This month we'd like to share with our friends and colleagues, why the ideal moisture content in green beans is so important to developing a roast that will deliver a high quality and flavorful cup.
First, a little background following the moisture content of the beans from the fresh picked cherry through to the bean's final processing.
In wet processing, coffee beans are harvested from the farm by selectively picking the ripe red cherries. These beans go through several stages. The first major process occurs at the factory, where the beans are separated ripe from unripe, de-pulped to remove the outer cover, and soaked in a fermentation tank to remove the remaining mucilage. After an adequate period of time, the coffee is soaked and thoroughly washed to remove the covering sugars and prepare for drying. The wet beans are fully dried and taken to the hulling plant, where the parchment is removed. Next, the beans go through the polisher to remove the sticky silver skin. Next, the clean beans go through a process where they are graded by screen size. For improved quality, the graded coffee is often passed through a density separator to separate heavy beans from lights.
THE SIX STAGES OF DRYING
At the factory, after removing the outer covering (outer skin & mucilage/fruit) the parchment coffee beans will have a moisture content of 55%.
During the process of drying coffee, six stages are considered to be of prime importance:
1. Skin drying (outside moisture); moisture content of the bean goes from 55% to 45%.
2. White stage (of the bean inside the parchment); moisture content goes from 44% to 33%.
3. Soft black stage (bean is now very elastic); moisture content goes from 32% to 22%.
4. Medium black stage; moisture content goes from 21% to 16%.
5. Hard black stage (bean is very hard and goes from black to the natural color according to the quality of the bean); moisture content goes from 15% to 12%.
6. Fully dried parchment coffee before hulling; moisture content of the bean goes from 11% to 10%
.Throughout this process, the beans are in parchment form. However, after hulling when the parchment is removed, the green coffee beans should maintain the final moisture content as shown above.
At the Coffee Research Foundation in Africa, experiments were conducted to test the suitability of mechanical drying at each stage in the process. The results revealed that at stages 1, 2, 4, 5 and 6, mechanized drying had positive results, with no material affect on the quality. But, at stage 3, sun drying was found to be mandatory in order to maintain high quality beans.
WHY 9.5-10.5% MOISTURE CONTENT IS OPTIMAL
Because raw coffee beans are very susceptible to volatile environmental changes, it's been proven that a moisture content between 9.5% to 10.5% enables the beans to be resistant to negative conditions that can affect the quality (Ref: Coffee Research Institute).
During ocean transport, coffee with ideal moisture content will normally gain 1% and then go back to its original level, while still maintaining its quality and color. If the moisture content is too high, 11%+, the beans will gain 1.5+% causing the beans to become moldy, fade and lose color. As these fungal laden beans dry, they will retain the green water damage and mold-based toxins, resulting in musty and woody flavors in the roast.
An increase in moisture during transport or storage can cause the beans to become moldy, change color and become dark and mottled. When this occurs, a chemical reaction takes place resulting in the beans becoming toxic with Ochratoxin A. Often these molds are inside the beans and not readily visible to the naked eye.
Ochratoxin A (OTA) is a mycotoxin produced by fungi and occurs naturally in moldy raw coffee beans. It's been found to be highly toxic to the kidneys and potentially carcinogenic in humans, as well as having genotoxic properties.
In coffee, it can be found in raw coffee beans that have not been dried or stored properly. It can also be found in warehoused coffee that has been stored for long periods of time.
**More important to the coffee roaster is that Ochratoxin A can also be found in brewed coffee and isn't completely eliminated when the beans are roasted!**
IMPACT OF MOISTURE CONTENT ON THE ROAST
Roasting of coffee at the ideal moisture content (9.5% to 10.5%) will typically give a dark chocolate color at a medium roast, with a percentage weight loss between 8% and 12%. During the process of brewing, beans of this type will give you a proper extraction which will represent balanced acidity, body and flavor. Beans with a high moisture content (11+%), usually take longer to roast, but more important, the beans will first "bake" prior to roasting.
This baking process creates a light colored, soft bean, which forces over-roasting, giving a light acidity, full body and negative flavors in the cup. Beans that have been baked become soft and permeable. After grinding these beans will give an uneven grind that is highly solvent in water, creating dissolved impurities that build excessive high body in the cup. During this process, the beans will lose desirable acidity and natural flavors.
Brewed coffee from high quality beans with proper moisture content will be clearer to the eye, more like a thick honey color because of having fewer impurities. This type of coffee usually has a sweet balanced cup with good acidity, quality and taste.
WHAT CAN YOU DO?
1. Always check the moisture content of the sample during cupping before you buy and look for coffees with target moisture content from 9.5% to 10.5% as a general guideline.
2. Keep an eye out for light colored and mottled colored beans, and internal molds containing Ochratoxin A, which result in soft beans that will first bake, leaving dissolved impurities in the cup.
3. Store green coffee in their original bags in a low moisture environment where there is air circulation. Quality green coffee at the ideal moisture content should stay relatively fresh for up to twelve months in their original bags.
This article is the third in a five-part series describing general guidelines on how to determine quality characteristics of green coffee beans before the cupping analysis.
Co-authored by Steve Josephs and Jackson Kanampiu
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