BEST PRACTICES: Coffee Wetmill Practices – How They Affect Your Coffee Quality
by Ken Sheppard, September 29, 2008
Objective: The purpose of this workshop is to discuss wet mill practices with a particular emphasis on the fermentation process.
Food for thought:
If we accept that the coffee on the tree is the best it can be and that our drying and storage systems are properly set up and there still remains some coffee quality issues then we need to look at the wet mill system for answers.
Arabica coffees processed by the wet method are the highest priced in the market because they exhibit the most desirable characteristics of a gourmet coffee. Washed coffee means the coffee has undergone a fermentation step including a thorough cleaning after fermentation. Washed coffee will always have a higher value (if done right) because it produces a finer acidity, and cleaner taste.
However, wet coffee processing requires know-how and meticulous care to produce a high quality coffee. A lot can go wrong along the way. But if you avoid the pitfalls, you will have the best coffee in the world. Of course, quality starts in the field and to have a world class coffee you need a quality orchard that is well fertilized, maintained, and free of pests and diseases.
It has been said many times before, after coffee is picked, only bad things can happen. It is as good as it can be straight off the tree. How you process it after that can either maintain that quality or diminish it.
Particularly on small farms the wet mill process starts with harvesting. It is essential to pick cherry carefully, only the red ripe coffee cherries should be harvested; larger mills have separating systems to somewhat correct for poor harvesting. The fully ripe cherries will run smoothly through all steps of wet processing, and produce the correct flavor and aroma when roasted without flavor defects. Green and unripe cherries develop a peanut like, grassy or green taste in the cup. Never include black beans, from cherry that has dried on the tree or collected from the ground. Never include beans that are overripe, rotten or have started to ferment. The picked cherry must be processed promptly so that it does not start to ferment in an uncontrolled manner. Rule of thumb is same day, or at latest early the next day.
Keep the cherry cool until it is pulped, even under cold water if pulping is to unavoidably delayed.
Tip each bag of cherry out and look at them. If there are too many green and/or already fermented/rotten cherries (more than 10 to 15% say), it may be bad to process this cherry at all as it has the potential to spoil the whole coffee lot. Low quality cherries should be rejected at the wet mill and sorted out as early in the process as possible. Later removal of defects is time consuming, expensive and not all defects will be eliminated. Not all farms do pre-cleaning and floating. If your pickers are good or you pick yourself, you may not need to do this step. But if you see more than 10% defective beans, then floating the pick in water and scooping out the defects (and leaves, sticks etc if they are there) will be essential.
Simple siphon systems can be built, and at least one farm in Kona uses a screw auger to elevate the cherry from a water filled hopper whereby the floaters can easily be removed before they enter the pulper.
Pulpers must be maintained well to make sure they are pulping correctly. They wear in use, need to be readjusted as the bean size changes and can be put out of calibration during off-season maintenance. Check and look at your results. If you are getting nicked or cut beans, they will be highly susceptible to deterioration in fermenting, drying and storage, as well as not looking good when roasted. Think of nicked beans as an open wound inviting infection. A few skins in the pulped coffee is not a major issue, some skins may be necessary to retain “body” in the coffee (Sivetz experiment).
1. Always remove the parchment that floats in the fermentation tank***.
2. When water is added for the tank for rinsing stir up the beans to release more floaters, they will constantly come to the surface, remove until only just a few keep coming up
3. Throw the floaters away. On a small farm there is nothing to be gained by trying to process them separately.
4. Floaters potentially contain defects which will taint the main body of the coffee.
*** Even if you make the decision to float the cherry too there will still be floaters in the fermentation tank, it is an ongoing process of removing defects at every stage.
Another way to rinse coffee in small quantities is to make a long tray from 2 X 4’s (or 6’s) with a 1/4 inch wirecloth bottom. The coffee can be blasted with a hose nozzle and moved around, it can also be left to drain on this screen before being put on the deck speeding up the drying process.
Natural fermentation is a biological way to remove the slippery mucilage layer from the parchment, it is a mixture of fairly complex bio-chemical process and I don’t pretend to understand all of them. It is not necessary to understand them fully if a few basic rules are followed, for those who want a more detailed explanation the Internet has abundant, but sometimes conflicting, information. I have used information from Sivetz and Jan von Enden.
During the preparation of fully washed coffee, the pulped coffee is put into a fermentation tank where it is left from 12 to 36 hours, depending on the climatic circumstances, the shorter the better. During this time, enzymes of fermenting bacteria will grow in the wet and warm conditions of bulked parchment and turn simple sugars into acids. The removal of the mucilage cover of the bean appears to be closely related to the increasingly acid conditions during fermentation. Therefore, slightly acid conditions (pH 4,5 to 5) are needed in the tank to achieve a full removal of the mucilage during fermentation. However, too acid conditions need to be avoided.
If conditions for fermentation are not right (too cool ambient air or water, exceedingly acid environment, too little available oxygen), fermentation will happen only slowly or not at all. The temperature of the processing water should not be less than 68 F, sufficient oxygen should be dissolved in the water and the acidity of the water should not be lower than pH 6 at the beginning of pulping operation. These conditions will permit bacteria and enzymes to create the desired environment for fermentation. These conditions usually occur naturally in the Kona coffee belt but the higher altitude farms sometimes have problems fermenting coffee because of temperature. If you have difficulty fermenting coffee then take a look at some of these possible causes
There are two basic parts to fermentation, one part is good and the other part is bad and the process must be halted before it gets too far into the second phase hence speed and short times are essential.
The microbiological actors are soft rot bacteria, liquid enzymes, bacterial pectolytic enzymes, yeast and yeast enzymes among others.
First Part (GOOD)
Pectinase (liquefying pectins), Galectoronic acid> pectic acid.
Second Part (BAD)
Sugar> Alcohol > Acetic acid (vinegar).
Off tastes, fruity> sour. stinker
When the parchment feels crunchy (a rough and pebbly feel) when you grip a handful together, it needs to be washed to remove remaining fermentation byproducts like acids, alcohols and disintegrated pectins. If you do not do this promptly then the liquefied mucilage in the tanks will discolor the parchment, and also be a good growing ground for moulds and yeasts. Thoroughly rinse the parchment until the water runs clear or close thereto.
This is an alternative to fermentation in which the friction of beans moving against each other is used to remove the mucilage. It can save water and time, but the beans do not fully develop the acidity that connotes a quality coffee. Some experts can detect a flat grassy taste. Also, some mucilage will remain in the centre cut of the bean where it will slowly disappear at later stages but will give the centre cut a slightly dirty appearance at roasting.
Latest research suggests that not the fermentation itself develops acidity and aroma in the coffee cup but rather the time in which the pulped, wet parchment is left in fermentation or storage tanks before drying. The time during fermentation with warm and moist conditions might stimulate processes in the bean which cause a biochemical restructuring, leading to the development of desired acidity and aroma carriers. Therefore, wet demucilaged coffee should be left in bulk for “finish” fermentation” (e.g. overnight in fermentation tanks, etc.) before drying and thorough washing, quality characteristics like acidity will increase and the green/grassy taste will be reduced.
In terms of cup defects, demucilaged parchment is less prone to fruity flavor, sour and stinkers caused by overfermentation because mucilage and sugars are removed mechanically rather than by microorganisms.
Personal Note: The standard wet process has been around for a very long time so is somewhat fixed and understood. Demucilaging is relatively new and I don’t think it has yet settled down to an absolute “best” method in terms of additional fermentation before or after demucilaging. It is close but researchers are still working on it, Good cuppers can still detect differences, the average coffee drinker probably not.