In this block every week I choose 3-5 questions about specialty coffee, and answer them, trying to make it clear, short and simple.
This week the question will be ALL ABOUT COFFEE FERMENTATION AND PROCESSING – definitely not an easy theme to explain in two words, but I will do my best. I’m gonna touch the topic of carbonic maceration, anaerobic fermentation and other pretty hot stuff. We will start with fermentation, and move towards the carbonic maceration.
(Follow the category “common questions” for the previous posts, where we talked about arabica and robusta, caffeine, home coffee roasters and others)
What is fermentation?
I like to see fermentation as a process done by microorganisms in order to get some energy, some food. In other words, fermentation is a metabolic process when sugars and other compounds are converted by microorganisms (a whole bunch of different types of yeast and bacteria) into energy for their own use (the same way people do with oxygen), leaving behind by-products like ethanol and acids. This way fermentation can be alcoholic (ethanol and CO2 as by-products) or lactic acid fermentation (lactic acid as by-product) (or even acetic acid fermentation (acetic acid as by product))
Fermentation is a way for yeast and bacteria to get their energy.
What is fermentation in coffee?
It is weird to think about it, but coffee is a fermented product. Fermentation has been the part of coffee processing, unintentionally and intentionally, since the very beginning. The fermentation process begins as soon as the coffee cherries are harvested. Afterwards, depending on the process used to separate the seed from the cherry (pulping), the fermentation process duration will vary.
First it has been used as a technique that helps to get rid of the mucilage easily. And only in the last 5-7 of years more attention has been paid on how the fermentation process influences the taste of coffee, and if and how it can be modulated. During the fermentation in coffee, enzymes produced by the yeast and bacteria (which either make a part of coffee cherries original microbiome, or has been added artificially) start breaking down the sugars in the mucilage.
The sugars degrade its lipids, proteins, and acids, and convert them into alcohol and acids. The source of fermentation in coffee is the sugars that the mucilage contains, plus the bacteria and yeast.
Are coffee processing and coffee fermentation the same?
No they are not. Coffee processing is a set of agricultural practices and techniques in order to prepare coffee for being exported or transported, which combines various processes, that include fermentation. Processing and fermentation are not interchangeable.
What is anaerobic fermentation in coffee?
It is important to notice that all types of fermentation can occur without the presence of oxygen. So when we are calling the fermentation anaerobic (without the presence of oxygen), we are repeating the same thing twice. All fermentations are anaerobic.
When you see “anaerobic fermentation” on a bag of coffee, it means that fermentation occured in an environment with no oxygen (stainless steel sealed tanks, or plastic sealed bags, for example). The farmer intended to create a sealed environment for the coffee to ferment, which is usually a sealed tank with a one-way valve to stop the in-flow of oxygen.
Usually the coffee is pulped (the skin is being removed, but not necessarily), and then put into a sealed container to ferment. The temperature and PH levels are being controlled. It does not mean that the temperature is always the same – lately there has been a lot of experiment done with different temperature profiles for fermentation. Important is that it is being controlled.
After the fermentation is finished, coffee can be treated in different ways: coffee can be washed and mucilage removed (washed processing), mucilage can be partially removed (honey) or can be left at all, letting the coffee to dry with the mucilage attached (natural processing).
What is carbonic maceration coffee?
Carbonic maceration is basically a fermentation in an environment with no oxygen (anaerobic), but with one detail – after the tank is sealed, the carbon dioxide is being injected, thus expelling the rest of oxygen from the tank. The duration of the process can be from a couple of hours to several days. Why carbon dioxide? It slows down the fermentation process, so sugars are broken down slowly. It influences the nuances of taste, and it is believed that coffees that undergo carbonic maceration have winy profiles and an increased sweetness and body. Usually for the carbonic maceration the whole cherries are being used, inclidung the skins (no depulping).
It is important to say that both coffee processing and fermentation are complex topics that have just started to be studied. So by no means we know everything about it at the moment, there are still lots of things to be done, to be experimented with. And what adds to the complexity of the topic is the amount of variables to control – because all these processes can be done with or without water, or using other liquids as juices, kombucha, kefir, champagne, at different temperatures, with a different duration, and using an almost infinite amount of combinations of yeast and bacteria.