Post Harvest Processing

Post Harvest Processing

Perhaps the single most important factor in determining coffee quality is the care taken during post-harvest processing. A single mistake can have serious implications, at times, capable of spoiling en entire batch of coffee. But before we discuss the various processing methods, lets take a look at the coffee bean itself. Anatomy of the coffee bean The heart of the bean lies deep within three layers of covering. On the outter most side, we have what is called the 'mesocarp' or otherwise known as the pulp of the cherry. Depending on the plant's variety, it can be vary in color from a deep red or yellow depending on ripeness. Below this layer, we have the 'endocarp' or parchment, which mainly consists of a slimy/gelly-like layer covering the bean. It i this layer rich in sugars that serve as an essential energy source during the fermentation process. Immediately below this layer, we whats called the 'silverskin' and consits of a thin paper-like tissue surrounding the bean. For most of us, this is what we see come off the bean during roasting and is otherwise known as 'chaft'. The Wet Process Step 1: Initial Separation Typically in those countries near the equator where rainfall is consistent and water is readily available, the wet method is the preferred method of processing. In this method, ripe coffee (along with some unripe beans) are harvested, collected  and placed in large water tanks for initial separation. Because of the differences in bean densities, ripe and unripe beans are easily be separated into two lots. Typically overfermented, unripe, or partially dry beans tend to "float" to the surface where they are siphoned off and separated from the "good" coffee - see picture below. These lower density beans are severely inferior in quality and are typically referred to as "floaters". In most countries it is this coffee that is separated and sold to locals for internal consumption. Those beans heavier in density sink to the bottom of the tank where a series of tubes then transport them to the next step of processing.  Although relatively simple, the water separation process works quite well at separating unripe beans, and also serves to separate out sticks, rocks, and any other foreign material that would otherwise contaminate the batch. Step 2: Depulping Now that only ripened beans have been separated out, the next step is to remove the outer skin from the cherry. In this stage, beans are pressed between rotating cylinderical screens thereby tearing the outer skin away. Although most depulpers are mechanical, smaller farms occasionally employ more traditional hand operated depulpers - see picture below. In both cases the friction exerted on the bean allows for the removal of the cherry pulp, leaving only the sticky mucilage layer behind.  The mucilage layer is typically 0.5-2mm in thickness and rich in pectin, sugars and a number of micro-organisms that aid in the subsequent stage of processing - see picture below. Step 3: Fermentation  Now that the beans have been removed of the mesocarp (outer skin) there still remains a sticky mucilage attached to the bean. In order to succesfully remove this layer we'll employ the help of microbes. As the beans remain in solution microbes on the suface begin to ferment and eat away at the sticky mucilage surrounding the bean. The actual lenght of fermentation depends on a number of factors, but is generally  signaled when the farmer takes a sample and rubs it between his hands. If sticky, the coffee is allowed to ferment further - if not, the coffee is ready and proceeds to the washing stage. Although the method is not truly scientific, it works quite well - being a mixture of both art and intuition developed from the farmer's experience. Fermentation not only aids in the removal of the mucilage but also increase the total acidity in the coffee. By allowing small microbes and enzymes to ferment, a great deal of  both acetic and lactic acid are produced thereby increasing perceived acidity in brewed coffee.  The rate at which fermentation occurs is highly dependent on ambient temperature which of course depends on the location and altitude of the farm. Typically, those farms at higher altitude (due to its cooler temperature) require more time in the fermentation tank than one at sea level. Depending on conditions, the beans can be fermented for 18-36 hours, or until the mucilage easily tears away from the bean. In general coffee processed by the wet method is generally have a cleaner cup and higher acidity than the dry processed. As such, they usually tend to fetch a higher price due to the increased amount of labor associated with it. The Dry Process Typically those countries such as Brazil where water is not readily accessible to farms, the dry process is the preferred method of processing. In this process, the coffee goes thru a cursory separation to remove any debris then the cherries are set out on drying patios for anywhere between 2 to 6 weeks. During the process its important that the ripe and semi ripe beans be separated before spreading on patios since this adversly affects quality. For example, moisture contents between unripe beans can be almost double its ripe counterpart. Once the beans have been spread on concrete patios, the layer of coffee must be rotated daily to ensure uniform drying and prevention of mold development. Once dried, the pacrchment is removed with machinery, polished and bagged.  Perhaps one of the most significant difference between a wet processed coffee and a dry is the leel of acidity and body observed. In general dry processed coffees will always have a lower perceived acidity level, but much more body than wet processed coffees. Why? Well, since a concentration gradient exists between the mesocarp (the pulp) and the endorcarp (bean), we see a slow migration of sugars, phenolics, etc. towards the bean. The bean readily absorbs these additional components and causes the bean to have a corresponding greater level of soluble solids content. As such, dry processed coffee are commonly used for espresso blends. Semi-Washed A relatively new processing method was introduced in Brazil during the early 1990's. Typically referred to as the "semi-dry"  this coffees takes elements from both the wet and dry processing methods. Well discuss this process in greater detail later, but in general the beans are separated, depulped and the beans are left to dry on patios with the mucilage on. Unlike the wet process where fermentation was used to separate the mucilage, no fermentation takes place in this method. The bean along with the drid mucilage is then taken to the mill where the layer is removed before being shipped.