The final product of roasting is only partially a function of the roast. As with any cooking, the ingredients are also of major importance. In this case, there is only one ingredient, so its quality will have a major effect on the final coffee
The only way to determine the quality of coffee, even among professional roasters, is to actually taste the coffee. A formal process called "cupping" is performed by professionals who usually have to evaluate several samples. This is a good experience for anyone interested in coffee and many roasters offer public cuppings. However, there are some physical measurements that can be taken to determine how to best roast the green coffee.
The main physical attributes that concern the roaster are:
- Density. This is especially important in predicting how a coffee will roast. If the roaster has a scale with an accuracy of 0.01, the density of the green coffee can be measured by (1) weighing a 500 milliliter beaker or cup, (2) filling it with the green coffee, (3) flattening off the top, and (4) weighing the beaker with coffee. Figure the density by subtracting the weight of the beaker from the to total weight and dividing the result by 500.
- Moisture. This is also important in predicting the timing of the roast since it is mainly the moisture that is responsible for conducting the heat into the bean. Too much, however, and much energy and time will be necessary to drive off excess moisture.
- Color. This can give a clue as to the amount of moisture and the type of processing the coffee went through in the growing country.
- Bean size distribution. Larger beans are more conductive due to their larger surface area, but it will take longer for the heat to penetrate to the center. As a result, the smaller beans roast darker given the same amount of time. All green coffee lots have a variety of bean sizes. In combination with density, the bean size distribution determines how quickly the bean will roast.
How are green beans different?
Checking out the green coffee sources, one can see that there are many coffees from different countries ("origin coffees"). What is the difference between these coffees and how will they taste once they are roasted?
As an agricultural product, different varieties of coffee are grown under different conditions. This results in different compositions that become flavor attributes when roasted. Some of the most important factors are:
- The species and variety of coffee are often selected in response to existing growing conditions. The Cofea Arabica species is referred to the Cofea Canephora (also known as robusta) species. Within the Arabica classification are different varieties. The Bourbon and Typica are the oldest varieties and are the source of other mutations and hybrids: the Caturra variety is a dwarf variety of the Bourbon, the Maragogipe (known for its large beans of low density), and the Mara-caturra is a hybrid between the two. The Catimor types were developed for disease resistance and are generally considered to be inferior (Wintgens, 2004).
- The altitude at which the coffee is grown affects its taste profile and physical attributes. Beans grown at higher altitudes have a slower rate of maturation resulting in a denser bean and a bluish-green color (Wintgens, 2004, p. 794). Use of shade has a similar effect on flavor (possibly also due to slower rate of maturation).
- The ripeness of the cherry during harvest has an influence on the amount of sugar in the green bean (Bradbury, 2001) and, as a result, the sweetness and sugar-browning capacity of the green coffee.
Once the coffee has been grown and harvested, the outer fruit and an inner substance called "mucilage" is removed from the seed and dried to a moisture content at which the coffee can be shipped, about 10-13%. This is called "processing" the green bean and happens in three main ways (and at least one additional unique way).
- In the wet process, enzymatic action is allowed to remove the outer layer of mucilage, usually but not always under water. In some cases, the mucilage is removed mechanically using water under pressure. Such coffees are usually more acidic and have more delicate aromatics. The color is usually green to blue-green.
- The natural process allows the cherry to dry to the seed and then removed. The coffee seed is allowed to dry to the final moisture content. Natural (also sometimes called "dry processed") coffees have less acidity and aromatics but a heavier body and often have a "fruity" slightly over-ripe quality. These are often chosen for espressos. The color is yellow to yellow/green.
- The semi-pulped process is somewhere in between the wet and natural process. The fruit is removed and the mucilage is allowed to dry to the bean. The resulting coffee has a little more acidity than the wet process, but is mainly noted for its body and sweetness. The color is green/yellow to deep green.
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