The best coffee moisture analyzer for field use

Besides having an impact on storage life, there are definite financial considerations that need to be considered relative to moisture levels in coffee, starting with the farmer, who, like everyone else gets paid by the pound.

Generally the buyer will establish parameters for acceptable levels of moisture in the coffee beans they buy. It behooves the farmer to sell coffee with higher moisture levels, because they get paid by the pound. But if the coffee is too moist, the truck gets sent back. Still many smaller farmers rely on ."bean biting" to give a quick on site analysis as to the moisture level in the coffee bean.

Usually only a senior employee or company principal, are allowed to indulge in the risky practice of verifying bean moisture by placing a bean between ones teeth, and biting gently to gauge the hardness.

With the advent of the specialty and gourmet coffee industry, quality control has become more precise. Laboratory grade equipment is now used to measure moisture levels in just seconds. Portable moisture analyzers like the SINAR MOISTURE SPEAR, and MOISTURE MAC are used along the transport trail of the coffee bean. Buyers have a vested interest in whether they are buying product or water. Low moisture levels in coffee can also shed insight in to improper storage or, "old inventory". High levels of moisture create a bacteria friendly environment, allowing mycotoxins to develop. Moisture levels of between 8 and 13% are recommended for safe transportation and storage of coffee. (Clarke, 1985; Reh, Gerber, Prodolliet, & & Vuatez, 2006)

This 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)

In the study, LOSS ON DRYING, or Gravimetrics, was used as a comparison to Karl Fischer. Some of the problems encountered were loss of moisture from grinding (This can be easily overcome by weighing the sample before and after grinding, and factoring in the % loss), loss of volatiles (oils) during heating, evaporation rate variances from not grinding,

as well as KF Reagent Inadequacies. 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. Advances in 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 would have great 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.

Coffee, because it is free flowing, and hygroscopic, is an ideal sample type for a capacitance based analyzer

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.