Direct measurement of rice bran lipase activity for inactivation kinetics and storage stability prediction Christoph Brunschwiler a, b , Daniel Heine a , Stefan Kappeler a , Beatrice Conde-Petit a , Laura Nyström b, * a Bühler AG, Corporate Technology, Gupfenstrasse 5, CH-9240 Uzwil, Switzerland b Institute of Food, Nutrition and Health, ETH Zurich, Schmelzbergstrasse 9, CH-8092 Zurich, Switzerland article info Article history: Received 11 February 2013 Received in revised form 19 June 2013 Accepted 21 June 2013 Keywords: Rice bran Lipase/esterase activity measurement Inactivation kinetics Storage stability abstract Rice bran is a rich source of valuable nutrients and has potential for high-value applications. Endogenous lipases catalyze the hydrolysis of rice bran oil to free fatty acids, which initiates lipid oxidation. The evaluation of the success of rice bran stabilization processes in terms of the degree of lipid oxidation and shelf-life has so far relied on the measurement of free fatty acid content over a storage period of 3e6 months. In the present study, a photometric and a titrimetric pH-stat method for direct lipase activity measurement immediately after debranning were adapted to rice bran. The photometric method was further applied to determine rice bran lipase/esterase inactivation kinetics, which are useful to optimize stabilization treatments in order to prevent overprocessing and retain maximum level of nu- trients. Rice bran was heat-treated in a specialized, hermetically sealable reactor at controlled holding times (5e40 min), temperatures (70e145  C) and moisture contents (10e20%). Temperature dependency of the lipase/esterase inactivation rate could be described by the Arrhenius equation. Empirical findings on the importance of moisture content for effective rice bran stabilization could be quantified. Furthermore, the results demonstrate the great potential of the method to predict the shelf-life of sta- bilized rice bran without time-consuming storage tests. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Rice bran is a side stream product of rice milling with an annual global production volume of about 70 million tons (FAOSTAT, 2012). It consists of the pulverized outer layers of the rice kernel and the germ and accounts for 10% of the processed brown rice. The vast majority of nutrients are concentrated in the bran fraction: oil including essential fatty acids, proteins, fibers, vitamins, antioxi- dants (e.g. g-oryzanol), and other micronutrients. Presently, rice bran is mostly used as cost-efficient ingredient for animal feed or as raw material for oil extraction (Orthoefer, 2004). But there is an underestimated potential for high-value rice bran products for human nutrition, and several studies have been conducted to produce ingredients from rice bran that are rich in (soluble) dietary fibers or proteins. However, after the whitening step, rice bran oil (18e22% of rice bran) is exposed to the endogenous lipases, which catalyze the hydrolysis of rice bran oil. This leads to an increase in free fatty acids and to the subsequent generation of off-flavors (Orthoefer, 2004). A stabilization step to decrease lipase activity is therefore indispensable but should be as gentle as possible to retain a maximum level of nutrients. As free fatty acids produced through the activity of lipase are also better substrates for lip- oxygenase enzyme and lipid autoxidation compared to the fatty acid moieties of glycerol esters (Gardner, 1995), inhibition of the initial formation of free fatty acids is a key to controlling rice lipid oxidation. Moreover, Orthoefer (2004) reports a lower inactivation temperature for lipoxygenase than for lipase and similarly, Vetrimani et al. (1992) found that lipase was more heat-stable than lipoxygenase when heat-treating rice bran. So far, two types of lipases (EC 3.1.1.3) and two types of esterases (EC 3.1.1.1) have been purified from rice bran. Rice bran lipase I (relative molecular mass M r 40’000) was purified by Funatsu et al. (1971) and later, Aizono et al. (1976) detected rice bran lipase II (M r 33’300). In addition, lipases present in rice bran might also origi- nate from microbes that proliferate upon grain storage especially under humid storage conditions (Huang, 1993). A rice bran esterase (M r 27’000) was identified and characterized by Chuang et al. (2011) and another esterase (M r 25’000) was isolated from rice bran by Hamada et al. (2012). Although it is not unambiguously proven whether esterases also contribute to the development of * Corresponding author. Tel.: þ41 44 632 91 65; fax: þ41 44 632 11 23. E-mail addresses: laura.nystroem@hest.ethz.ch, lnystrom@iki.fi (L. Nyström). Contents lists available at SciVerse 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.2013.06.007 Journal of Cereal Science 58 (2013) 272e277