There are three major methods in use for the processing of coffee: dry, wet, and semi-dry. The oldest method, also known as natural, or unwashed, is the dry method. Before the beans undergo this method, they are sorted and cleaned. Dirt, twigs, soil, undesired beans, and other debris are removed by winnowing with a sieve and/or by placing the harvest in a washing channel and allowing the ripe cherries to sink to the bottom and the rest to float to the top. Once clean, the coffee cherries are placed on a patio surface such as concrete, or on a raised table allowing hand access. Here they are allowed to dry in the sun. They are raked or turned by hand for even drying. The length of time it takes to reach the right moisture content will depend on the weather. Mechanical dryers are used by some processing facilities to speed up the process once the coffee has spent some initial drying time in the sun, usually a few days. It is important that the cherries are neither too dry nor too moist as too dry makes the beans brittle and easily breakable, and too moist leaves them vulnerable to fungi and bacteria. Once the cherries are dry, they are stored in silos until they are sent to the mill for the next processing steps which include hulling, sorting, grading, and bagging. Whereas the outer layers of the dried cherries are removed by the hulling machine in these later stages of the dry process, in the wet (or “washed”) process, the fruit covering the bean is removed prior to drying. In the wet process, the cherries are soaked in containers of water. As in the cleaning stage of the dry process, the ripe red cherries sink to the bottom while the unripe float to the top. The fruit is pressed through a screen, removing the cherry skin and some of the pulp. Not all of the pulp is removed from the beans at this stage so further steps are required. There are various methods to do this, including the ferment-and-wash method. The wet processing method is more expensive, requiring extensive use of water and special equipment. A layer of skin and parchment still remain on the bean after the de-pulping. At this time, the beans are dried either in the sun or by machine, or both. They are brought to the right moisture content. Once dry, the parchment is crumbly and removed easily during the hulling process. The extra work and costs that go into this method of processing the beans are justified because the connoisseurs of coffee consumption find the “washed” coffees to provide a smoother and rounder cup of coffee, and beans processed by this method generally can demand higher prices. The outside layer of the coffee fruit is a skin called the pulp. The next layer is mucilaginous and surrounds the two beans. In addition to this layer surrounding each bean individually, there is a tougher “parchment” as well as an inner, more delicate “silver skin.” The beans of Arabica and Robusta plants have two distinguishable shapes: The Arabica is a flat, oblong bean with a crooked furrow, while the Robusta is rounder and convex with a straight and even furrow through the center of the bean.