A Field of Beans
Some Basics on Processing and Grading Coffee

by James Vaughn

(from Ground Control Issue #16)

Whether picked by hand or by machine, coffee cherries have an inherently wide range of ripeness, size, and density. In the processing of coffee these differences are narrowed and refined, resulting in grades of coffee. One major goal in processing and grading the coffee is the uniformity of the beans for consistency in roasting throughout the batch. Same weight, same density, same size, etc.

Once the beans have been harvested, the very first choice is between the wet or dry processing methods. In many locations where there is a natural abundance or natural lack of available water, the choice is easy. Good examples for discussing issues in grading can be found in the basics of wet processing, so we will focus there in this article.

Wet processing coffee begins with a ready supply of coffee cherries and some water. The process is similar whether you are working with a bucket in the backyard or on a 3400 acre automated plantation. Step One. Dump the beans in enough water that they either float, or sink. Voila! You've just completed an important step in the classification of the beans. The overripe cherries that you don't want float on the surface, while the ripe cherries, plus the immature beans, sink to the bottom. In a matter of moments you have just separated all the overripe cherries out of the batch.

Since we're going for high grade coffee here, we don't want to be stuck with only ripe and under-ripe cherries. Collect all the beans that have sunk and take them to station #2.

If you are automated you have something that looks like a very wide exercise wheel for a gerbil called a "Green Ripe Seperator." Its a stationary screen with a rotating core. The core has a couple ridges in it in a slow spiral similar to the beater in your vacuum cleaner. As you feed the ripes and immatures in the space between the core and the screen, the core rotates, pressing the beans against the screen. The ripe cherries get mushed through the bars while the under-ripe cherries are too hard to mush and stay inside. Eventually, because of the spiral, all the under-ripes get fed out the end of the machine. Everything that was pushed through the cage that is not a bean is called "mucilage" the sticky pulpy material, and everything else outside the cage is a bean, now called "parchment." The bean itself is a little like a pistachio at this point, in that it is currently surrounded by a thin, close fitting husk.

Moving along to the third point of separation is the "classification channel." This is special as it is our first point of real classification. Here we have a recirculating channel of water that immediately draws away the floating mucilage while simultaneously moving and tumbling the parchment along the botton. The heaviest beans sink first, followed by the lighter beans, so the beans naturally stratifiy into layers. Just siphon off the bottom layer which are the heaviest (best) beans, and continue on to the "Criba."

The Criba is another gerbil-wheel like device but this one is half submerged in water. A long wire tube slowly rotates, and the heaviest beans we just collected are fed into it. This time we're looking for a generalized size and to eliminate any other floating material. This is also another chance for the beans to fail a test. The beans we want will sink to the bottom of the wheel and fall through the holes. Everything else gets washed out the end of the machine. We are now able to work with first quality coffee. Everything else is collected and sent to the second quality line for its own processing.

Following the best beans to the next station, (which can sometimes be skipped) we arrive at the fermentation tank. Here the beans sit in water for about a day, where they ferment ever so slightly. The natural bacteria present in the bean helps to remove any remaining mucilage that is still sticking to the beans. But be careful, there are lots of ways to ruin coffee, and overfermentation is one of them. If you happen to be automated, just step on over to your "Aquapulper" and let a little bean on bean friction remove all that mucilage. Kind of like a rock tumbler, the drained beans tumble around on themselves for a short time and then are once again rinsed. You should at this stage have clean parchment coffee ready to be dried.

Drying

The pre-drier gets the biggest droplets off. The beans are drained, blown dry,

and then fed evenly across a fluidized bed to remove any surface moisture. You can sun dry your beans, or you might be use a Berico dryer, similar to a corn silo but with perforated walls. Just create a void down the center of your silo and pump thousands of cubic feet of warm air through the openings while dribbling coffee beans from a conveyor belt high above. As the bean column fills up, the warm air (110-115 degrees) filters through to the outside carrying bean moisture with it. The alternative is get a rake and a patio, and keep the beans moving. When you hit about 11-1/2 percent moisture, you're done. You can store the beans for later, or take them on to be milled.

Milling

Step one in milling is the de-stoner. Not as hippy-dippy as it may sound, the destoner removes tiny rocks, thus preventing them from completely destroying your thousand dollar coffee grinder later on. This in itself would be a real buzz-kill. Little rocks and stones can be picked up during harvesting and drying and are suspiciously similar in size and weight to green coffee beans. Enough said. Next step: The Huller.

It's time to remove the parchment from the bean. The parchment is now very dry and easy to remove by gently breaking it away from the bean. At home it's just a matter of rubbing the beans between the palms of your hands and breaking it away. The next layer exposed is the "silverskin" which later on will become chaff. To lessen or eliminate the silverskin, the beans may go on to a third step which is called polishing. Polishing can be especially useful in accurate grading of the coffee, but over-aggressive polishing will allow the bean to "fade" prematurely, that is, lose its sparkle as when taking any product out of its protective wrapper.

But, let's return to the issues of grading. We now have dried, polished beans of a relatively uniform size and density. The next step, "sizing" is where classification really happens. The "sizer" takes these beans and through a series of eight screens which divides them in increments of 64ths of an inch. When you hear the reference to screen size 16/17 that's in 64ths of an inch, between 16/64ths (a quarter of an inch) and 17/64ths. By the way, 1/64 of an inch in decimals is only 0.0156 of an inch from one screen to the next. This is about the thickness of a sheet of newspaper. OK, grab all the screen size 18 beans and meet at the density table.

What was once a very large quantity of beans is now a fraction thereof. But let us say we have a good collection of perfectly sized 18 screen beans. Being the same size does not necessarily mean the same density. The "Oliver," or "density table," helps give us that equivalent precision in terms of density, or weight. The Oliver is a slightly inclined table that vibrates madly, and even comes comes complete with a blast of air. This is big fun for the beans. This time, the heavier beans rise to the surface due to the vibration, while the lighter beans drift to the bottom. The air blast from below helps to move things along. Since the table is inclined, the heaviest beans find their way not just to the surface but all the way to the top of the incline and off the end while the lighter beans find their way to the bottom, and off the end.

If you don't have a home color sorter, this next step can be skipped, but if you do: drop the beans one by one in front of the color analyzer. This machine looks for a segment of the spectrum which you can adjust at will. When a bean is defective or out of the color parameters, a puff of air will kick the bean out of the batch. You will examine every single bean of every single batch of every single crop, so this may take a while.

The last step is the mixer to assure consistency from bag to bag when you are working in large volumes. Dump all your coffee from all the processing into one big hopper, mix it around, and measure it back out, finally, into the familiar burlap bags. Sew the top shut.

And there you have it. Absolutely consistent size and density beginning with a field of beans. Wet processing is of course just one way to go. There is also the dry process and even semi-washed.

Washed coffee is not necessarily better, but it does offer extraordinary consistency in taste, aroma, and roast grade. Dry processed coffees have their own benefits, such as complex flavors; but that is another story.

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