Determining Moisture Content

Credit: METTLER TOLEDO

Moisture content influences the taste, texture, weight, appearance, and shelf life of foodstuffs. Even a slight deviation from a defined standard can adversely impact the physical properties of a food material. For example, substances which are too dry could affect the consistency of the end product. Conversely, excess moisture may cause food material to agglomerate or become trapped in the piping systems during production. Also, the rate of microbial growth increases with total water content, possibly resulting in spoiled batches that need to be disposed of. However, water is also an inexpensive ingredient adding to the weight of the final product. Hence, obtaining an optimal analytical value for moisture is of great economic importance to a food manufacturer. For these reasons, food analysts engage in the delicate balancing of moisture and total solids to ensure consistent product quality, safety, and profitability.

Legal Requirements

International and national standards define the permitted thresholds for moisture content in commercially sold products. Regulatory bodies such as the BRC (British Retail Consortium), IFS (International Featured Standards), or GFSI (Global Food Safety Initiative) heavily influence the production, processing, and sale of foods. For food manufacturers, this translates into increased workload around quality assurance and the development of efficient and cost-effective solutions. According to the stated legal requirements, methods of analysis and procedures must be clearly described and tested. Many food producers themselves have strict criteria for measurement accuracy, reliability, and traceability to ensure the consistent quality of their products. These standard operating procedures encompass the entire measurement process, including sample volume, number of required measurements, maximum tolerable deviation, and procedures for correcting errors.

Water Properties in Food

As mentioned in chapter 6 of Food Analysis by S. Suzanne Nielsen, official methods and procedures for moisture analysis are important since the method used to determine moisture may lead to varying results for moisture content, depending on the form of the water present in a food. In the simplest scenario, water retains its properties by existing “freely,” i.e. it is only surrounded by other water molecules. Free water (also known as bulk water) can be adsorbed on surface particles, held in narrow capillaries, or stored in the pore systems deep within the food material. For instance, dried fruit or meats have complex cellular structures where water is bound by adsorption to the surface or transported deep within the cells by capillary action. Adsorbed water can also become physically bound to other elements present in the food material such as proteins, or exist as chemically bound water (e.g. certain salts such as Na2SO4·10H2O). In a bound state with other molecules, water most often evaporates at a higher temperature compared to free water molecules. Consequently, physically or chemically bound water takes on varying physicochemical properties, making it very challenging for the food analyst to accurately measure the moisture content.

Technologies for Moisture Analysis

A summary of technologies used for moisture determination are listed below.

• Thermogravimetric analysis (oven drying, halogen/IR drying, microwave drying, etc.)
• Chemical analysis (Karl Fischer titration, calcium carbide testing)
• Spectroscopic analysis (IR spectroscopy, microwave spectroscopy, proton nuclear magnetic resonance spectroscopy)
• Other (e.g. gas chromatography, density determination, refractometry, etc.)

This article focuses on thermogravimetric analysis (TGA). Moisture content is derived from the loss of product weight during drying by measuring the change in mass of a sample while being heated at a controlled rate until no more change in weight is observed.

Balance and Drying Oven

The drying oven, commonly used for commercial purposes, is the established reference method for loss on drying (LoD) by TGA. In this procedure, a sample is weighed and subsequently heated to allow for the release of moisture. Following this, the sample is cooled in the desiccator before reweighing. Moisture content is calculated by the difference in wet and dry weight. In this process, measuring accuracy and the resolution of the balance are extremely important. Careful consideration must also be given to maintain identical conditions, where temperature and duration are vital for generating precise and reproducible results.