Determining Moisture Content

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.

Advantages. Two important advantages gained from using a drying oven are sample throughput and flexibility in regards to sample volumes/sizes. This method also produces very precise results while being cost effective (see Table 1).

Disadvantages. This method requires extended heating periods and cooling phases, meaning it usually takes hours to produce results. Procedures are laborious and tedious, involving many manual steps. Therefore, the potential for error is high, since weighing is performed manually and in separate stages during the drying process as opposed to a moisture analyzer with an integrated precision balance which allows for the continuous and automatic recording of results. Typical pitfalls include mixing up samples, manual transcription errors or the miscalculation of weighing results. The risk of committing such errors increases with large sample volumes (see Table 1).

Moisture Analyzer

Moisture results can be obtained more rapidly using a moisture analyzer. The measurement principle does not differ from that of the thermogravimetric method. The main distinction lies with the type of heat source used: In the oven, samples are heated by convection while a moisture analyzer heats samples via the absorption of infrared energy.

Advantages. The most important advantage is the rapid measurement time, thanks to the efficient heat source. Results can be obtained within 2–10 minutes. Samples are heated quickly and evenly, and obtained measurements show good repeatability. Handling is also straightforward and the risk of error is reduced (see Table 1).

Disadvantages. All thermogravimetric methods, including the moisture analyzer, carry the risk of decomposing constituents or the loss of volatile components during heating. This results in a further decrease in weight, which is not explained by the release of water. Finally, samples can only be measured one at a time and the automation of measurements is not feasible (see Table 1).

Halogen Technology

The technology of halogen drying can measure moisture content in virtually any substance. Halogen technology uses a halogen heating device in combination with an integrated precision balance for the measurement and recording of sample weight before, during, and after the release of moisture. Thanks to its innovative heating technology, halogen moisture analyzers (HMAs) are capable of producing fast and precise measurements.

Furthermore, automated moisture determination eliminates transcription and calculation errors. Most HMAs offer a number of predefined methods, which can be stored and easily accessed via the display menu. Some manufacturers also allow users to set individual user rights to ensure that quality criteria are met. The calculated results are stored in the instruments or can be printed out or transferred to PC via USB or other interfaces.

Reference Methods

Reference methods are of much use to food manufacturers who must comply with legal requirements for the maximum or minimum amount of water present in diverse foods. Up until now, moisture content determination in a drying oven is the established reference method. Values determined by other methods must, therefore, always be referenced against the LoD method in the drying oven.

METTLER TOLEDO, as an example, has a library of validated measurement methods for over 100 food products saving users time in developing methods for different food specimens. If a substance is not included in the library, it is possible to adapt a method from a comparable food sample. For instance, Table 2 compares procedures and results for moisture analysis in ground hazelnut using a drying oven and METTLER TOLEDO HMA. Based on six measurements, the mean value of the moisture result was calculated. The results reveal that a moisture analyzer produces identical results to a drying oven. In addition, the standard deviation for both methods is comparable and very small.

Conclusions

Moisture content is a critical indicator of food quality, safety, and shelf life, thus moisture analysis serves an important quality control function in various stages of the food production chain, from raw material testing in the laboratory to incoming goods inspection. Several analytical procedures are available to measure moisture content in diverse food samples. Selecting the correct procedure for a particular sample or application is pertinent to the food industry’s success since the accuracy of moisture measurements are highly dependent on the analytical method used.

Compared to the traditional drying oven, faster determination of moisture content via LoD can be achieved using alternative methods. For example, an HMA is straightforward to operate and produces reliable results in just 5-15 minutes, compared to 2-4 hours when using a drying oven. In addition, the automation of weighing measurements and calculations allows for fully compliant and reliable results.

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Dr. Appoldt is head of strategic product group moisture at METTLER TOLEDO. Reach her at [email protected]. Raihani is global lab marketing at METTLER TOLEDO. Reach her at [email protected].