ARS | Publication request: Gas cell stability in bread making. II. Mechanism of stability of the gluten-starch matrix

Submitted to: Journal of Cereal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 8, 2008
Publication Date: N/A

Interpretive Summary: It is the unique properties of the gluten proteins of wheat that allow bakery products such as breads, pizza, and tortillas to be made. This research project examined the role of gluten and starch in stabilizing gas cells in bread, which are important in determining product quality. Fractions of gluten proteins with different molecular weight distributions were prepared and added back to flour, then strain hardening and mixing parameters measured. Doughs with higher strain hardening index were sufficiently extensible to respond to gas pressure but also had sufficient strength to resist collapse. The phenomenon of strain hardening appeared to depend on the balance between strength and extensibility of the entangled network of polymeric proteins of wheat flour. The optimum balance seemed to exist when the relative proportions of polymerice proteins greater and smaller than the optimum molecular weight were roughly 60:40. A shift in the balance to either side was related to a decrease in loaf volume. The smaller polymers (less than the optimum molecular weight) may decrease the stability of the gluten-starch matrix. On the other hand, an increase in strength conferring polymeric proteins may prevent sufficient expansion of the gluten-starch matrix required to increase loaf volume.

Technical Abstract: A key parameter in the primary stabilizing dough film (gluten-starch matrix) is thought to be the property of strain hardening. The hard red winter wheat, Jagger gave higher test-bake loaf volume than a soft wheat and higher strain hardening index for dough. Rheological properties of doughs were varied by the addition of flour protein fractions prepared by pH fractionation. Fractions were characterized by SE-HPLC and MALLS. The molecular weight distribution (MWD) of fractions progressively shifted to higher values as the pH of the fractionation decreased. Changes in mixograph peak development time paralleled the changes in MWD. However, the strain hardening index and the test-bake loaf volume increased with increasing MWD up to a point (optimum), after which they declined. At a given strain rate, the behavior at the optimum is thought to result from slippage of the maximum number of statistical segments between entanglements, without disrupting the entangled network of polymeric proteins. The shift of MWD to molecular weight higher than the optimum results in a stronger network with reduced slippage through entanglement nodes, whereas a shift to lower molecular weights will decrease the strength of the network due to a lesser number of entanglements per chain.