Flour is mostly starch. In the endosperm of each grain of wheat, starch molecules are so tightly coiled together into tiny granules that they don't readily absorb water. This would be a problem for the baker if it weren't for the fact that during the milling process some of these granules are split or chipped. This opens them up and makes it easier for water to penetrate. Millers and bakers call this
Too little starch damage means that the enzymes that are so crucial for baking will not be able to access and break down enough starch for the yeast to metabolise. As a result, fermentation will be poor and the dough won't rise enough. If there is too much starch damage, the activity of the enzymes will be too rapid, producing a sticky dough that rises too quickly. Luckily, millers are expert at optimising the amount of starch damage by carefully controlling the process by which cereal grains are ground into flour.
Want to find out more? Look at the chapter on ingredients in the book flour and water.
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