How Green Coffee Quality Affects Roasting
MOISTURE CONTENT
In roasting, the moisture content of the green bean plays an important role. Under normal conditions, green coffee beans have a moisture content of 10–12 percent. The moisture content will fluctuate freely with the relative humidity content of the ambient air. In cities like Amsterdam and San Francisco, relative humidity levels throughout the year are nearly perfect for storing green beans over a length of time, and for slowing down the aging process of green coffee. This also reduces the likelihood that the roaster operator has to change roast profi les to compensate for possible variances in green coffee moisture.
The moisture inside the green beans is partially free or is present as bound moisture and contained in the carbohydrate molecules. bWe can summarize the roasting process as a three-stage cycle:
The drying phase is when the moisture content of the coffee is reduced to about two percent. During this phase, the “free” moisture—the residue of the process from cherry to green bean—evaporates. Free moisture also plays a role in the heat transfer during roasting. As soon as the beans are energized with heat, the bean’s moisture conducts this heat through out the bean. When the internal bean temperature approaches 212 degrees F or 100 degrees C., the free moisture starts evaporating. In the second phase, from the first crack to the second crack, coffee beans develop their specific aromas and flavors, which, as coffee tasters know, can produce a very complex taste profile. At the end of the second phase, all free moisture has evaporated. The length of the second phase depends on the roasting degree, which can vary from region to region and from product to product. With very dark roasts, there is also a third phase which starts when the second crack is almost completed. During this phase, carbonization takes place and the bound moisture is destroyed.
Beans with a moisture content of less than 10 percent have a sharply reduced free moisture level and will tend to roast much faster, especially in the first phase. In this case, the roaster operator needs to change the roasting profi le by initiating the roasting process at a lower heat level and by maintaining a lower amount of energy supply (less BTUs) during the first roasting phase.
Beans with a high moisture content (fresh crop coffees can have a moisture content in excess of 14 percent) often require that the roaster operator includes a pre-drying phase before starting the first phase of the process. During pre-drying, it is recommended that the roaster maintains a drum temperature of 300 degrees F or 148 degrees C. with the objective to slowly remove the excess free moisture. The actual phase one of the roasting process can begin as soon as the beans start losing their deep green color.
CELL STRUCTURE DENSITY
Lower grown beans generally have a less solid bean structure than higher grown beans. The density of the bean structure is revealed by the shape and the position of the center cut. Bean from Kenya, which was grown at an altitude of at least 5,500 feet or 1650 meters. The center cut is tightly closed and almost seems to be floating in the upper layer of the bean. In sharp contrast, Robusta bean, grown at almost sea level. In this case, the center cut is widely opened and draws like a deep crevasse through the coffee bean.
What is the relationship between bean density and roasting? High-density beans have a denser cell structure and more cells per cubic millimeter than low-density beans. As a result, high-density beans are more resistance to heat, which will be especially noticeable during the first phase of roasting. After the evaporation of free moisture, the color of the coffee beans starts changing from (light) green to yellow to light brown. During this color change, the bean starts expanding. With lowerdensity beans, the centercut will open more quickly, allowing for a faster transfer of heat, which will accelerate the process even further.
Source : ROAST MAGAZINE
JAVA PREANGER COFFEE ADDICT
Fadillah Satria
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