Measuring moisture content: It’s more complicated than you think

Moisture content is the percentage of water in a food product or other material, expressed relative to the wet or dry weight of the material.

The amount of water present in a material affects product quality and shelf life, but moisture content alone isn't enough to predict product safety or stability. To determine whether a product is safe or stable, you also need to know about water activity.

What is moisture content?

Moisture content is simply the amount of water in a material. It is expressed as a percentage of the total material weight (wet basis) or a percentage of the dry matter (dry basis).

Though both methods measure the same material, they give different results because they use different reference weights. For wet basis, the reference weight is the total weight (water + dry matter). For dry basis, the reference weight is only the dry matter. This means dry basis values will always be higher than wet basis values.

In food, moisture content affects texture, weight, viscosity, appearance, and how susceptible a food is to microbial spoilage. Moisture is typically the most prevalent component of food, making it a primary variable in food composition.

How to measure moisture content

Moisture content is typically measured using one of these three primary methods:

• Loss on drying (also called thermogravimetric methods)
• Near infrared spectroscopy (NIR)
• Karl Fischer Titration

Each method uses a different principle and is suitable for different sample types.

1. Loss on drying

This method works by measuring the weight of a sample before and after drying. The sample is dried at a set temperature until it reaches constant mass. The moisture content is determined by calculating how much weight was lost during drying.

Loss on drying is a common analytical method for moisture content in solids and semisolids. The oven method, described in AOAC methods 930.15, 935.29, 950.46, 984.25, and others, is the most widely used method for moisture measurement in foods because it requires minimal infrastructure.

One downside of oven methods is that the heat used to dry the sample can drive off other volatile compounds, like short-chain fatty acids and alcohol. This means oven methods can overestimate moisture content in some samples.

Moisture analyzers (sometimes called moisture balances) are a modernized version of the oven method that automatically measures and records the weight loss. They are widely used in quality control because they're simple, fast, and don't require separate weighing equipment.

2. Near infrared spectroscopy

NIR spectroscopy identifies the composition of materials based on how they absorb and reflect light in the near-infrared spectrum. Water absorbs NIR light in characteristic ways, so NIR can be used to quickly estimate moisture content.

The main benefits of NIR for moisture measurement are speed (results in seconds), non-destructive testing, and easy deployment in-line or at-line during manufacturing.

NIR is an indirect measurement method, meaning it estimates moisture content based on a statistical model that has been trained with known samples. Accuracy depends on the quality and relevance of the calibration model. NIR systems must be calibrated for each specific product type and validated against a primary method before being deployed for routine use.

3. Karl Fischer Titration

Karl Fischer Titration is a chemical method that directly and specifically measures water, even at very low levels. Unlike oven methods, it is not affected by the presence of other volatile compounds.

Karl Fischer Titration is the reference method for moisture content in many industries, including pharmaceutical manufacturing (per USP 921). It is well-suited to products where high accuracy is needed or where oven drying would be inaccurate or impractical.

Moisture content formulas

The formula you use depends on which basis (wet or dry) you're reporting.

Wet basis moisture content formula

Wet basis moisture (%) = [(wet weight – dry weight) / wet weight] x 100

Wet basis is more common in food and agriculture because it expresses water as a fraction of the material as-received. It is also sometimes called "as-is" moisture.

Dry basis moisture content formula

Dry basis moisture (%) = [(wet weight – dry weight) / dry weight] x 100

Dry basis moisture is common in research and in industries like grain and feed, where comparisons between samples of different moisture levels are needed. One consequence of dry basis reporting is that values can exceed 100%, which sometimes causes confusion.

More accurate moisture analysis

Measuring moisture content is a useful first step in characterizing a material. But moisture content can't tell you whether the water present is free to support microbial growth or drive quality-degrading reactions.

That requires a water activity measurement.

Water activity (aw) tells you about the energy state of the water in a material—how available that water is to act as a reactant or solvent. Products with the same moisture content can have very different water activities, and different water activities mean different spoilage risks and shelf-life behaviors.

To fully characterize moisture in a product, measure both moisture content and water activity. The combination gives you both the quantity and the availability of water—and lets you make informed decisions about formulation, processing, packaging, and shelf life.

Frequently asked questions

What is moisture content?

Moisture content is the percentage of water in a material, expressed relative to either total weight (wet basis) or dry weight (dry basis). It tells you how much water is present, but not whether that water is free to cause spoilage or quality problems.

Is moisture content the same as water content in food?

The terms are used interchangeably in food science — both describe the percentage of water in a material by weight. The meaningful distinction is not between the two terms but between the reporting basis: wet basis expresses water as a percentage of total weight, while dry basis expresses it as a percentage of dry matter only.

What moisture content level makes a food shelf-stable?

There is no reliable moisture content threshold for shelf stability, because two products at the same moisture content can have very different water activities depending on how tightly water is bound to sugars, salts, or proteins. Water activity — not moisture content — is the measurement that predicts whether microbial growth, spoilage, or texture degradation will occur.

Can I compare moisture content values from two different suppliers if they use different methods?

Not reliably, because different methods measure different things and are not interchangeable without a validated conversion factor. Even if both suppliers use direct methods, one using air-oven drying and the other using Karl Fischer will produce systematically different results for the same product.

Does moisture content alone determine whether a food product is microbiologically safe?

No, because moisture content cannot distinguish between water that is tightly bound to solutes and water that is free to support microbial growth. Water activity is the measurement that determines microbial risk, and it is what food safety regulations like FSMA are built around.