Moisture measurement in coffee and cocoa beans

Picture: Portable moisture meter, suitable for fast, multiple measurement. The instrument simply is placed on top of the sample

Microwave technology provides a very quick and accurate way for frequently repeated moisture checks with easiest sample handling.

Moisture measurement in foodstuffs – The water content in coffee and cocoa beans, as well as in many other foodstuffs and pre-products, is crucial for quality, processibility, suit­ability for storage and last but not least for the product’s value. Several methods are available to quantify the water content in solid products, each of them with advantages and disadvantages. A basic knowledge about indus­trial moisture measurement is helpful for any person who is in charge of food production or quality control.

Direct measurement by loss on drying or chemical reactions – Calculating moisture from the loss of weight of a sample during drying is a widespread method. In addition to weighing out before and after heating, many products need to be ground to speed up the drying process or to enable drying at all. Other moisture measuring methods use a chemical reaction which consumes all the water in a sample. Here again, samples have to be ground to access the water inside beans or grains. The time necessary for sample preparation and drying often pre­vents from monitoring moisture as closely as desired.

Indirect measurement with electromagnetic and optical sensors -A completely different approach to quantify moisture is to measure material properties which are influenced by the sample’s water content. Several methods utilize the interaction between electromagnetic fields and water molecules and measure e.g. electric conductance, dielectricity or attenuation. The product sample has to be filled into a sensor vessel or has to lie on top of a planar sensor. Instruments require cali­bration, which may vary for different products. As the field penetrates the product and quasi looks into the core, no grinding of the sample or expelling of water is necessary. Measurement is fast, some instruments are suitable for online applications. However, most indirect methods are very sus­ceptible for variations of sample quantity and product density. Process-related compression of the product can cause a falsely increased moisture reading.

Near infrared (NIR) measurement can be calibrated to detect water content as well as fractions of protein, fat or fibre of a food product. As an optical method it has the advantage of non-contact operation. The disadvantage is that only surface moisture is measured and thus the method is not very suitable for whole beans and other chunky foodstuffs.

Combining advantages –A method for fast and accurate moisture measurement with very easy handling is provided by microwave technology.

A moisture sensor generates a low-power microwave field which penetrates the sample. Different to conventional methods, two parameters of a sensor-specific microwave resonance are measured and analyzed by the instrument.

Thus a moisture reading is obtained which is independent of varying product density or sample quantity.

Picture: Microwave moisture sensor for samples of green coffee beans. No weighing, grinding or other preparation of samples is required, moisture measurement is independent of the sample’s density.

The water content is detected on the surface and also inside the product.

No weighing, grinding or other preparation of the sample is required. The moisture reading is instantly available. Samples are not heated, consumed or damaged and can be used for further processing or analyzing. Installation of sensors is possible also at conveyers, in chutes and silos.

Microwave resonance moisture analyzers are established with many applications for cocoa beans with a moisture range of approx. 2% - 6%, green coffee beans with 8% - 14% and roasted coffee with 2% - 6%. In addition to desktop sensors for samples, online systems for installation in process lines are available, e.g. for automatic continuous moisture measurement in large scale coffee roasting facilities. Green coffee beans can be measured also with portable instruments with good accuracy.

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