Introduction to the chemistry of the wet process (part 1)

This article is a simplified version of what was researched and written for the attempted coffeed study group. Sadly that project didn’t take off – however I am grateful to people like Andy Schecter and Jim Schulman for publishing the papers they wrote regardless. The article doesn’t focus much on cup quality and the specific effects of the process – more on the process itself.

The Wet Process

The primary goal of the wet process is to remove the sticky layer of mucilage that surrounds the beans and parchment before the coffee is dried. The practise stems from a simple goal of improved consistency and reduced defects in a lot. When this layer is removed there is a lowered chance of problems and flaws in the coffee. However it should be made clear that this does not mean that it will be of a higher quality as that is both subjective and also needs to be balanced by what characteristics are desired from the coffee. As the process uses up a lot of water it is only the higher grades of cherries that go through this process – in terms of ripeness or varietal.

There isn’t much on the history of the wet process available. The earliest printed source I have is Ukers, and the process of loosening the bean from its “closely adhering saccharine coat” is documented in reasonable detail. 1 I’ve heard various different things about its origin, and Costa Rica is the most commonly quoted country to me (though I currently have nothing solid to back this up).

Coffee bean covered in mucilageA coffee bean after pulping, still coated in the mucilage

The mucilage layer is primarily carbohydrates – a variety of simple and complex, long chain sugars 2 , and is between 0.5mm to 2mm thick 3. The washing process, broadly speaking, is considered complete once the layer easily comes free from the parchment. The simplest test is to take some of the beans and rub them – if they retain a slimy texture then they are not ready, though if the mucilage easily comes free in the hand then they are ready to be removed, rinsed and dried.

The carbohydrates that we want to break down are celluloses keeping the cell walls together, the most common of these in coffee is also found in many other fruits: Pectins.

Pectins

The structure of pectinThe Structure of Pectin

The key to successful fermentation of coffee is balance of the methods of breaking the pectin down: bacteria and yeasts.

Bacteria can produce enzymes like pectinase and pectase that are specific biological scissors that break the pectin down. However some research done claims that the most commonly found bacteria in the process do not produce the right kind of enzymes to effectively break down the mucilage. 4

Yeasts break down the pectins too, but the byproducts of those reactions are typically ethanol and lactic acid. In the right conditions the balance is right and the mucilage is broken down quickly with no negative characteristics being developed.

The source of the enzymes is even more of a mixture than that – there are plenty already within the cells of the fruit. These enzymes are gradually softening the fruit as it ripens. Once the bean is pulped they become much more active, due to the oxygen and the presence of other bacteria.

Of the three sources of enzymes it is important to note that yeasts prefer oxygen free conditions, whilst the bacteria are more effective with oxygen around. For this reason it is important not to let water tanks stagnate, as then the yeasts take over causing negative flavours. Different types of fermentation – open, water covered or a mixture – will have a different balance of reactions for this reason, creating a very different cup profile.

Once pectin breaks up, in an environment with sufficient calcium it can start to gel – this is useful if you are making jam for example. This also explains a rather amusing test of fermentation done in some parts of Costa Rica – a stick is put upright into the tank and if it stays upright (held by the gelatinous water) then the fermentation is done.

There are various factors that affect the balance and speed of fermentation:

Temperature

This is the key variable in fermentation, and is the key variable dictating the time it takes. Enzymatic reactions are directly linked to temperature so at higher altitudes the process takes longer as the ambient temperature is usually lower. To increase the speed of fermentation it is possible to preheat the water in various ways before the cherries arrive at the station to be pulped, but I am not sure how common this is.

Acidity and pH

Again sources here seem to disagree about whether pH should be close to neutral or quite acidic 5”. It can often get down to a pH of 4.5 towards the end of fermentation – it is worth noting here again the pH is a logarithmic scale so a pH of 5 is 10 times more acidic than a pH of 6, and 100 times more acidic than a pH of 7. At lower pH the yeasts do better than the bacteria, though I’ve read that low pH can stall a fermentation.
Work was done in Nicaragua on monitoring pH to see if it was an accurate predictor/indicator of the state of fermentation 6 The sharp drop to around ph 4.5 near the end of the fermentation was also matched by an increase in lactic acid and ethanol implying that the yeasts had taken over the bulk of the breakdown of the carbohydrates at this point, though it seems unlikely that they were the cause. More likely the products created by the bacterial and natural enzymes caused a drop in pH and also slowed down their own reactions.

pH during fermentationpH during fermentation in Nicaragua

There aren’t many other studies widely published using pH as a tracker, though I’d be interested to see more.

Part 2 will cover more about specific cup qualities linked to the wet process, including the generation of off flavours like vinegar and onion. Any questions, corrections or things that don’t make sense then please leave a comment (I lost a few of my original papers since I started the original paper).

  1. Ukers, “All about Coffee”, The Tea and Coffee Trade Journal Company, 1935  ↩︎
  2. Redgwell & Fischer,”Coffee Carbohydrates“  ↩︎
  3. Illy & Viani, “Espresso Coffee, The Science of Quality”, Elsevier, 2005  ↩︎
  4. Sylvie Avallone, Jean M Brillouet, Bernard Guyot, Eugenia Olguin, Joseph P Guiraud (2002),”Involvement of pectolytic micro-organisms in coffee fermentation”, International Journal of Food Science & Technology 37  ↩︎
  5. Ken Calvert, “The Microbiology of Coffee Processing  ↩︎
  6. Susan C. Jackels, Charles F. Jackels (2005), “Characterization of the Coffee Mucilage Fermentation Process Using Chemical Indicators: A Field Study in Nicaragua”, Journal of Food Science 70 (5), C321–C325  ↩︎

4 Comments

  1. Wow, a great article and a massive bit of luck that i’m processing coffee at the moment.

    In fact i’m just in the middle of writing a practical assessment of the semi wet/dry process!

    Fancy a trip to Cornwall?

  2. This is truly a great piece of information!

    For additional reading I suggest the book “Coffee: Growing, Processing, Sustainable Production” by Jean-Nicolas Wintgens, published by Wiley-VCH in 2004.

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