Endogenous and Enhanced Oxidative Cross-Linking in Wheat Flour Mill Streams

Cereal Chem. 88(2):217–222 A better understanding of the variation in oxidative cross-linking potential among flour mill streams can improve flour quality through the formulating of superior flour blends, and hence end-product attributes. The purpose of this research was to study the variation of oxidative crosslinking among 10 Miag Multomat pilot mill streams from 31 hard, soft and club wheat cultivars using Bostwick viscosity measurements. Flour slurries were made with either water alone (to measure the endogenous oxidative cross-linking viscosity) or with added hydrogen peroxide-peroxidase (to measure the enhanced oxidative cross-linking viscosity). Hard cultivars tended to produce more viscous flour slurries for both water and peroxide-peroxidase viscosity measurements in most mill streams. Mill streams with high oxidative cross-linking potential were those with the largest differences between water and peroxide-peroxidase viscosity including 1st Break, 1st and 2nd Middlings, and 1st Midds Redust. Conversely, mill streams 3rd Break and 4th and 5th Middlings were the least likely to form oxidative cross-links. The ability to form oxidative cross-links is dependent on the availability of ferulic acid and tyrosine residues. Therefore, the arabinoxylan and protein polymers in mill streams that have a high oxidative cross-linking potential have a structure that is more conducive to forming oxidative cross-links. Wheat (Triticum aestivum L.) grain processed by roller milling produces mill streams with distinct functional characteristics. These functional characteristics are dependent on several factors including but not limited to wheat cultivar, kernel hardness, tempering conditions, and mill settings. Break and early reduction mill streams are primarily composed of pure, starchy endosperm. Later reduction streams tend to have higher ash content due to bran contamination. The concentration of protein and ash increases as extraction rate increases (Hinton 1959; Nelson and McDonald 1977; Posner and Hibbs 1997). To produce a commercial flour, selected mill streams with similar functional characteristics are combined to produce flour blends destined for a particular enduse; blends can produce a wide range of flour grades. The flour grades include high purity blends (low ash) such as fancy patent, first patent, and short patent as well as lower purity (high ash) blends like fancy clear and first clear (Posner and Hibbs 1997). High-ratio cakes, pastry products, white salted noodles and Japanese sponge cakes are produced from lower extraction, lower ash, patent flours. Other products such as bread, cookies, crackers, and pancakes are generally produced from higher extraction straightgrade flours. Consequently, flour stream blending is based on the desired characteristics needed to produce a certain product. The blending process can benefit from a greater understanding of the functional characteristics of each flour stream. Further, more optimal blending of functional flour fractions may eliminate or reduce the need for further treatments and hence decrease labeling requirements. One minor constituent that significantly affects the functional characteristics of flours is arabinoxylans. Arabinoxylans are nonstarch polysaccharides that comprise ≈2–6% of the wheat kernel. These polymers are structural components of cell walls; they form a matrix with protein to entrap cellulosic material as well as glucomannans (Stone 2006). Similar to protein and ash, arabinoxylans are not uniformly distributed in the wheat kernel (Ramseyer et al 2011). Arabinoxylans are more highly concentrated in the bran, pericarp, seed coat, and aleurone layers (Pomeranz 1988). The concentration of arabinoxylans in the central endosperm is significantly less compared to the outer layers of the wheat kernel. Furthermore, arabinoxylans are also not homogenous in physical conformation. Generally, arabinoxylan conformation consists of a β,1-4xylose backbone with arabinose residues branching from the 2or 3-carbon position. More specifically, arabinoxylans from the outer layers of the wheat kernel have a higher degree of arabinose substitution compared to the inner endosperm arabinoxylans (MacArthur and D’Appolonia 1975; Ciacco and D’Appolonia 1982; Delcour et al 1999). The degree, pattern, and frequency of arabinose substitution on the xylose backbone affects the water extractability. Thus, total arabinoxylan (TAX) content can be empirically separated into water-extractable arabinoxylan (WEAX) or waterunextractable arabinoxylan (WUAX) fractions. Arabinoxylans from both water-unextractable and water-extractable fractions have ferulic acid residues esterfied at the C (O)-5position to a variable number of the arabinose residues. Ferulic acid residues of WUAX are covalently cross-linked to other cell wall materials and are unavailable for further chemical reactions (Mares and Stone 1973). Ferulic acid residues of WEAX are unlinked and can participate in chemical reactions (Fausch et al 1963). In the presence of free radicals, oxidative cross-linking between adjacent unlinked ferulic acid residues occurs to produce large networks of WEAX (Fausch et al 1963; Morita et al 1974; Neukom and Markwalder 1978; Vinkx et al 1991). In a batter food system, this network entraps water, resulting in the formation of a shear-thinning gel (Izydorczyk et al 1991). The availability and concentration of unlinked ferulic acid residues is an important factor that affects the gel characteristics (CarvajalMillan et al 2005; Niño-Medina et al 2010). A similar oxidative cross-linking mechanism can also occur between adjacent unlinked tyrosine residues on proteins, as well as between ferulic acid and tyrosine residues and result in an arabinoxylan-protein network (Neukom and Markwalder 1978; Oudgenoeg et al 2001; Tilley et al 2001; Wang et al 2002; Takaski et al 2005). Oxidative cross-linking of polymers affects the end-use quality of wheat flour. Bettge and Morris (2007) studied the oxidative cross-linking (also known as oxidative gelation) viscosity of soft and club wheat flours and related the findings to variation in sugarsnap cookie diameter. Kweon et al (2009) found that oxidative gelation viscosity increased as a consequence of extensive chlorination in soft wheat flour. Sarker et al (1998) found that oxidative cross-linking between wheat arabinoxylans and protein increased foam stability and concluded that the increased foam stability 1 School of Food Science, Washington State University, Pullman, WA 99164-6376; affiliated with the USDA-ARS Western Wheat Quality Laboratory. 2 USDA-ARS Western Wheat Quality Laboratory, Washington State University, 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. 3 Corresponding author. Phone: +1.509.335.4062. Fax: +1.509.335.8573. E-mail: