Although the wheat grain comprises 82 - 86% starchy endosperm, only approximately 76 - 78% is able to be separated using current milling technology. Suitability for primary processing depends on three main characteristics that are in turn influenced to varying degrees by the genetic origin of wheat and agro-climatic conditions during plant development: the endosperm to bran ratio, kernel hardness and ease of separation of bran and endosperm. Among these, hardness appears to be an essential factor in wheat milling behavior also important to the food processing industries. Accumulated evidence supports the prominent influence of the puroindoline (pin) proteins on grain hardness though additional factors influence milling quality.
The general aim of this project was to increase the understanding of grain characteristics in hard wheat varieties that influence flour yield. The first research question involved the validation of the rheological information derived from the Single Kernel Characterisation System (SKCS) as the most suitable indicator of milling quality for small sample quantities. The methodologies initially involved correlation of fragment/particle size distribution of conditioned and unconditioned hard wheat varieties produced by first break rolls and the SKCS. The use of the SKCS for studies in milling quality was validated (Chapter 4).
This was followed by a process relating qualitative observations gained from light microscopy, conventional and environmental electron microscopy with the crush response profiles produced by the SKCS. This study generated an hypothesis involving starch granule size distribution (SGSD) which was tested by correlating quantitative data produced by a laser diffraction technique with small scale milling and SKCS data. SGSD has generally been disregarded as having a significant influence on hardness or milling quality with much investigative effort instead directed towards the puroindolines. However the results presented indicate the contrary with significant correlations with hardness and flour yield across a range of hard varieties. This is partially a reflection of positive relationships between bonding area and cohesion within composite materials (Chapter 5).
Finally, standard DNA sequencing methods were used to screen puroindoline sequence variation in 120 varieties from the Australian Winter Cereal Collection harvested and milled from two different environments. Further SGSD analysis indicated significant correlations with flour yield though differing between pin genotypes. In addition quantitative analysis of starch granule-bound puroindoline using a micro-capillary electrophoresis method (Lab-on-chip) demonstrated a significant negative correlation with flour yield in the premium milling quality varieties with pin genotype, Pina-D1a, Pinb-D1b (Chapter 6). Several alternative influences on milling quality such as lipid content, cell wall characteristics, the regulation of the hardness locus (Ha) and function of the grain softness proteins require further investigation. Also as expected results suggested that seed weight itself has a significant influence on flour yield.