Irrespective of the harvesting method, green coffee beans and overripe coffee cherries inevitably end up mixed with the perfectly ripe cherries and must be separated during coffee processing. Overripe coffee cherries, undeveloped coffee cherries, sticks and leaves float in water. Ripe coffee beans and green coffee cherries are dense and sink. Therefore, the first step in coffee production consists of separating the “floaters” from the “sinkers.” The coffee floaters are usually sent directly to the patio to be dried and are often slated for internal consumption. The ripe and green cherries can be sent to the patios to be dried using the natural process of preparing coffee or can be sent to the coffee pulping machines.
Coffee Processing Equipment
The first stage of coffee pulping is used to remove the green coffee cherries from the ripe cherries. In the coffee pulping machinery, the internal pressure is monitored to push the coffee against a screen with holes only large enough for a coffee bean (not cherry) to pass through. Since the ripe cherries are soft, they break and the coffee seed is released through the screen.
The green cherries are hard and cannot be pulped. Instead of passing through the screen, the green coffee beans pass to the end of the barrel system and are separated from the ripe coffee beans. The pressure inside the barrel controls how many cherries will be pulped. A very high pressure will cause all of the cherries including the green beans to be pulped. It is necessary to continuously monitor the pressure so that about 3% of ripe cherries are not pulped and are removed with the green cherries. This margin of error ensures that no green cherries are mistakenly pulped. The pulp and coffee beans are then separated by centrifugal force and a barrel screen system.
For more information about coffee pulping, view this video showing the inside of the pulp separator: medium (1.24 Mb). Video showing another pulp separator in action: medium (4.2 Mb) or small (985 Kb).
The coffee beans covered in the slippery mucilage can be sent to the patios to dry as pulped natural coffees or can be sent to coffee fermentation tanks. The coffee fermentation tanks are used to remove the mucilage before drying. The pulped coffee beans are put into cement tanks with water and are allowed to ferment for 16-36 hours. On the way to the fermentation tanks, another density separation can occur. The highest quality coffees are the densest and should be separated and fermented in a different tank.
The coffee fermentation time depends on a number of factors including the amount of coffee fermenting, water temperature, and humidity. The mucilage is made up of pectin materials including protopectin (33%), reducing sugars including glucose and fructose (30%), non-reducing sugars such as sucrose (20%), and cellulose and ash (17%) (Wrigley, 455). Protopectin is not water soluble and will hydrolyze to pectinic acid in the fermentation tanks (Wrigley, 455). Hydrolysis of the protopectin and degradation of the pectin by enzymes is the process that occurs to remove the mucilage during fermentation (Wrigley, 455). Currently, the best way of determining the end of coffee fermentation is to feel the coffee beans to determine if they are still encased in mucilage. If the coffee beans are fermented for 36-72 hours, stinker beans develop. Lactic, acetic, and propionic acids are produced in this process and are believed to prevent the traditional fermentation taste by inhibiting mold growth that regularly occurs during drying on a patio in humid conditions (Wrigley).
For more information about coffee fermentation, view this video showing the coffee in the coffee fermentation tanks: medium (1.1 Mb).
From the coffee fermentation tanks, the beans are moved to drying patios and dried to 11-12% moisture content. See coffee drying section for more details. A small portion of the lot is hulled and milled by a mini-huller. Three hundred grams of coffee is classified for defects (100 grams is often used), and the percentage of each screen size is determined. Then, 200-300 grams of coffee is roasted in a sample roaster and cupped to determine coffee quality. Ideally no lots will be mixed until the coffee has been classified and cupped. The coffee remains in pergamino until shipment time to help protect the flavor and aroma of the coffee.