Damaged Starch Caused by the Milling Process
Damaged starch affects the behavior of dough during processing as well as the quality of the finished product (see "Damaged Starch - Impact on Baking"). When it comes to damaged starch, the following rule of thumb applies: neither too much nor too little! But how does damaged starch occur?
Starch damage occurs during the milling process, either at the industrial level or in a lab. Damaged starch is an unavoidable byproduct of any traditional dry milling process. Millers also play a key role in the level of starch damage. There are 3 variables that affect the amount of damaged starch in flour:
- Grain hardness (genetic criterion)
- The preparation of the wheat for milling (mechanical criterion)
- The settings of the mill (mechanical criterion)
Grain hardness is a genetic criterion that mainly depends on the variety of wheat. It is therefore a fixed value. Under equal milling conditions, "hard" wheat almost always produces more damaged starch than "soft" wheat. Some countries label wheat as "resistant to grinding" or "not resistant to grinding" instead of "hard" and "soft." This notion of "grinding resistance" is interesting because it allows us to consider that a more resistant wheat might break down into larger but also more damaged particles. Fortunately, "hard" wheat is also usually higher in protein, which compensates (in part) for the damage to the starch. In any case, grain hardness is something all millers have to look at.
For any given sample of wheat, millers have two main ways to control the level of damaged starch:
- The preparation of the wheat (conditioning). Conditioning consists of mixing the wheat with a specific amount of water and then letting it rest. This process makes it easier for the endosperm (floury kernel/albumin) and the outer layers of the grain (bran) to be parted during milling. The goal is to obtain a maximum amount of flour while optimizing the ash content - and thus the amount of bran. Water softens the endosperm, making it easier for the starch to be separated from the protein matrix. This results in less damaged starch.
- The settings of the mill. The main job of a miller is to obtain a maximum amount of high-quality flour from a given batch of wheat. This job is made easier by a complex tool comprised of rollers (fluted and smooth) and sifters (plansifters).
Starch damage occurs when the wheat passes between the 2 rollers, which are designed to reduce the size of the particles. The mill setting criterion is purely mechanical. Instead of getting into a complex milling diagram, here are some of the main points:
- The narrower the gap between the rollers, the more likely the starch will be damaged.
- The higher the pressure on the rollers, the greater the damage to the starch.
- Fluted rollers work in a shearing motion and "break" the granules.
- Smooth rollers are more like crushers and "crack" the granules.
- As the milling continues, the flour is subjected to more mechanical stress and the damage to the starch increases.
- Although the break and conversion passages do not produce flours with the highest levels of damaged starch, these passages do produce large quantities of flour so ultimately contribute the most damaged starch to the finished product.
- On the other hand, although the end-of-milling (i.e., final reduction) passages produce flours with extremely high levels of damaged starch, they only produce small amounts of flour so their impact on the finished product is less pronounced (Figure 1).
It is a known fact that millers themselves play an important role in the production of damaged starch. Each of their actions contributes to more or less damaged starch, from the choice of wheat to the execution of the various steps in the preparation and milling processes. We also know that these actions have a direct impact on the rheological properties of the flours (hydration, resistance, stickiness, etc.) and the characteristics of the finished products (volume, color, cracks, etc.).
We can therefore conclude that measuring starch damage on the mill is just as important (if not more important) as measuring protein content. And while it is true that traditional methods were often inaccurate and complex, we now have a quick, simple, and internationally recognized means of measuring - and thus optimizing - one of the parameters that directly affects the quality of flours.