One of the principal postharvest processes is drying that gives rise to the formation of the characteristic color, flavor and taste of coffee brews.
There are many quality parameters for coffee trading. Among them, moisture content has been regulated as one of the quality standards for coffee green bean by most of importer and exporter countries. The appropriate range of moisture content is 8.0 to 12.5% based on fresh matter.
Green coffee that is high in moisture (greater than 12% wet basis) can deteriorate due to bacteria, mold, or yeast, especially if the seed is killed. If the seed remains alive, enzymatic activity will cause the cupping quality to change.
When the moisture content of the coffee bean is below 8%, it can be shrunken bean and unwanted physical appearance. In any case, the parchment coffee moisture level should be lowered to below 12% soon after harvest.
Therefore, an accurate and fast moisture content determination for the coffee bean is essential for the coffee sectors.
The coffee cherries are dried immediately after harvest. This is usually sun drying on a clean dry floor or on mats. The bed depth should be less than 40mm and the cherries should be raked frequently to prevent fermentation or discoloration.
The initial moisture content of harvested coffee is about 55-60% and after drying lowers the moisture content to around 12%.
When the ambient relative humidity is about 70 percent, coffee beans will gradually equilibrate to about 12 percent moisture. Thus green bean coffee is generally dried to 12 percent, and bought and sold at this moisture percentage.
After fermentation, the coffee is known as 'parchment coffee', since the seed retains its endocarp layer. It must be dried to about 10-12% (wb) moisture content to ensure stability.
The gravimetric method is widely accepted as a reference method to determine the moisture content for all coffee trading countries. A convenient sample size is 100 g initial weight. The samples are placed in the oven at 220°F for 24 hours. They are then weighed again (final weight). The difference between the initial weight and the final weight is the water, and it is also the percent moisture in the sample if started with 100 grams.
Alternative methods for rapid determination are also done and one of them is Near Infrared Spectroscopy (NIRS). NIRS is an increasingly growing non-destructive technique due to its rapidity, simplicity, and safety.
Moisture content of coffee bean
