Case Study

Moisture in Coffee Equals Quality

by Jason Coleman
CSC Scientific Company, Inc.

Moisture in coffee beans matters. Moisture content indicates the maturity of the beans, the amount of drying the beans require, and the weight loss of the green beans during storage and roasting, all of which decide the quality of the finished product.

Determining the Ideal Picking Time
If the bean is not picked when its moisture content is at an optimum level, there is the risk of deterioration from environmental factors. For example, the risk of bean discoloration (loss of the green tint and the development of a musty, whitish color) can be greatly reduced, if not eliminated, when beans are harvested within their optimum moisture range (20-30% moisture). Measuring moisture in the fields to determine harvest schedule is an important element in this first step of the process and has implications for subsequent production techniques.
Picking the coffee beans during harvest

Growers need to constantly monitor moisture content in order to determine the best drying time and procedures. For example, green beans in Indonesia will typically be higher in moisture content than green beans in Columbia because of that region's higher humidity. Consequently, different schedules and techniques are needed to improve the drying results.
The concrete patio drying method

Frequently, two loads of green beans from the same region have different moisture contents. This may result from different drying methods or varying weather conditions. For example, three different growers advertise that they use the gentle drying process. However, the grower in Kenya uses a concrete patio, the grower from Ethiopia uses netting, and the grower from Ghana uses a drying table. If the beans' moisture is not monitored, wide variations in the resulting product will occur.

Shipping and Storage
Exporters and importers know that one of the keys to cost effective purchasing is knowing the moisture levels of the product at the shipping destination. The moisture level in green coffee at this juncture not only affects processing and taste but also is a significant factor in price.

During shipping, coffee beans are exposed to wide variations in temperature and humidity. These environmental changes often result in condensation and other exposure to the source of absorbed moisture. Because it is difficult to control these environmental factors, obtaining the correct moisture levels at the completion of the drying process is of paramount importance. If the beans are too dry (below 9% moisture), they will immediately begin to lose flavor. If they are too wet (above 13% moisture), the beans will mold easily.

Getting the Best Roast out of the Bean
The moisture content of the green bean determines how the roaster is set, which decides the taste and consistency of the final blend. For example, roasting a Colombian with a moisture content of 9.5% with roaster settings of 400 °F for 2 min produces a standard product. If the next Colombian batch had a moisture content of 12%, the same setting would produce something different from the standard product. With knowledge of the second batch's moisture content, adjustments in roast time and temperature can be made to produce the standard product.

Measurement of moisture content is a principal way to determine shrinkage, which determines bottom line. Knowledge of shrinkage data provides realistic factors for determining cost pricing structures.

The impact of moisture in the grinding process is twofold. At high moisture levels the beans will not pass through the grinder uniformly and can cause the unit to clog. At lower moisture levels, dry grounds can create static-electricity problems. The grind size also affects the extraction rate of the coffee. The perceived taste of the coffee is affected significantly if the grind-size is larger or smaller than it should be.

The Sinar Moisture Analyzer
The Sinar AP moisture analyzer from CSC Scientific offers coffee producers a fast and convenient way to measure moisture content at every step in the production process. Battery operated and weighing less than 5 pounds, the analyzer is extremely portable, allowing measurements to be made in the field, at the shipping dock, and in the roasting plant.
The Sinar AP Moisture Analyzer

When the cell is filled and the appropriate commodity program selected, the moisture content (percent) is clearly shown within 6 seconds on the liquid crystal display. In a series of up to 254 samples however, the average moisture content was measured in one second. The unit provides readouts of temperature and bushel weight and offers an RS232 printer output to generate a hard copy.

The design concept of the Sinar AP is based on the simultaneous sensing of capacitance, weight and temperature of the sample being tested, and providing a corrected moisture (percent) reading in a few seconds. This technology allows whole grains, cereals, oilseeds, etc. to be loaded directly into the measuring cell without any sample preparation.

Mark Hopkin, product specialist for CSC Scientific, noted that the Sinar AP requires the sample to be free flowing and not saturated. And some chemicals, salt for example, will interfere with the readings. Because the device is designed to measure moisture between zero and 30%, it is especially suited to these food commodities, rather than applications with moisture rates below 1%.

Up to seven calibration programs can be held in the memory, and these can be individually adjusted when required to optimize accuracy. A security password ensures that only authorized users are able to make calibration adjustments. Once calibrated, the instrument can go anywhere and provide measurements as accurate as a similar benchtop instrument.

Thick polyurethane construction makes the unit a good choice for farmers and growers who operate in harsh and demanding environments. And in terms of maintaining the unit, Hopkin said, "the Sinar AP is essentially maintenance-free."

Additional Technical Specifications:

  • Dimensions: 325 mm x 164 mm x 120 mm
  • Weight: 1.8 kg
  • Operating environment: 0°C to +55°C
  • Storage temperature: -20°C to +55°C. Humidity up to 95% non condensing
  • Construction: Polyurethane Reaction Injection Molding
  • Display: 3.5 character. LCD 15 mm high.
  • Power Supply: Four C size 1.5 volt alkaline batteries
  • Processor: Intel 80C31 microprocessor
  • Memory: EPROM and RAM
  • Measurement range: 1-35% moisture, wet basis depending on applications
  • Accuracy: Typical standard deviation 0.3 for moisture (dependent on application and moisture level) and 1.0 for bushel weight
  • Repeatability: Standard deviation 0.05 to 0.15 (dependent on application)
  • Weight balance: Oscillating weight balance which also operates in non-horizontal position. Measures and corrects from weights between 20-240 grams.
  • Temperature correction: Thermistor sensor connected to microprocessor. The correction is software programmable. Temperature read out and correction 0-50°C
  • Full cell: 290 ml volume
  • Bias facility: Moisture content and bushel weight readings can be biased in increments of 0.1
  • Temperature: °C or °F
  • Specific weight: Hectoliter or Bushel weight readings

For more information contact: CSC Scientific Company, Inc., 2810 Old Lee Highway, Fairfax, VA 22031. Tel: 703-876-4030; Fax: 703-280-5142.