Moisture plays a critical role in many aspects of food production, from getting the right consistency to achieving proper shelf life. Too little moisture can lead to products that are crumbly, hard, or that have palatability issues, says Ian R. Olmsted, PhD, a product manager in the process control division at CEM Corporation in Matthews, N.C., while too much moisture can lead to spoilage.
A variety of moisture control methods are currently available, and each has its pros and cons. Here’s a closer look.
Loss on Drying
Loss-on-drying instruments, such as ovens, thermogravimetrics (TGA)/infrared, and microwaves, are a simple and robust way to measure moisture in foodstuff. The general principle is that a sample is weighed initially, then dried in an apparatus such as an air oven, under a heat lamp or via microwave energy, Dr. Olmsted says. Once a sample is completely dry, it is re-weighed and the amount of loss on drying is calculated. Air ovens, a low-cost option, work well for many sample types but require up to eight hours to completely dry samples.
TGA is a very precise method of analysis, but to get reliable data a sample must be heated at a slow, controlled rate, Dr. Olmsted says. Therefore, TGA isn’t good for rapid process control.
Microwave moisture analyzers use microwave energy to dry samples; an integrated balance automatically measures sample weight during a test, Dr. Olmsted says. Microwave moisture analyzers are the fastest way to measure loss on drying, with typical testing times taking as little as two minutes.
Claas Boerger, head of the strategic product group at Mettler-Toledo GmbH in Greifensee, Switzerland, concurs, and says that microwaves are indeed fast. However, they can be used only for samples with high moisture contents approximately above 10 percent, limiting their applications. Typically, they cost more than an infrared moisture analyzer as well.
Infrared moisture balances provide a more rapid approach to drying samples. However, most instruments don’t have active ventilation so high-moisture products can take as long as 20 minutes to dry, Dr. Olmsted says. Infrared moisture balances with active ventilation can reduce testing times to around five minutes.
Claas says that infrared moisture analyzers are fast and easy—providing results in minutes. “They are easy to use for untrained personnel (e.g., shift workers), and results match the official method of oven drying,” he says. Their versatility enables them to be used for all samples with moisture contents ranging from 0 percent to 100 percent. On the downside, method development needs to be performed in order to get the same results as a drying oven, which is why modern instruments support the user with integrated method development functionality.
Karl Fischer Titration Method
To avoid the loss-on-drying method’s main shortfall of not being specific to water, the titration method instead relies on a wet chemistry to detect the amount of water, says Brady Carter, PhD, senior application scientist at Novasina AG, based in Morgan, Utah. The concept involves creating a reaction chamber containing the sample plus a solvent that will help release the water from the sample, and creating the necessary conditions needed for the reaction to proceed.
Then, iodine is titrated into the reaction chamber and the amount is closely tracked. Iodine and water are both needed for the reaction to proceed. When all of the water is consumed by the reaction, the reaction stops, and iodine starts to accumulate in the reaction vessel, Dr. Carter says. This causes a change in the solution’s electrical properties detected by an electrode inserted into the solution.
When the change is detected, the test is stopped, and the amount of iodine added is directly proportional to the amount of water present in the product. This amount of water is then divided by the wet weight to give a wet basis moisture content, or divided by the dry weight to give a dry basis moisture content, Dr. Carter says.