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.


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:


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  ↩︎

Colombia 2007 – Armenia

Occasionally being jetlagged can be a good thing.

Anette and I arrived into Bogota on Sunday evening, and collected by our host – Luis Velez – and dropped at our lovely hotel with the worrying news that in order to catch the 6.15 flight to Armenia we would need to be up at 4.30am. Thankfully we slept
straight away and the as we were still 5 hours ahead internally it wasn’t too painful to wake up then.

Four of us travelled to Armenia – Anette and I, Martin Velez (Luis’s son) and the Mexican barista champion Salvador Benitez. The flight across is surprisingly short, possibly the shortest I’ve ever had – you only just get up to cruising altitude before you plummet back to earth. In Armenia we were hosted by Jaime Raul from Agrado. Agrado is an extremely interesting place. It is the focal point for the coffees in that region – Quindio – and the local FNC organisation have decided that for Quindio the only way to go is towards quality. So at Agrado – a medium sized farm – they have set up an impressive cupping lab and research facility. I’ve never seen anything like it. Talking with Jaime Raul gives you a very different perspective towards coffee. He dislikes the idea of a coffee chain, a very linear path for coffee to take. He would rather that the ends of the chain met to create a circle through which knowledge is traded and coffee improved. It was great to wander amongst the trees on the farm and taste the cherries at different stages of ripeness (the difference is amazing!) It was important for me to better understand the raw materials and the growing and picking.

Anette amongst the trees

Anette amongst the trees

Jaime Raul also turned my focus onto coffee pickers. The quality of the crop that they pick determines so much, but picking only the ripest cherries is hard work especially when you are paid by weight and there will always be some level of temptation to pick indiscriminately. At Agrado they not only pay a premium for a better quality harvest but try to look after the pickers as much as possible and get them as involved in coffee as possible. I don’t know of anywhere else in the world that provides espresso and cappuccino from a Linea in their lab for free to all the pickers – brewing coffee from the trees they harvest.

La Marzocco Linea at Agrado

La Marzocco Linea at Agrado

We toured the farm a little and then had a beautiful coffee break. They have a large section of bamboo forest and in the middle, down by their water source, they have built a place to sit and have coffee. The lab hosts growers from all over the region – to teach them to cup, to teach more about agronomy and when groups visit they all head down to this patch to drink coffee and talk about what they taste. It is probably the most incredible place I’ve had a cup of coffee.

Coffee break at Agrado farm in Armenia

Coffee break at Agrado farm in Armenia

We spent most of the afternoon cupping – first coffees from other regions, some familiar (like Huila) and some not. We did three flights, and then we played with the espresso machine for a while. One of their staff was practicing as she planned to enter the National Barista Competition this week. After that we cupped more coffees – this time from Quindio and there was some really lovely stuff on the table. I confess it was not a region I knew much about, but I think their drive towards quality is paying off and it is a name that will become well known in the next few years.

Anette and Jaime Raul at Agrado

Anette and Jaime Raul at Agrado

After this the rain came – the huge, torrential rain that we never get in the UK, and we headed back through the washed out roads to our hotel in Armenia. The next morning we went to the local FNC headquarters to talk more with them about driving towards quality in Quindio and to get our reactions to Agrado. From there we headed over to Almacafe to see the parchment coffee being processed. I’d seen something similar in El Salvador but you always learn something new and it is always interesting.

Parchment warehouse at Almacafe

Parchment warehouse at Almacafe

We also cupped in their lab a little too – Anette’s razor sharp tastebuds picking up a little phenol in a couple of cups, though it wasn’t very strong. I wish I was better at defect cuppings! (Something we did a little more of in the afternoon). Having stolen fruit from a tree growing outside the warehouse we headed back to Agrado to cup some more and also see some processing that they do there.

Cherries being weighed and checked at the mill

Cherries being weighed and checked at the mill

I had missed the harvest when I was in El Salvador so this was the first time I had seen processing of coffee cherries up close. As much as you can understand from books and pictures nothing beats seeing unripes float, or watching a pulper squeeze the seeds from the flesh. They do quite a rigorous pulping and selection before the coffee hits the fermentation tanks at Agrado, but the coffee they end up with is great. The are also constantly using their own crop for experiments – be it different drying methods or different shade systems. They log everything with great detail at the cupping table. I find it very exciting and promising for coffee in the future.

Pulping coffee

Pulping coffee

And this was all we had time for – we had to head back to the airport for another bumpy hop across the mountains to Bogota so we could land in more torrential rain! Up next will be a post about the barista competition happening here. I am judging and Salvador and I did some demonstrations but I will post more with pictures soon.

I know I’ve missed a load out but I will try and update when I get the chance!

There are more photos at Flickr or have a browse below: