determine if there is an optimum temperature for esterification; to compare formation of mono-esters and di-esters for evidence that their synthesis is under similar control; to determine whether there is a Abbreviations: YAN, yellow alkaline noodle; DM5685*B12-HFL, bread wheat line DM5685*B12 with a relatively high content of free lutein; DM5685*B12-HLE, bread wheat line DM5685*B12 with a relatively high content of lutein esters; BHT, butylated hydroxytoluene; MTBE, methyl t-butyl ether. * Corresponding author. Tel.: þ61 8 83137291; fax: þ61 8 83137480. E-mail address: daryl.mares@adelaide.edu.au (D.J. Mares). Contents lists available at ScienceDirect Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs 0733-5210/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jcs.2013.08.004 Journal of Cereal Science 58 (2013) 408e413