During the summer of 2015, Alltech tested the North American wheat crop and found an average of 3.2 mycotoxins per sample. Mold and the mycotoxin it produces represent a serious economic threat to wheat growers and flour producers. Molds are spread throughout the crop as grain mixes in storage. Often, pockets of high moisture develop, allowing mold to grow. Moisture continues to be an issue as grain is tempered, processed into flour, and stored.
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Moisture content is monitored at various points in this process. However, susceptibility to mold and microbial growth is better correlated to water activity than to moisture content because fungi cannot grow (and mycotoxin cannot be produced) in grain and flour below a 0.70 water activity. Moisture content is not directly related to mold and microbial growth rates, and the currently suggested moisture content levels for flour and farina correspond with water activity levels right at the cutoff point for mold growth. Any uncertainty in the moisture content measurement can allow contamination of the product.
Water activity is a measure of microbial susceptibility accepted by the USDA and a key component in many Hazard Analysis and Critical Control Point plans. To prevent mold and microbial contamination, it behooves the flour industry to consider including a water activity specification to ensure microbial safety.
How Does Contamination Occur?
According to Cereal Foods World’s study “The Case for Water Activity as a Specification for Wheat Tempering and Flour Production”, it appears that through trial and error, those who have set current recommended moisture levels for grains and flour have fortunately, if unknowingly, pinpointed the right water activity level to maximize stability. However, as current moisture content specs result in water activities very close to the microbial growth limit, problems can result from even small uncertainties in the moisture content measurement.
In fact, because flour is hygroscopic, it can easily take on moisture that pushes its water activity above the microbial growth limit. If flour or farina is exposed to high humidity (more than 70 percent), its water activity will quickly become unacceptable. Soft flour will reach unsafe water activity levels the fastest, but all products will have reached unacceptable water activities and moisture levels in only 24 hours. (This change in water activity and moisture content only applies to product exposed to ambient conditions. Moisture movement through a mass of bulk stored product once the surface has come to equilibrium and the subsequent changes in water activity and moisture content are not part of this discussion.)
Using the Right Measurement for Every Step
Weighing. Wheat products, both flour and grain, are sold on a weight basis. When you buy grain you don’t want to pay for water. Thus, there are restrictions about what moisture contents are allowed, and these restrictions have become a useful standard of identity for grain and flour to make sure that purchasers are buying what they expect. Moisture content is commonly required for any flour specification sheet, with 13.5 percent ideal for soft wheat and 14 percent ideal for hard wheat.
Tempering. Before it is milled, grain must be tempered with moisture to soften the endosperm and toughen the bran, which facilitates grinding of the grain and separation of the bran and germ from white flour. Liquid water is added to raise the moisture level of the grain from 12 percent to 17 percent. The grain then equilibrates for 16 to 24 hours before it is milled. However, standard practices do not include a test to determine if moisture equilibrium has been achieved, but instead depend on pre-set soaking times. The 16 to 24 hour hold time for tempered wheat is typically not long enough for mold to grow. Longer hold times, however, are likely to cause tempered wheat with water activities higher than 0.70 to experience mold growth. Since mold, microbial spoilage, and lower rates of rancidity are better correlated with lower water activities than moisture contents, it would make more sense to focus on optimizing the water activity level and then confirm that the moisture content is acceptable rather than to rely on just a moisture content specification.