Grain endosperm microstructure and milling quality of hard wheat

Milling quality is considered to be a combination of the potential yield, bran content, colour and starch damage of flour. It is a complex trait because these factors interact with one another, with the milling process and with the requirements for the end product to be made from the flour. Despite considerable gains in wheat flour milling yield through conventional breeding strategies and milling technologies, the theoretical maximum yield still has not been attained. Attempts to unravel the genetics of flour yield are made especially difficult by the number and complexity of linkages between gene expression and grain processing. Our approach was to improve the understanding of the role of grain endosperm microstructure in determining high flour yield. A lack of vitreousness in hard wheat causes microstructural weaknesses characterised by low endosperm strength and stiffness that result in fracture through cell contents with a consequent high first break release and low overall flour yield. The SKCS provides a method for stress-strain analysis of wheat grain in which better milling quality is characterised by increased endosperm strength and stiffness. These properties were used to discriminate wheat samples according to their milling yield so that the differences in microstructure between contrasting groups, within each of the two hardness genotypes present in Australian wheat germplasm, could be examined. The Pina-D1a, Pinb-D1b set was characterised by a higher average flour yield than the Pina-D1b, Pinb-D1a, although the ranges of flour yields showed considerable overlap between the genotypes. For the Pina-D1b, Pinb-D1a genotype, which does not express starch bound puroindoline protein, starch granule type accounted for up to 47% of the variation in flour yield between varieties. For the Pina-D1a, Pinb-D1b genotype, starch granule type accounted for no more than 24% of the variation in flour yield between varieties, with 31-35% accounted for by starch bound puroindoline protein content. The combined effect of starch granule type and starch bound puroindoline accounted for 68% of the variation in flour yield in the Pina-D1a, Pinb-D1b set.