Why Moisture Content is not Enough
Moisture content is the most common moisture-related measurement made in the grain and flour industry. Though it has a purpose as a standard of identity, over the course of my PhD research, I came to believe that water activity is a more effective way to evaluate moisture in grain and flour, and that its use would ensure a consistently stable and safe product.
During the summer of 2015, Alltech tested the North American wheat crop and found an average of 3.2 mycotoxin per sample[1]. 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.
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 U.S. Department of Agriculture 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?
based on a survey of the water activity of wheat milling constituents (Carter et al. 2015), 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 (>70%), 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.[2]