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.
The Karl Fischer titration method is specific to water, so it’s a more pure determination of water content, says Dr. Carter in noting its positive attributes. It isn’t impacted by ambient conditions, making it more reproducible. However, this method does require using hazardous solvents that must be handled, stored, and properly disposed of. It’s a complicated process, which requires training and understanding, Dr. Carter says. Furthermore, the equipment is typically more expensive than that for loss on drying. Because no independent standard is possible for moisture content, the measurement is completely empirical—making it impossible to determine accuracy or its true value.
Near Infrared (NIR) Moisture Meter
NIR moisture meters use calibrations to convert an optical signal into the percent of moisture. They can be quite accurate if regularly calibrated, but users must be aware that as the optics of an instrument age, they change in a way that affects the signal. “NIR instruments are commonly used as inline detectors, but they need to be calibrated to a primary method to maintain accuracy,” Dr. Olmsted says.
On the positive side, Claas says that these meters provide very fast measurements in 30 to 60 seconds, and multi-parameter options (e.g., fat, moisture, proteins). On the negative side, the meter requires calibration; precision and accuracy depend on the quality of calibration. In addition, the meters are expensive.
Since moisture content is subject to many sources of error and doesn’t have an independent standard, it’s impossible to know the true value, Dr. Carter says. Given this, he prefers the loss-on-drying method because it’s the cheapest and easiest to perform. That said, he would not use moisture content to monitor moisture control, but instead would use water activity testing.
Dr. Olmsted prefers a method that’s direct, easy, and rapid. For these requirements, a microwave moisture analyzer with secondary infrared heating is his instrument of choice.
Claas says his choice depends on the application and workplace. For food production in regular operation (e.g., goods-in, in-process control, final quality control) infrared moisture analysis is his choice because it’s versatile and can be used for many different samples (e.g., liquids, creams, powders, granulates), it’s easy to use, results are provided quickly, and it offers great precision and accuracy.
Innovations in Moisture Control
CEM Corp. combines numerous technologies to provide a loss-on-drying analyzer that’s faster than any other primary method on the market—the SMART 6. “Not only does it combine microwave and infrared energy for faster heating, but it also uses active ventilation to both speed the process and to give SMART 6 the capability of operating safely outside a fume hood with no odors being released into the test area,” Dr. Olmsted says. For customers using inline NIR sensors, the SMART 6 offers the ability to calibrate the NIR sensors in only a few minutes, instead of hours with an air oven.
Dr. Carter works with Novasina, a provider of water activity meters, and Neutec Group, Novasina’s distributor for the U.S. market. “Although water activity meters are not new, many food manufacturers are now switching to releasing product solely on water activity values and relegating moisture content to only a measure of purity and standard of identity,” he says. “Some producers have eliminated moisture content testing completely and only measure water activity. By tracking water activity throughout the production process, these producers are able to catch changes in production that may lead to problems before they are widespread.”
In addition, by releasing on water activity, they are able to maximize moisture levels but assure product safety and stability using the water activity. “Since most products are sold on a weight basis, releasing based on water activity makes it possible to maximize profits, eliminate waste, save on energy costs, and release a safe product with optimal shelf life,” Dr. Carter says.
It’s important for food industry organizations to keep their members educated on moisture control methods, and what innovations are happening.
The American Dairy Products Institute showcases equipment at its annual meeting and technical symposiums where instrument suppliers can present new or updated products. They can also network with users to get answers to specific questions and training issues. Instrument manufacturers may have workshops for specific instruments as well as preventive maintenance training.
Software training for calibration adjustment or development is done via webinars or online training. “Most instruments have modem and interface capabilities, which allows the manufacturer or service technician access to an instrument in the event of a needed repair or updates,” says Dean Tjornehoj, a dairy industry quality and food safety consultant and Center of Excellence resource professional at the American Dairy Products Institute in Chicago. Control system manufacturers may exhibit control packages with integrated infrared or NIR instruments for moisture control. Webinars are used for company training sessions, with a manufacturer’s technical service joining in online to answer questions.
The majority of educational material that the American Association of Meat Processors (AAMP) provides and sponsors are in the form on hands-on demonstrations or presentations for processor members on the preparation and techniques used to create safe and wholesome meat products, says Nelson J. Gaydos, outreach specialist for the AAMP, which is based in Elizabethtown, Pa. The organization presents several state and regional conventions throughout the year nationwide.
Tjornehoj says the American Dairy Products Institute may share particular issues with a type of instrument or test method with its members. “Generally, because accuracy and precision have a significant influence on production efficiencies and profitability, further detail is usually proprietary,” he says. “NIR in general works well for powder and condensed products with appropriate procedures. A discussion about methods is usually limited to those points, as well as the general reliability or ease of adjusting product calibrations.”
Service is critical for NIR or infrared systems, especially those instruments integrated into a control system that may not be as flexible as a benchtop offline unit, Tjornehoj says. Less common are questions about networking instruments or networking software, which is important for companies that are large enough to purchase and take advantage of such software.
AAMP shares information in two major food areas: food safety and how preparation/cooking affects the final product. “Food safety, specifically prevention of pathogen growth and destruction, is of the utmost important when it comes to any food product,” Gaydos says. “Cooking with humidity has been shown to be an extremely important factor when it comes to killing pathogens such as Salmonella and E. coli. Secondly, it’s also important to understand how processes like cooking and humidity (or lack of it) affects the final flavor, texture, and appearance of products so processors can make products consistently the same and to their desired preferences.”