Whole polished rice grains were ground using cryogenic and hammer milling to understand the mechanisms of degradation of starch granule structure, whole (branched) molecular structure, and individual branches of the molecules during particle size reduction (grinding). Hammer milling caused greater degradation to starch granules than cryogenic milling when the grains were ground to a similar volume-median diameter. Molecular degradation of starch was not evident in the cryogenically milled flours, but it was observed in the hammer-milled flours with preferential cleavage of longer (amylose) branches. This can be attributed to the increased grain brittleness and fracturability at cryogenic temperatures, reducing the mechanical energy required to diminish the grain size and thus reducing the probability of chain scission. The results indicate, for the first time, that branching, whole molecule, and granule structures of starch can be independently altered by varying grinding conditions, such as grinding force and temperature.

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Milling or grinding of cereal grains to produce flour is frequently accompanied by damage to starch granules, disruption of starch crystalline structure, and degradation of starch molecules. Amylopectin (AP) molecules are more susceptible to degradation than amylose (AM) molecules as AM molecules, present in amorphous conformation in native starch granules, are more flexible than the rigid double helical crystallites made of AP branches. The changes of starch structures caused by milling alter starch properties, including gelatinization, pasting, swelling, solubility, and digestibility, and thus affect flour quality. The changes in starch structures and properties, however, are different between the grinding of cereal grains and that of isolated starch granules as the latter ignores the influences from non-starch components in grains. Hence, the grinding of isolated starch granules, commonly used to understand the effects of milling, does not represent the grinding of cereal grains. This review allows a better understanding in the changes of starch structures and properties by milling, which is important in the production of flour with desirable properties.

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