Oxidative Gelation Measurement and Influence on Soft Wheat Batter Viscosity and End-Use Quality

Cereal Chem. 84(3):237–242 Viscosity is an important end-use attribute for some soft wheat flour formulations. Specifically, in formulations with minimal gluten development, such as batters (as in cake, pancake, and doughnut) and coatings (as in tempura), viscosity is important to leavening gas retention and flow characteristics. Current tests for predictors of viscosity leave considerable unexplained variation. The potential for water-extractable arabinoxylans to form oxidative gels through ferulic acid dimerization may represent an important component of viscosity variation. A method was developed to identify variation in viscosity due to oxidative gelation. This method, comparing viscosity of flour slurries made with water, a peroxide-peroxidase system, and a system with xylanase, indicated that two, and likely three, types of oxidative gelation were contributing to viscosity. Predicted viscosity due to inter-arabinoxylan gelation through ferulic acid dimerization, dityrosine formation among proteins, and ferulic acid-tyrosine bond formation varied among wheat cultivars. Oxidative gel formation increased batter viscosity probably due to water sequestration; this effect was correlated with reduction in the sugar snap cookie spread (diameter). Results indicate that oxidative gelation is an important contributor to batter viscosity and also contributes to the quality attributes of dough systems. Wheat flour product formulations encompass a large range in water content, from relatively dry, stiff doughs to low viscosity batters. In bread doughs, sufficient water and mechanical work are used to develop gluten and form an elastic network capable of holding fermentation gases. In many soft wheat products, such as cookies, and batter-based products such as cakes or donuts, the desired product performance and consistency is attained through the use of chemical leavening. Extensive gluten development in soft wheat flour products is generally undesirable because it reduces textural quality. Consequently, gluten development is minimized in most soft wheat products and other factors become more important as influences on end-use quality. For product formulas with a high water content such as cakes, coatings, pancakes, waffles, wafers, donuts, etc., batter viscosity is one of these critical factors (Morris and Rose 1996). Batters for coatings (e.g., tempura batter) must be viscous enough to adhere to the product but without clumping or sheeting off. Pancake and donut batters must be viscous enough to retain leavening gasses and prevent settling but without being so viscous as to inhibit flow and spread. Batter viscosity also relates to the control or degree of water sequestration. Variation in batter viscosity has, in part, been attributed to arabinoxylans (also known as pentosans, hemicellulose, or nonstarch polysaccharides). Arabinoxylans consist of a β-1,4 xylose backbone, variously substituted at the 2and 3-carbon position with arabinose. The degree, pattern, and frequency of the substitutions determine the water extractability of the polysaccharide. As such, arabinoxylans can be empirically separated into water-extractable (water-soluble) and water-unextractable (water-insoluble) fractions (Courtin and Delcour 2002). The water-unextractable fraction can interact with ≈10× its weight in water in such a way that water activity is reduced. In addition to interacting with water directly through hydrogen bonding, water-extractable arabinoxylans can form gels. Water-extractable arabinoxylan (WEAX) has varying numbers of unlinked ferulic acid moieties esterified to the arabinose side groups. Under a conducive chemical environment, usually involving the presence of free radicals, dimerization between ferulic acid moieties results in a large network of arabinoxylan polymers (Morita et al 1974; Neukom and Markwalder 1978; Vinkx et al 1991). The resulting matrix entraps or sequesters water leading to a gel (Izydorczyk et al 1991; Carvajal-Millan et al 2005). Similar free-radical-induced cross-linking can also occur between tyrosine residues contained in proteins both among proteins (Neukom and Markwalder 1978; Oudgenoeg et al 2001; Tilley et al 2001; Wang et al 2002; Takasaki et al 2005) and between arabinoxylans and proteins through ferulic acid-tyrosine esterification (Neukom and Markwalder 1978; Oudgenoeg et al 2001; Wang et al 2002). The potential for oxidative gels to form in aqueous environments has long been known. Durham (1925) published a paper indicating that an unidentified water-soluble fraction from wheat flour formed a gel when exposed to hydrogen peroxide. Further work (Morita et al 1974; Ciacco and D’Appolonia 1982) identified the water-soluble fraction as being composed of arabinoxylans, and the mechanism as ferulic acid cross-linking. Water-extractable arabinoxylan and protein polymers create viscous suspensions at low concentration due to their large size (Izydorczyk and Biliaderis 1992; Lu et al 2005) but form oxidative gels under appropriate conditions that exhibit shear-thinning (Izydorczyk et al 1991; Lu et al 2005). This rheological trait indicates that their impact would likely be less important in mechanically mixed doughs (e.g., pan bread) and more important in cake and other batter formulations where there is less shear thinning. Further, the effect of di-cysteine bonds in gluten formation likely overshadows the effect of di-tyrosine bonds in bread dough formation. Limited research on the influence of arabinoxylans and viscosity has been conducted. Izydorczyk et al (1991) and Moore et al (1990) measured viscosity using capillary viscometers or spindleand-plate or spindle-and-cylinder type instruments that produce variable amounts of shear. However, variation in viscosity due to arabinoxylans was observed, even using the higher shear instruments. Trough-style consistometers (Bostwick) measure viscosity as flow distance under low shear conditions that better measure oxidative gels associated with batter systems that can exhibit shear thinning. Other current tests for flour constituents that contribute to viscosity include the lactic acid solvent retention capacity (SRC) and SDS-sedimentation tests which identify the contribution of protein components (Approved Methods 56-11 and 56-70, respectively), and sucrose SRC (Approved Method 56-11) (AACC International 2000) that identifies the contribution of arabinoxylans and, to an extent, gliadins (Slade and Levine 1994a,b). 1 USDA-ARS Western Wheat Quality Laboratory, E-202 Food Science & Human Nutrition Facility East, P.O. Box 646394 Washington State Univ., Pullman, WA 99164-6394. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Corresponding author. Phone: 509-335-4062. Fax: 509-335-8573. E-mail: abettge