Methods and Instruments for Moisture Testing
Water is a pervasive liquid. It has an ability to attach itself to most materials. This moisture can stick to the outside of the substance. This is known a free or surface moisture. Moisture can also get locked up in the molecular structure. This is known as bound moisture.
These differences in moisture content often require different moisture analysis methods. In addition, the composition of the material to be tested will influence the measurement technique needed.
Further complicating the selection of a method or technique is the length of time available to complete a moisture content test.
In order to provide solutions to most of these measuring problems, we offer instruments based on the following methods:
The following are the basics of each of these methods:
The LOD method is the most widely used way of doing moisture analysis. This technique is based on the following steps: 1) weigh a sample of the substance, then heat the sample until the moisture is all driven off; 2) at this point, the sample is weighed again; 3) the difference in weight represents the amount of moisture that was in the original sample.
Limitations of this method involve two conditions. The first is when a material includes volatiles that are released at the same time as the moisture. Examples of such volatiles are the presence of sugars and fats. The second problem is with water that is tightly bound so that the heat energy needed to release the moisture will burn the sample first.
The Karl Fischer test is water-specific, overcoming the problems of volatiles. The technique is based on using a solvent to separate water from the substance. This is done in a container closed off from the environment. The presence of the water is detected by an electrode. A reagent is introduced that converts the water to a non-conducting chemical. The electrode detects when the water is fully converted. The amount of reagent necessary to accomplish this is measured and convert to units of water.
For some materials, it is necessary to expose the sample to high temperatures and measure the amount of water in the resulting vapor. This process is accomplished in a Karl Fischer Oven.
These techniques have the advantage of nearly instantaneous results. As these are indirect relationships of electrical type changes in a product and moisture changes, the effectiveness/accuracy is dependent upon the development of calibration charts. These calibrations need to be made individually for each product to be tested as the relationship of moisture to electrical characteristics is different from material to material.
The calibrations are made by testing a substance at different moisture levels and recording and relating the electrical characteristics at each level. Depending on the technique used, there are limitations on the maximum percent moisture that can be measured.
The NIR technique of determining moisture content is based on the fact that water is associated with specific wavelengths of Near Infrared radiation. In this method, a light that includes the NIR frequencies is used to illuminate the sample. The amount of energy that is reflected in the water by the associated wavelengths is measured and directly related to moisture level. These measurements are instantaneous.
NIR and several of the Capacitance and Radio Frequency instruments are suitable for in-line moisture content analysis.
Water Activity (aW) is an increasingly popular measurement technique. It is a critical factor in determining quality and safety of foods. It has significant impact on shelf life, texture, flavor, color and aroma. In addition, measurement of water activity has a growing application in areas of pharmaceutical and cosmetic development and production.
Water activity is not moisture content. It is the measurement of how much water is given off by a product at a given temperature. The technique consists of placing a sample in a closed chamber. As water is released from the sample, the relative humidity (RH) in the chamber increases. Water Activity Instruments measure this RH. When the RH stops changing, the aW is calculated as a function of the RH.
The Water Activity Scale is from 0.0 to 1.0 aW.
Standards are set for different aW level of product conditions such as mold and bacterial growth, color change, and other measures of spoilage.