Moisture Content and Analysis - An Overview

It is recommended that time is taken when trying to select a weighing instrument. A model may draw people in with an interesting design or novelty functions. Yet, it is the measurement of speed and accuracy that matters.  The methodology is the most crucial issue when it comes to accuracy.

Hi-tech weighing solutions are ordinarily supplied with appropriate methodology either in the form of a document or a face-to-face training package, which then facilitates a mutual exchange of information and experience. Such training helps to win over the customer's trust and form a strong bond between the purchaser and the manufacturer. The distributor is expected to have a thorough knowledge of all technical aspects of their product.

The correct measurement methods are required by moisture analyzers, which are an example of a complex device. A lack of knowledge of the methodology required in combining weighing and drying processes can bring about extremely surprising results. Such results could lead the operator to draw incorrect conclusions concerning the instrument used. As such, it is necessary to explain how a moisture analyzer operates with focus on the usage, metrological and legal aspects.

Significance of the Moisture Content in the Industry

Many raw materials and food products are made up substantially of water. The quality, nutritious value, food flavor, consistency and shelf-life period of products are all determined by water content. As such, there are respective standards in place to enforce the correct amount of water along with food monitoring procedures. Water is held in many products and can take one of two basic forms:

Unbound water - water appears mainly on the very surface and is not bound with the product subsurface. The rate of chemical reactions that occur within a particular product is influenced by unbound water.

Bound water - water is permanently bound with the product and higher temperatures are required for it to evaporate. It has different characteristics to those of unbound water. Plaster stone can be used to test bound water content.

The moisture content of a product is not in itself of interest. However, the analysis of it may lead to interesting conclusions concerning the following features:

  • Clump and cake properties (powders)
  • Microbiologic stability
  • Flow properties, viscosity
  • Dry mass content
  • Product concentration and purity
  • Class (conformity with standards)
  • Possibility to use the material in a manufacturing process
  • Product's nutritive value
  • Conformity with technical, standard and legal regulations

To determine a sample’s moisture content, various methods can be used within the industry. Specific technical solutions are enforced regardless of the method, as speed and efficiency are both required. With food products, it is possible to use:

  1. Thermal drying methods
  2. Azeotropic distillation method
  3. Densitometric methods
  4. Refractometric methods
  5. Chemical methods
  6. Electrical methods
  7. Nuclear magnetic resonance method (NMR)

Technological development and innovations have resulted in a diversity of methods. Results should be comparable to the real value regardless of the procedure and how much the mechanical designs of the devices used to differ. Proving that a specific procedure is correct requires the results to be compared with results obtained using reference methods (standard-specified). This is all part of the validation process. The tests are carried out for the same sample, prepared and stored as specified by respective regulations. For determining moisture content, there are standard-required methods that need to be used including:

  • Moisture analyzer method (convection heating of the sample)
  • Karl Fisher titration method

The moisture analyzer method is a process of weighing, drying and reweighing the analyzed sample. A weighing device and an oven with adjustable temperature are both required for this. With regards to sample preparation, the method is characterized by its own set of rules. Coulometric or volumetric titration is applied to determine the number of water molecules using chemical reaction.

There are many solutions using microwave in addition to the two described above. The drying process is based on rotating the sample's dipolar molecules (water mainly). The molecules align in parallel with the electric field forces. Dipolar molecules relocation is caused by enforced rotation of the sample or electric field alternation, which, in turn, leads to molecular friction. Thermal energy is emitted within a sample as a consequence and moisture content is reduced.

All of the obtained results are required to be comparable and, likewise, regardless of the moisture content analysis method used. As such, it is believed that the method-related technique may be a crucial issue, especially when it comes to moisture analyzers. As portable universal devices, these are operated in the laboratory, production hall and numerous institutes, organizations and places.

The Notion of Moisture Content

There are multiple approaches to the notion of moisture content dependent on the method used. Methods based on thermogravimetric analysis are characteristic for the process of volumetric heating of the sample. Components such as water, fat, aromatics, organic dissolvents and chemical additives and components are released due to heating. In practical terms, it is not possible to differentiate between the loss of unbound water from loss of other substances. As such, it may be concluded that with this approach moisture should be defined as a group of all sample components that have evaporated during the heating process. A realization of the above enables an objective assessment of the usefulness of the moisture analyzer operated as a device intended for moisture content analysis.

To determine moisture content, it is also necessary to determine dry mass content. When water is evaporated the substance left behind is called dry mass. It provides information on the real state of the sample upon performing the drying process.

Moisture Analyzer as a Versatile Device

Mass measurement is required to be carried out at least twice in the thermogravimetric analysis of moisture content determination. With some standard-specified methods, the balance and oven are treated as separate devices. This balance-oven set is not a portable instrument and this may be a big disadvantage for some users.

A dual-function solution to this is to use a moisture analyzer is a device comprised of two integral modules - a weighing module and a heating module. The operator can then use this to precisely determine the mass of any sample, the one to be dried and the one to be weighed without being subjected to the drying process. However, the device is drastically different from typical weighing instruments offered on the market as the drying chamber influences the moisture analyzer design. This then imposes some limitations.

It is possible to determine moisture content automatically with the heating module turned on. Two interrelated operations can be performed simultaneously with the moisture analyzer - mass measurement and temperature measurement. Numerous specialists operating in many areas of economy, for example, chemical industry, agriculture, etc., have long desired such functionality.

Moisture analyzers of MA.R, MA.3Y series

Photo 1. Moisture analyzers of MA.R, MA.3Y series
(in the background: IR – emitter, HAL – halogen lamp)
Image Credit: Radwag Balances & Scales

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Last updated: Nov 29, 2019 at 9:53 AM

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