Variability in Arabinoxylan, Xylanase Activity, and Xylanase Inhibitor Levels in Hard Spring Wheat

Cereal Chem. 90(3):240–248 Arabinoxylans (AX), xylanase, and xylanase inhibitors have an important role in many cereal food processing applications. The effects of genotype, growing location, and their interaction (G × L) on AX, apparent xylanase activity, and apparent xylanase inhibition activity of Triticum aestivum xylanase inhibitor (TAXI) and xylanase inhibiting protein (XIP) were investigated for six hard red and six hard white spring wheat genotypes grown at three locations. Difference in total AX level among genotypes was not determined to a significant level by genotype. Instead, variability in total AX content was largely dependent on G × L. However, total AX content was significantly different between the two wheat classes. For bran xylanase activity, 25% of the variability could be attributed to G × L interaction. Moreover, there was significant difference between the bran xylanase activities in the two wheat classes. Bran TAXI activity and XIP activity were significantly different among genotypes. Genotype contributed 72% to the variability in TAXI activity and 39% in XIP. However, no significant difference was observed among the two wheat classes for TAXI or XIP activity. These results indicate that TAXI might be a stable parameter in segregating wheat genotypes with varying xylanase activity. Arabinoxylans (AX), xylanase, and xylanase inhibitors have an important role in many cereal food processing applications, including breadmaking (Courtin and Delcour 2002), gluten–starch separation (Frederix et al 2004), refrigerated dough (Courtin et al 2005; Poulsen and Sorensen 2006; Simsek and Ohm 2009), malting and brewing (Dornez et al 2009), and feed conversion of animal feeds (Bedford and Schulze 1998). Microbial xylanases are routinely added in some food processes to hydrolyze AX. However, the effectiveness of these additions is influenced by grainassociated xylanase and xylanase inhibitors occurring naturally in wheat kernels in variable amounts in different cultivars (Bonnin et al 2005; Gebruers et al 2010). One of the major problems in wheat milling and processing industries is the variability in end-use quality of the wheat (Dornez et al 2008). Minor constituents such as lipids, enzymes, and nonstarch polysaccharides contribute to this variation (Dornez et al 2008). Of the nonstarch polysaccharides, AX are of great importance, as they are the most abundant component in the cell wall of wheat (Goesaert et al 2005). Even though they are a minor constituent of the grain, their water-holding capacity and viscous properties impose considerable impact on the functional properties of grain-based product processing. Furthermore, consumption of AX has been associated with beneficial health effects in humans (Garcia et al 2006). Xylanases, endo-β-(1,4)-D-xylanase (EC 3.2.1.8), are the main enzymes involved in hydrolysis of AX. They cleave AX by internally hydrolyzing the 1,4-β-D-xylosidic linkage between xylose residues in the xylan backbone (Collins et al 2005; Dornez et al 2009). This cleaving causes drastic changes to AX molecular weight, water extractability, and functional properties (Courtin and Delcour 2002). Thus, grain-associated xylanases in wheat kernels are capable of imparting significant effects on processing. Apart from AX and xylanase, wheat kernels also contain high levels of xylanase inhibitors. These inhibitors inactivate grainassociated xylanases of microbial origin and hence affect the net functionality of xylanase in wheat processing (Dornez et al 2010). In wheat, three proteinaceous xylanase inhibitors have been identified to date: TAXI (Triticum aestivum xylanase inhibitor) (Debyser et al 1999), XIP (xylanase inhibiting protein) (McLauchlan et al 1999), and TLXI (thaumatin-like xylanase inhibitor) (Fierens et al 2007). The total content of these inhibitors is high in wheat (Dornez et al 2009), and considerable differences exist in the levels present in different cereal types and varieties (Gebruers et al 2010). Xylanase inhibitors in plants have been suggested to play a role of plant defense to disease (Dornez et al 2010). This theory is mainly supported by the fact that TAXI, XIP, and TLXI do not inhibit endogenous wheat xylanase but inhibit microbial xylanases. Some microbial xylanases are routinely added to wheat flour or whole meal to achieve the desired level of AX degradation. However, their effectiveness is in many instances affected by the level of inhibitors naturally occurring within the grain and the sensitivity of the microbial xylanases to these inhibitors. The increase in bread loaf volume upon addition of xylanase can be cancelled by adding TAXI (Debyser et al 1999). Xylanase inhibitors have been found to negatively affect the activity of added xylanase in pig and poultry feeds (Sørensen et al 2004). However, xylanase inhibitors have positive effects in some food processes. In refrigerated dough, addition of TAXI-type inhibitors has been shown to reduce syruping up to 50% after 10 days of storage at 10°C (Poulsen and Sorensen 2006). XIP inhibits both glycoside hydrolase (GH) family 10 and GH family 11 xylanases, but only those of fungal origin (Dornez et al 2009), whereas TAXI-type inhibitors inhibit xylanases of fungal and bacterial origin from GH family 11 and not GH family 10 (Goesaert et al 2001). This understanding about the inhibition specificity of the xylanase inhibitors has been exploited to determine the levels of each inhibitor when both are present. Based on this specificity, Dornez et al (2006b) used Bacillus subtilis xylanase (GH 11) to determine TAXI activity and Penicillium purpurogenum xylanase (GH 10) to determine XIP activity in wheat grains. Although AX, xylanase, and xylanase inhibitors play an important role in wheat processing, little research has been done on the contribution of genetic and environmental factors to their variability in hard spring wheat. Studies in these areas are important * The e-Xtra logo stands for “electronic extra” and indicates that Figures 2, 5, and 6 appear in color online. 1 North Dakota State University, Department of Plant Sciences, Cereal Science Graduate Program, P.O. Box 6050, Department Number 7670, Fargo, ND 581086050, U.S.A. 2 USDA-ARS Hard Red Spring and Durum Wheat Quality Laboratory, Harris Hall, North Dakota State University, Fargo, ND 58108, U.S.A. 3 Laboratory of Food Chemistry and Leuven Food Science and Nutrition Research Centre (LFoRCe), Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium. 4 North Dakota State University, Department of Plant Pathology, P.O. Box 6050, Department Number 7660, Fargo, ND 58108-6050, U.S.A. 5 Corresponding author. Phone: (701) 231-7737. Fax: (701) 231-8474. E-mail: [email protected] http://dx.doi.org/10.1094 / CCHEM-08-12-0103-R © 2013 AACC International, Inc. e-Xtra*