Moisture and Water Content Analysis in Food

Moisture analysis plays a vital role in all areas of the food industry, from goods-in inspections, quality control, production, and storage to developing new products. Moisture content also determines the cost of materials and often influences the financial margin of finished goods. In nearly all cases, the prepared and raw ingredients have an optimum moisture content that provides the best possible recipe, taste, consistency, appearance, and shelf-life.

Substances that are too dry may cause static discharge problems in production or affect the end product's consistency. On the other hand, substances that are too moist may agglomerate and clog pipes and machinery or lead to reduced shelf life.

Moisture determination ensures consistent product quality. The food industry is under constant pressure to keep costs at a minimum while producing enormous quantities of products with uniform quality. A reliable and fast moisture determination method enables food technicians in production to intervene promptly and avoid interruptions or even pauses in production.

The environments, properties, and physical states of water can vary drastically from sample to sample. Temperature, pressure, and bonding can change the physical condition of water within a sample, increasing the complexity of each analysis and opening the door to confounding results if the data is analyzed incorrectly. For example, carbohydrates can decompose at temperatures as low as 50°C, at which point they undergo a chemical reaction that releases water and adds to the measured total water content. However, this "added water" should not be considered as the water content of the food product.

International and national standards define the permitted moisture/water content threshold for commercially sold products. Regulators and standards such as the BRC (British Retail Consortium), IFS (International Featured Standards), GFSI (Global Food Safety Initiative), ISO (International Organization for Standardization), and AOAC (Association of Official Agricultural Chemists) are increasing regulations on the production, processing, and sale of foods. This increases the work needed for quality assurance and quality control and demands more modern and efficient solutions.

Many food producers have strict criteria for measurement accuracy, reliability, and traceability to ensure product quality. These SOPs (Standard Operating Procedures) describe sample preparation and measurement processes, including the sample volume, number of required measurements, maximum tolerable deviation, and procedures when scientists find errors.

QC scientists use gravimetric, chemical, and physical methods to determine moisture content or water content.

• Thermo-gravimetric methods measure changes in the mass of a sample being weighed before and after drying. The term loss-on-drying refers to the total loss of mass. With further analysis, scientists can determine whether water, other volatiles, or only decomposition products have been released.
• Chemical methods involve a chemical reaction that takes place between water and a water-specific reagent. The amount of water converted during the reaction is proportional to the amount of reagent consumed. The most well-known method based on this principle is Karl Fischer's titration.
• Physical methods utilize isolation techniques or physical properties of water to analyze water content. These indirect methods of measurement require calibration against a reference standard or primary method (e.g., Karl Fisher titration) before being used to measure moisture. A secondary method (e.g. Near Infrared Spectroscopy) can only be as accurate as the precision (repeatability) of the method used to perform the calibration.

METTLER TOLEDO offers instrumentation and software for thermo-gravimetric and chemical reaction-based methods.

The drying oven method and the moisture analyzer work according to the thermogravimetric principle referred to as loss-on-drying. They measure moisture or dry content, and the results are not specific to water.

The principle of measurement with the drying oven is simple. It compares the change in mass of a sample before and after drying and assumes that any weight loss is moisture.

METTLER TOLEDO MS-TS and XPR balances contain a differential-weighing application and can streamline the loss-on-drying workflow.

A moisture analyzer consists of two components: a balance and a heating unit. The measurement process starts with recording the sample's start weight. Then a halogen lamp heats and dries the sample while the integrated balance records the sample weight. When the sample stops losing weight, the instrument automatically shuts off and calculates the moisture content using the weight loss. Technicians can quickly obtain results in 2–20 minutes with a moisture analyzer.

METTLER TOLEDO HS 153 or HC 103 Moisture Analyzers are easy-to-operate, provide fast and reliable moisture content results, and are robustly built to ensure a long lifespan. They are ideal for use in the laboratory and in production. The HX204 Moisture Analyzer is the best option for samples with low moisture content and/or highly regulated industries.

Loss-on-drying using a drying oven is often the reference method for moisture determination. With a Halogen Moisture Analyzer, scientists can quickly determine moisture content using the gravimetric principle. A Halogen Moisture Analyzer can replace the drying oven method if the results of the two methods are comparable.

Thermogravimetric analysis (TGA) is another option for moisture determination. TGA utilizes a furnace and an ultra-micro balance to continuously weigh a sample while heating it in a controlled atmosphere. It offers data as a function of time or temperature as the sample is heated, which provides much more information than the loss-on-drying. A stand-alone TGA can only determine the moisture content of a sample, but it is possible to determine a material's water content or to identify the gases released during the drying process by coupling a TGA with an evolved gas analysis system.

The Karl Fischer method is an accurate and recognized reference method that is selective for water. It is suitable for trace analysis (coulometric titration) and high-water content (volumetric titration). With the opportunity for extensive automation using suitable sample changers, the process allows for very high sample throughputs.

• The Titrator Excellence T5, T7, and T9 accommodate a wide range of titration applications, including coulometric and volumetric Karl Fisher.
• Volumetric Compact Karl Fischer Titrators accommodate a wide range of water content applications, with fast and precise determinations from 100 ppm to 100% water.
• Compact Coulometric Karl Fischer Titrators offer particularly fast and precise titration of samples with a low water content from 1 ppm to 5%.
• EasyPlus KFV specifically targets the needs of the food industry for volumetric Karl Fisher titration, with precise determination from 100ppm to 100% water.
• The In Motion™ Karl Fischer Oven Autosampler enables automated handling of samples and gas phase extraction for Karl Fischer titration

Selecting the most suitable method depends on the physical and chemical properties of the sample, such as the type, volume, number, presumed moisture content, application needs (e.g., measurement speed, automation), regulatory requirements (e.g., accuracy), and further criteria specific to the product or manufacturing process.

Gravimetric measurements are always loss-on-drying methods and yield samples' total volatile content (including water). In contrast, Karl Fischer's titration or physical methods are specific to water and determine the water content of samples.

Gravimetric methods are more suitable for materials where moisture is only weakly bound to the material. In honey, for example, water is strongly bound to sugar molecules by hydrogen bonding. As a result, completing a gravimetric determination of the moisture content is time-consuming, whereas Karl Fischer can determine the water content in honey in just a few minutes.

Choosing the proper method of moisture determination can be delicate, and there is no one-size-fits-all solution. However, if you answer yes to the following questions, then Karl Fischer is a better option than loss-on-drying.

Do you need to measure only water content?

Does your product contain more than trace amounts of volatiles, which can distort the moisture reading?

Does your product have a very low moisture content?