Oxidative Gelation – Its Influence on Processing and End-Use in Soft Wheat Flour

1. Oxidative Gelation – Its Influence on Processing and End-Use in Soft Wheat Flour A.D. Bettge USDA-ARS Western Wheat Quality Laboratory Pullman, WA USA

2. Quality, Variation and Control • Controlling, or understanding variation in quality is important, scientifically and economically. • Defining the biochemical influence(s) on quality and developing reliable and informative testing methods allows this control or understanding • Allows development of new wheat varieties that require reduced processing input to obtain desired quality. Also shortens product label. • Viscosity is a little-studied quality attribute.

3. Viscosity and End-Use Quality Variation • Viscosity is an important consideration in batter-based processes, i.e. cakes, batter coatings, donuts, pancake mixes, etc • Some biochemical components may contribute both to viscosity and to end-product quality • Substantial unexplained variation exists

4. Variation in Viscosity • Protein, especially HMW glutenins, can influence viscosity. Lactic acid SRC or SDS-sedimentation are tests • Other water-absorbing compounds also contribute: arabinoxylans (aka pentosans, hemicellulose, non-starch carbohydrate). Sucrose SRC is the test • Leavening gas (bubbles) affect viscosity • Focus here is arabinoxylans and protein

5. Arabinoxylans and Viscosity • Xylose backbone, variously substituted with Arabinose • Degree of substitution and pattern of substitution determines water solubility • Broadly, functionally categorized as water soluble or water insoluble • Water soluble arabinoxylans also have ferulic acid moieties attached to arabinose side-chains (insolubles already have ferulic acid cross-linked as part of cell walls)

6. Arabinoxylan

7. Arabinoxylan Structure Stone, 1996, Cereal grain carbohydrates in: Cereal Grain Quality eds: Henry and Kettlewell

8. Arabinoxylans and Oxidative Gelation • Under appropriate conditions, ferulic acid moieties can cross link with other ferulic acid moieties or protein side chains forming a large polymeric oxidative gel that sequesters water and creates viscosity • Water soluble arabinoxylans form oxidative gels; insoluble arabinoxylans do not appear to do so (already cross-linked) • Insoluble arabinoxylans adsorb water (not sequester) and affect Aw

9. Arabinoxylan cross-linking Arabinoxylan – Protein cross linking Protein – Protein (di-Tyrosine) cross linking Neukom and Markwalder, 1978: Cereal Foods World

10. Modifiers of Arabinoxylan Concentration and Activity • Endogenous (sometimes exogenous or added) enzymes • Inhibited by TAXI (Triticum aestivum xylanase inhibitor) or XIP (xylanase inhibitor protein) – Courtin and Delcour. • pH – elevated pH solubilizes esterified arabinoxylans

11. Oxidative Gelation • Long known that gels form: • Oxidative arabinoxylan gels are mechanically fragile, hence limiting their impact on mixed doughs (bread, cookie, etc); protein gels may be more robust Cereal Chem. 1925

12. Oxidative Gel Impact on Quality • Viscosity can retain leavening gas in low-gluten products, allowing greater volume • Potential detrimental effect on low-moisture products such as cookies or crackers through sequestered water, low spread or high volume and “stickiness” • Potential beneficial effect on cakes, batters and batter coatings where viscosity prevents the “settling” that causes separation of flour suspensions and loss of leavening gasses

13. Oxidative Gelation • Gels form naturally from endogenous, uncontrolled presence of free radicals (i.e. auto-oxidized lipids and fatty acids) • Gels can be caused to form by adding free radicals in excess (e.g. H2O2 + peroxidase) • Gels are formed from both protein and arabinoxylan • Both likely have an end-use impact

14. Oxidative Gelation Test • Test developed to assess oxidative gelation as manifested in viscosity • Hydrate 20 min, with rocking, 1:2.5 w/v flour:peroxide (75 ppm) Hoseney&Faubion, 1981 • Add 60 U horseradish peroxidase • Decant into Bostwick Consistometer reservoir; let sit one minute • Measure flow distance at 40 sec

15. Equipment

16. Protein and Viscosity

17. Arabinoxylans and Viscosity

18. Antioxidants and Gelation

19. End-Use and Arabinoxylans • “MaxR” models constructed to obtain information on most influential factors • NOT to be used as predictive equations • Provides best model parameters based on combination of factors reflected in F-values and R2

20. Straight Grade

21. Patent

22. Interpretation (?) • Oxidative gelation occurs between: Arabinoxylan polymers, protein polymers and a combination of the two • Elevated pH decreases viscosity through inhibition of cross-linking • Patent flours are distinctly more prone to oxidative gelation than are straight grade flours (unwitting breeding selection of club wheats?) • Flour has sufficient free radicals (likely from lipid auto-oxidation in flours over a couple weeks old) to initiate oxidative gelation without other sources. • Flour aging and new/old crop factors? • Other oxidative systems (i.e. PPO) may also have a role. Chlorination issues?

23. Interpretation (?) • Oxidative gels affect cookie, cracker and cake quality, but especially products with unconstrained viscous flow (pancakes or cake donuts) • Nutritionally and functionally, WEAX esterification to gluten proteins may add a form of fiber in hard wheats for bread • But at a cost to mixing quality (tolerence) Probably occurs due to reduction in elasticity due to presence of AX in gluten

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