In the import and export arena of the coffee sector, moisture levels in
coffee are important, not only because they have an impact on storage considerations, but becuase coffee is most often bought by the pound. A pound of coffee at 14% moisture may not be as good a value as the same coffee at 10% moisture, especially in large quantities.
Roasters also need to know the moisture level in green coffee, prior to roasting. Roast profile software is now standard with most commercial roasters, but if you apply the same program to samples with different moisture levels, the resulting product will be different. To attain true repeatiblility, all variables need to be eliminated.
With the advent of the specialty and gourmet coffee industry, quality control quickly became a hot topic.
Subjective methods like biting a bean to establish it's moisture content, or evaluating roasted coffee color by eye, are no longer acceptable methods. The amount of money that can and is lost from incorrect data, is small in small quantities, but big in big quantities.
Todays newer technologies provide effective moisture determination in seconds, and can be configured to accomodate almost any specific application. For coffee growers, importers and exporters, who store and process raw coffee, it is not practical to conduct oven tests when moisture determination is called for. Instead, capacitance based (INDIRECT METHOD) analyzers are often used.
The SINAR MOISTURE SPEAR , for example, is designed to be inserted in to large piles of grains, or coffee beans, either in bulk, or through the tops of sacks.
Capacitance based moisture analyzers like this hand held
Understanding some of the unique challenges in measuring moisture accurately is a little more of an elusive persuit. Many of the issues were brought to light in a study titled, "A comparative evaluation of methodologies for water content determination in green coffee", published by Elsevier LTD. 2006 Swiss Society of Food and Science Technology.
That study compared only direct methods of moisture determination. Indirect methods of moisture determination like capacitance, are popular because of their speed and accuracy, but the results they provide are based on direct methods. A capacitance based instrument, like any other indirect method of analysis, is only as good as the data programmed in to it. (See CREATING CAPACITANCE CALIBRATIONS)
LOSS ON DRYING, or Gravimetrics, was used in comparison to Karl Fischer. One of the problems encountered in LOD was the loss of moisture and / or volatiles from grinding (This can be easily overcome by weighing the sample before and after grinding, and factoring in the % loss
Also, the lines between bound, unbound, and total moisture become somewhat blurred with gravimetrics. Unbound moisture evaporates at a somewhat constant rate, relative in part, to the saturation of the materials surface. Evaporation of bound moisture occurs at a dramatically slower rate. This change over point is referred to as the critical moisture point, and is much more difficult to ascertain. In fact, most moisture balances are automatically default programmed to shut off at the critical moisture point, leaving unbound moisture unaccounted for.
Most, if not all of these issues have been resolved in the fairly complex standard ISO 1446:2001 E, titled,
"Green Coffee-Determination of water content-Basic Reference Method", which utilizes an enhanced Loss On Drying method.
KARL FISCHER TITRATORS, and reagents should provide similar results with the use of either an
in vessel homogenizer, or evaporator oven.
Either would be suitable as a reference method, but both require costly instrumentation and operator training, as well as being extremely time consuming. Research facilities, independant laboratories, and large corporate quality control labs might have more use for this type of instrumentation to establish benchmarks.
Similar results can be obtained by using infra-red moisture balances. Advances in heating technology, like the MID INFRA RED MOC 120 H, allow for effective heating of samples like coffee, with less sample degradation and reactance to color, by using specific wavelengths of Infra-Red Energy. Simpler methods like placing a non burning cloth like a glass fiber sheet on top of the sample to prevent surface burn, may also be employed. The drying proccess can be customized on these instruments to start out at a higher temperature, and then decrease automatically as the sample reaches a lower moisture level, where volatile burnoff becomes more of an issue.
By taking advantage of the electrical properties of water, Karl Fischer Titration avoids many of the pitfalls of gravimetrics, like volatile burn off and sample degradation.
Capacitance based analyzers do something very similar, by emitting a small electrical field in to a defined space, where the sample is located, and then measuring very accurately, albeit with it's own set of pitfalls, the reaction to available water. These instruments are very fast (1 second in some cases) and usably accurate, (Tolorences range between +/- .1% ot +/- .5%) as well as able to read total moisture, like Karl Fischer Methods.
However, because some coffee beans are larger than others, like many samples from Hawaii, the packing density as well as amount of available moisture can vary. By utilizing a fixed volume sample cell, and weight correction with a balance, like the Sinar AP 6060, issues with packing density are virtually eliminated. Fixed volume capacitance analyzers that do not incorporate a balance, may require a small calibration adjustment for beans that fall outside the parameters of the average size range.
Capacitance based analyzers also require almost no operator training, and are comparitavely inexpensive, ranging from around $400 to $3,000, depending on the features, capabilities, and versatility of the particular instrument.
PROFESSIONAL LABORATORY INSTRUMENTATION FOR COFFEE, TEA, COCOA AND AGRICULTURE
www.coffeelabequipment.com
Po Box 71
Weems Va 22576
Tel (804) 435-5522 Toll Free (866) 244-1578 Fax (703) 991-7133
www.coffeelabequipment.com
Low cost field analyzers like this COFFEE PRO MOISTURE MAC are an inexpensive, but effective way, to ensure coffee moisture levels are adequate for shipping Moisture levels of between 8 and 13% are recommended for safe transportation and storage of coffee. (Clarke, 1985; Reh, Gerber, Prodolliet, & & Vuatez, 2006)
This Sinar Moisture Probe is programmed to provide moisture results, for green, parchment, dried cherry, and roasted coffee beans, in seconds
THE MR DA8 Coffee Moisture Analyzer also works on most coffees, but the 6" probe tips are specifically designed to be inserted in to the side of a burlap coffee sack. In a large warehouse with hundreds of bags of green coffee, this could be especially useful.
