Complete Moisture Analysis for Wheat

Shelf life. To have value as an ingredient, flour must maintain its quality during storage. The degradative reactions that could potentially end its shelf life include microbial spoilage, caking and clumping, nutritional loss, color loss, and rancidity. The two extrinsic influences that will most significantly impact its rate of shelf life loss are temperature and moisture. Wheat and flour producers currently use moisture content as an indication of whether their product is going to be in danger of molding. However, water activity is not only a well-established predictor of mold and microbial growth, it is also more closely correlated with caking and clumping, nutritional loss, color loss, and rancidity. Water activity measurements are more repeatable than moisture content analysis, and can be verified using saturated or unsaturated salt solutions. In addition, water activity helps form the basis for the FDA’s definition of potentially hazardous foods. Consequently, including water activity in flour specifications is more critical to ensuring quality and shelf life than moisture content.

Moisture Content Methods for Wheat and Flour

The moisture content is a measure of the quantity of water in a product reported on either a wet or dry basis. In theory, moisture content determination is simply a comparison of the amount of water in a product to the weight of everything else in the product. In practice, it is an extremely complex process to actually obtain reliable results.

There are many ways to analyze how much water is in a product. The Association of Official Agricultural Chemists, or AOAC, lists 35 different methods for measuring moisture content. These are classified as either direct or indirect measurement methods. Direct moisture content methods either force water out of a sample at elevated temperatures and track the weight change or involve a chemical reaction with water and titration. The most common direct moisture methods include air-oven drying and Karl Fischer titration.

Indirect methods do not remove the water from the sample but instead try to predict the moisture content based on either testing under accelerated heat conditions or by correlating another measured property to the moisture content. These secondary methods require calibration to a primary or direct method. Examples of indirect measurement methods include halogen or infrared-based moisture balances, near infrared absorption, and dielectric capacitance.

The advantage of direct methods is that they are a primary measurement typically with superior precision, but they are also labor intensive and have long analysis times. Indirect methods are typically much faster than direct methods, but are not primary measurements based on accepted standards and consequently suffer in reliability. Due to the absence of a scientific definition of “dry,” all moisture methods lack a standard that would allow the comparison of methods or determination of accuracy. These issues can cause problems when determining shelf life because current wheat moisture content specs are so close to the mold-growth limit.

Loss-on-drying methods are also affected by the ambient conditions under which samples are analyzed. It is difficult to obtain reproducible results with all loss-on-drying methods, including both the moisture balance and convection oven, because relative humidity in the lab environment affects the moisture content measurement. See Table 1.

The ideal moisture method would combine high throughput testing with a primary measurement, eliminate variability due to changing ambient conditions, and provide a scientific standard for dry. Table 1 shows how much more accurate moisture content measurement can be when using this type of method.

Which Water Activity Specs Should Be Used?

Mold. A suggested water activity specification for flour and farina would be 0.62-0.68 aw. As indicated in “The Case for Water Activity as a Specification for Wheat Tempering and Flour Production” study, this water activity range corresponds with ideal moisture levels for hard and soft flour. In addition, this water activity range would prevent mold growth and minimize the rate of rancidity.