Moisture Content Analysis

Like many products, nuts have a moisture content (MC) “sweet spot” where they’re dry enough to meet customer quality specs but not so dry that they break during shipment. To check quality reliably as the nuts are processed, shipped, stored, and used as ingredients, manufacturers need a moisture method that is:

• Highly accurate: Traditional loss-on-drying methods only reach ±0.5% MC. In nuts, that margin of error is often greater than the difference between a nut that breaks because it’s too dry and one that contains too much water.
• Verifiable: Currently, as one quality control manager put it, “My suppliers tell me what the moisture content was when it shipped, but they can say whatever they want, because we don’t have reliable standards to measure against.”
• Simple to perform: Operators will vary in their education, training, and skill. The method should be one that any minimally trained person can perform accurately.

A new method of moisture analysis offers promise in meeting those needs. This method measures moisture content using the dewpoint water activity method.

Water Activity and Moisture Content

Water activity quantifies the energy status of water in a product and uses the notation aw. It is a measure of microbial susceptibility accepted by the U.S. Department of Agriculture and a key component in many hazard analysis and critical control point plans. It can be measured with ±0.003 aw accuracy using tests that are both simple and precise (precise meaning whether the instrument gives the same answer each time). But can it be used to find moisture content?

Since work by chemists in the 1930s, food scientists have known that moisture content and water activity are related. The relationship, called a moisture sorption isotherm, is a graph showing how water is adsorbed into, and desorbed from, a product held at constant temperature.

Isotherms are complex—unique to every product and surprisingly unpredictable. In the 1980s and 1990s, a lot of research went into making isotherms for common products and ingredients, but the process was painstaking—a single point on the isotherm could take a few weeks to produce.

Isotherms were so expensive and time consuming to make that they were beyond the reach of all but a few big labs. And their usefulness was limited because they didn’t have good resolution—a graph might be constructed from just six points, for example.

All that changed with the introduction of isotherm generators that can produce isotherms that include 50 to 100 points in just a day or two. With that breakthrough, it became possible to reliably determine moisture content from water activity.

One Instrument, Two Measurements

Let’s say a nut producer wants to measure the moisture content of pecans. The first step is to collect a data set of water activity and moisture content values to construct an isotherm. Then using a model, several of which are available, the pecan isotherm is converted into an equation. This equation will take the water activity reading of a pecan sample and convert it into a moisture content reading.

The process could be simplified by programming the equation into a special instrument. The instrument could determine water activity and convert the reading into a moisture content measurement using a preprogrammed equation.

After processing the pecans, the producer can put a sample into the instrument, run it for five minutes or so, and get both moisture content and water activity readings on his pecans.

Moisture content in pecans is a touchy thing. Their target moisture content is around 4.6%. At moisture contents over 4.8%, they are susceptible to mold. The problem: Most moisture content methods are not precise enough to distinguish between the two. The dewpoint measurement can be much more precise, especially in intermediate moisture foods like pecans. Intermediate moisture foods have an S-shaped isotherm with a flat spot in the middle. That flat spot tends to be the moisture range food producers care about, and, in that range, small changes in moisture content correspond to big changes in water activity.