In converting wheat into flour through the milling process starch granules are damaged through the pressure and shear forces generated as the kernels are reduced in the roller milling process (Morgan, 1995). The amount of damage varies with severity of the milling process (typically indicated by the roll gap used in the gradual reduction system and the differential of the rolls) and the hardness of the wheat (Lin, 1996). In general, hard wheats have around 6-8% starch damage and soft wheats around 3-5% (Mathewson, 2000).

In the milling process the smooth reduction rolls are responsible for the generation of starch damage. The control of the level starch damage can be accomplished through various measures including mill design and mill adjustment. If a longer roll surface is used in the reduction system a thinner layer of material can be fed to the to the roll allowing for more severe grinding of the endosperm particles. Also, finer flour cloth can be used in the sifter sections causing some of the overtails in downstream reduction stages to include flour which, as a result, will be over ground increasing starch damage. The miller can increase the reduction roll pressure or temper to a minimum level. Grinding drier wheat on smooth rolls also creates higher levels of starch damage (Posner, 1996). Obviously the reverse of these methods can be employed to produce a flour will less starch damage as well.

Damaged starch granules differ from sound granules in two important ways: (1) they are significantly more susceptible to attack by alpha-amylase, and (2) they have an increased ability to bind water and thus increase the absorptive properties of the bread dough (Morgan, 1995).

The importance of susceptibility to alpha-amylase digestion in the bread making process is that there results a production of maltose which can be used as a substrate for the yeast in the latter stages of fermentation when supplies of the readily fermentable sugars (sucrose for example) in the flour have been exhausted (Evers, 1984).

This dual nature of damaged starch granules (susceptibility to alpha-amylase reduction, and increased water binding capacity) can lead to complex problems in the baking process. Evers (1984) states:

…because the starch granules capable of absorbing the most water are also those which are most susceptible to amylosis, it follows that the longer the period for which damaged granules remain accessible to amylolytic enzymes, the more of those granules will be digested and the greater will be the reduction in water holding capacity.

This somewhat circular sounding process indicates that during longer fermentation times the release of water from digested granules can lead to sticky doughs. Thus control of the level of starch damage in a flour is critical and more so for bulk fermented doughs than straight dough or no-time dough formulations.

As indicated above starch damage increases with kernel hardness. Starch damaged in soft wheat flours is substantially and consistently lower. This is a fortuitous occurrence as high starch damaged is detrimental to most products made from soft wheat flours including cookies and cakes (Lin, 1996). The mechanism behind the differences in the level of starch damage lies in the protein-starch bonds in the endosperm. In hard wheat the starch-protein bond is substantially stronger than that found in the soft wheats. When the endosperm is subjected to compressive and sheer forces of roller milling the protein and starch bond in cleaved in the soft wheats whereas in the hard wheats the starch granule is cleaved instead of at the bond itself thus resulting in damaged starch (MacRitchie, 2000).