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How Is Coffee Fermented and How to Make It at Home?

26 May 2021
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Fermentation is a biological process where yeast, bacteria, and other microorganisms break down sugars into simpler substances. It is a key part of post-harvest coffee processing.

From seed to cup, you’ll be surprised at the role of fermentation and how its different stages open a new world of aromas, flavours, and depth to your coffee.

In this article, let’s learn about fermentation in a coffee bean’s journey and how it transforms your brew.

Coffee Processing – The “Regular” Fermentation

sun drying coffee cherries
In dry environments, coffee cherries are left to ferment naturally under the sun. Photo from @ngtghaivanamg.

Every coffee cherry, once harvested, will ferment naturally due to the high sugar content of the pulp.

This natural fermentation process is practised by all coffee producers worldwide in order to strip the pulp and remove a sticky mucous layer to get to the coffee bean.

This process is also critical as it shapes the coffee’s flavour, producing acidity and flavour notes as the sugars are broken down by yeast and bacteria.

Common Types of Fermentation

There are three approaches to coffee processing involving “regular” fermentation: wet processing, dry processing, and the honey method.

Before reading on, use this coffee wheel to understand how each process affects the taste notes of the coffee beans.

Wet Processing

In humid conditions, wet processing is favoured, where ripe coffee cherries are submerged in a water tank to allow fermentation to take place over 12 to 48 hours to remove the mucous layer.

After the fermentation is completed, the coffee beans are washed and ready to be dried. This wet method gives the coffee a pleasant acidity and a clear, articulate taste, which is related to the region of origin.

Dry Processing

In dry environments, coffee cherries are spread out and left to dry under the sun where fermentation takes place.

This method of dry processing uses the sun’s heat to soften the pulp for easier mechanical separation and removal.

It produces wine-like flavours that tend toward fruity notes.

Honey Method

The honey process involves first removing the pulp of the coffee cherry, leaving a small portion of the pulp behind during drying; this is known as the “honey”.

This gives the coffee bean a unique flavour profile as it ferments on the bean for about 18 to 25 days.

Some coffee producers adopt this method to reduce spoilage because the pulp has been mostly removed. This method often produces sweet and floral tasting notes in the coffee.

Take this quiz to find out your coffee taste profile!

Anaerobic-environment Fermentation

coffee cherries on a plant
The coffee cherry’s exterior is hard and waxy, with a juicy flesh lining the interior. This makes it hard to remove. Photo from @andres11hernandez.

So what sets this fermentation method apart? At the beginning of the fermentation process, oxygen is removed from the vessels containing the coffee cherries, hence the term “Anaerobic”.

This form of fermentation produces distinct acids, like lactic acids, that give the coffee a striking flavour.

Uniquely Fermented: Kopi Luwak

civet cat
The beans collected from a Civet are known to be the best or cream of the crop. Photo from Marjo Groenewegen.

This coffee is no stranger to coffee lovers. Kopi Luwak, also commonly referred to as cat poop coffee, is hailed as the best of all coffee because it goes through a unique kind of fermentation.

Find out why Kopi Luwak is hailed as “coffee beans for the crazy rich Asian”.

This fermentation of coffee cherries occurs inside the belly of a cat-like animal called the Asian palm civet, where its faeces are collected for making coffee. Civets are known to eat only the best fruits due to their keen sense of smell.

The beans then undergo heavy cleansing processes before getting roasted as usual. Kopi Luwak is appreciated as it has less acidity, it is easier to digest and it is full of amino acids.

A Secondary Fermentation Process

unroasted coffee beans
Green coffee beans after it has been harvested. Photo from @battlecreekcoffeeroasters.

As we have seen, coffee fermentation isn’t a new concept and it occurs before the roasting process. But some coffee producers have taken fermentation to the next level by directly fermenting the processed green beans instead of the berries, in order to have much more control over the flavour profiles.

In this process, coffee producers choose specific microorganisms for fermentation to change or add specific flavour notes. The resulting cup is unlike anything else, generally less bitter and with a richness of taste.

With more control over the fermentation process, this entirely new class of coffee can be lower in caffeine and made to taste sweeter, so there is less call for adding sugar or other sweeteners.

Make Your Own Fermented Coffee At Home

pouring syrup into a cup of iced coffee
Kombucha coffee is best served cold. Photo from TanerSoyler.

Coffee can also be fermented after brewing. This is done by using a culture starter, a mix of yeast and bacteria, and letting the coffee sit for days. You can use a scoby, which stands for “symbiotic culture of bacteria and yeast”, for the culture starter.

Coffee Kombucha anyone? This is very easy to do at home, and great for impressing your guests.

What you need:

  • 8 cups of brewed coffee
  • ½ cup of sugar
  • 1 kombucha scoby
  • A large jar
  • A cheesecloth to cover the jar

Instructions (makes 8 cups):

  1. Using the De’Longhi Maestosa Coffee Machine, brew 8 cups of coffee.
  2. Add ½ cup of sugar, and pour the coffee into a large jar.
  3. Add the scoby and cover the jar with cheesecloth.
  4. Allow it to ferment from 3 to 10 days. Taste it often until it reaches the carbonation you like.
  5. You can add more sugar to reduce the bitterness and let the scoby work on that for an extra day.
  6. Chill in the fridge before serving.

All the different fermentation methods will create a variety of flavour profiles. Of course, the final taste of the coffee is also affected by the roasting and how the coffee is extracted, which can completely change the taste.

We hope this deepens your appreciation of the production processes behind your favourite beverage and leaves you excited to explore the world of coffee fermentation for yourself!

This article features De’Longhi Maestosa Coffee Machine

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