Determining Moisture Content

Table 1 (click to enlarge)

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

Table 2 (click to enlarge)

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.