Coffee fermentation - Facts and things to know

Coffee fermentation - Facts and things to know

Calum Hill 5 mins Read

When we think of fermentation we think of kombucha, beer, wine, cheese or yeast. But it’s increasing use within coffee production has more people asking how? And where does it lay within the production process? In this post we will discuss the fundamentals of coffee production and the use of fermentation. Humans have used the process of fermentation for at least 10,000 years, through many different processes and variations. 

Firstly, what is fermentation?

Channel your school science classes where you’ve probably been told this before, and let's (re)discover what fermentation is before we get into its connection to coffee.

Fermentation is the process that allows substances to break down into simpler substances, micro-organisms like yeast and bacteria are commonly used in the fermentation process. Basically, all the delicious things like bread, yoghurt and wine, are all examples of the use of fermentation. When putting sugar and water together this is a natural process that happens - both combined will allow for fermentation to happen - coffee cherries have both of these.

So how are fermentation and coffee connected?

To be clear, fermentation only takes place during coffee processing.

As coffee fermentation is a very complex process, it’s important to maintain a strong consistency, as many issues can affect the crop with mould or even chemical flavours in the coffee. But when successful the process can enhance a coffee's best attributes.

Fermentation actually takes place within the coffee cherry soon after picking. The processing methods vary between geography, climate, logistics, and the farm's traditions. Controlling the fermentation is almost like a catch 22, the process is natural at first but without correct control the coffee cherry can undergo the complete opposite of the desired goal. For example let's picture the process, like a yoga teacher easing you into a sense of thought…. The plants are plotted, the plant grows, the cherry blossoms, when ready we pick the cherries, from this point fermentation begins. What might happen next is that the cherry is packed or stored. Depending on the weather, time of control and the air. Left too long this stage can lead to defects such as “sours” or the dreaded “stinker” bean, which can instil a pungent smell and foul flavour across few kilograms of finished coffee beans. Ways of combating this are storing in water. 

Coffee fermentation

                                       Image by Café Granja La Esperanza, Colombia

Within the process there are two separate types of fermentation being:

- Aerobic fermentation: This is simply the process with oxygen, this is the most simple fermentation method. The recently picked cherries will simply be added to a tank or container and left to process.

- Anaerobic fermentation: In this case the coffee cherries are left in a tank with water and left denying the beans of oxygen, allowing different microorganisms to work.

Anaerobic fermentation process - coffee

                   Diagram of anaerobic fermentation process: Image by Sassa Sestic 

How coffee fermentation affects coffee flavour?

So how and why does fermentation affect the flavour of coffee? Some of the main attributes is that it can bring out refined flavours from the cherry - from sweetness to acidity  - and distinguishes the sensorial notes such as fruits, chocolate and caramel. If you imagine kimchi, before fermentation it remains a simple cabbage, after fermentation contains a strongly enhanced flavour. This is exactly what happens with the coffee cherry.

Explaining the starter culture

I remember reading how many cultures were in my kefir one day and thinking… What on earth does that mean? How does culture get into my drink? However it turns out starter culture means something quite different. When we discuss starter culture it almost resides like this. When we (human beings) understood the benefits of fermentation within our food/ drink we understood that utilising and adapting the fermentation process was something we could do to enhance flavour. In layman's terms a starter culture is a preparation of microorganisms used to make the chemical composition and the sensorial properties to make a more homogenous product. 

Some frequently asked questions about coffee fermentation…

How long does it take for coffee to ferment?

This can alter due to the producer's desires, the weather surrounding the crop and the process. When discussing washed coffees it generally takes 18-24 hours. This is the correct amount of time to allow the process to go through all stages, in short, enough time for the bacteria to get going, the enzymes will break down the mucilage. The timing is key, too long or too short the coffee can miss those perfect flavour spots.

Why is it important to ferment coffee?
There are two key fundamentals when fermenting coffee, firstly to clean the bean and secondly to add flavour and a unique quality.

                 Dry fermentation in Huehuetenango Guatemala. Photo from DRWakefield

All coffee is fermented a little bit, but here’s a little information on the differences between washed, natural and pulped.

Washed coffees are usually soaked in a tank for around 18-24 hours. During this time microorganisms eat away at the sugars which create enzymes, breaking down the mucilage (mucilage is the sweet sticky layer surrounding the coffee seed)This is the main process of the washed method, to remove mucilage.

With naturals the fermentation process is different, this is where the coffee ferments without water. Usually when the coffee cherry is plucked from the tree they are left to dry in the sun or the shade.Leaving the mucilage intact here is instrumental to the final flavour, so as fermentation occurs you get berry like and sticky sweetness. 

Another method is to throw the cherries into a stainless steel tank and fill with CO2… this is known as the carbonic maceration process.

This process is similar to natural with one key difference… the cherry is pulped (fully removed from the seed) and also left in the shade or sun to dry. The bacteria will feed on the mucilage until the environment becomes too dry for it to survive.

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