Oxidative Gelation

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

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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, 12.5 w/v
  flourperoxide (75 ppm) HoseneyFaubion, 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
Cookie (sugar snap) F/p-value (/-) Cake Volume F/p-value (/-)
Flour Protein 12.7 0.0052 14.0 0.0033 -
Px Pxase _at_ pH 15.1 0.0031
Total AX 12.1 0.0059 -
Water Extr. AX 5.3 0.0438
Carbonate SRC 6.4 0.0278 -
Lactic acid SRC 10.7 0.0075

Model R2 0.84 0.66
Model F-value 13.1 7.1
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Patent
Cookie (sugar snap) F/p-value (/-) Cake Volume F/p-value (/-)
Water SRC 6.4 0.0322
Px Pxase _at_ pH 8.2 0.0186 -
Xylanase 23.4 0.0009
Xylanase Px Pxase 16.5 0.0028 -
Feulic Px Pxase _at_ pH 18.2 0.0021
Lactic Acid SRC 7.7 0.0198
Px 10.3 0.0093 -
Water Brookfield 7.1 0.0240
Cysteine _at_ pH 6.3 0.0313

Model R2 0.81 0.71
Model F-value 7.5 6.2
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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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