What is the origin of grain hardness

It is interesting to trace the history of research on the origin of grain hardness because this illustrates how science works to increase 

However, there were no differences in hardness between starch from hard and soft wheats or between matrices from hard and soft wheats. To rationalize this observation, it was postulated that variations in hardness were due to differences in adhesion between starch granules and protein matrix. In hard wheats, strong adhesion caused a more compact structure in soft wheats, lower adhesion strength resulted in a more porous structure with flaws in the contact between starch granule and matrix. 

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