There are many ways to measure how much moisture is in a food or pharmaceutical product. But can you rely on the answers? Getting reliable moisture content numbers is hard, and here’s why:
It’s difficult to get all the moisture out of a product
The simplest way to measure moisture content is to weigh the product, remove all the moisture, and then weigh again. Methods that attempt to remove all the water from a product are called direct methods. These range from the simple (oven-drying) to the involved (Karl Fischer titration). Direct methods are considered the most reliable, but are labor-intensive and time consuming. And it’s difficult to remove water and only water from a sample.
Indirect methods are imprecise
Indirect methods involve measuring a property that changes as the moisture content of the food changes. They range from refractometry and IR/NIR to microwave adsorption, dielectric capacitance, and ultrasonic adsorption. These methods are often quick and easy, but they must be calibrated to a direct method and are thus less reliable and less precise. Calibrations can also change due to external factors. These changes are often hard to detect and can add uncertainty to the moisture measurement.
Challenges of reproducibility
Moisture content is often reported as a simple percentage without reference to the method used. However, values measured by different people at different times using different methods may not be comparable. For example, a measurement that is made by heating the sample can be very different than one made using a chemical analysis. A heated sample will frequently decompose. Heating may volatilize elements other than water. This can lead to a loss-on-drying moisture content reading 2-3% higher than one measured on the same sample with methods like Karl Fischer analysis, which do not use heat to remove the water.
Wet vs. Dry Basis
To further complicate the matter, moisture content can be reported on either a wet or a dry basis. If reporting on a wet basis, the amount of water in the sample is divided by the total weight of the sample. If reporting on a dry basis, the amount of water in the sample is divided by the dry weight of the sample. Either method is acceptable, and it is not difficult to convert between the two, but moisture content is typically reported simply as a percentage. If you attempt to compare your moisture content numbers to values from other sources, it may be difficult to tell whether you are comparing apples to apples.
Consistency in measurements
In your operation, you can overcome some of these issues by using a single method of analysis with a strict measurement SOP. For example, you might choose to make all measurements using oven loss-on-drying. There are still three main challenges you face in obtaining consistent, reliable moisture content data.
Every method has sources of variability. Loss-on-drying oven methods are accepted as correct, but even this method has many sources of variation. Over time, a single oven can vary in temperature significantly, and two ovens set to the same temperature can vary by as much as 40 degrees C. Vapor pressure inside the oven also varies from day to day. Fast moisture balances, which not only heat the sample but also use a calibration, are even more subject to error.
The term “dry” is scientifically undefined. As a result, you must determine how to dry the sample and when to call it dry. One commonly accepted endpoint is to dry a product until its weight stops changing. Weight loss levels off at different temperatures for different products, however, and methods that use heat can cause decomposition and volatilization. The AOAC recommends drying times and temperatures for some products, but research often shows that the recommendations do not lead to reproducible results
There is no standard against which to measure accuracy. This makes it difficult to evaluate precision, repeatability, and reliability, and impossible to measure or state an accuracy for any instrument or measurement method.
Costs of imprecise measurements
For any food manufacturer that sells product by weight, moisture content is an important way to monitor yield. In order to maximize yield, you need better precision and more context for your moisture measurements.
One good alternative is a total moisture measurement, one that includes both moisture content and water activity. Total moisture is based on fundamental principles of thermodynamics. It can give you the accuracy and context you need to operate with tighter specifications. You see both the yields and the safety/quality moisture data together in one place, tied to accuracy standards you can trust.
The moisture measurement best suited to measuring safety is water activity, a kind of “relative humidity” of food. Bacteria don’t grow in foods with less than 0.85 water activity (85% ERH). Molds and yeasts don’t grow at water activities less than 0.65 (65% ERH).
Water activity is also directly related to factors that affect quality like moisture migration, texture issues, lipid oxidation, caking and clumping, and vitamin degradation.
Using a total moisture approach, top-performing food manufacturers are able to optimize moisture on each batch, consistently increasing yields by 2-3%.