For those who have no idea what I am talking about I should explain.  There is a defect in coffee that is referred to as potato.  It is the result of a bacterial infection of the seed, usually after being bitten by an insect carrying that bacteria.  Once roasted that particular bean carrys very, very strong aromas of freshly peeled potato skins that is incredibly potent when you grind the coffee and when you brew it.

Unlike many other defects you can’t tell if a green or roasted bean has potato in it until it is too late.  The solution to the problem is very labour intensive.  When the cherries are picked any cherries that show signs of damage are discarded, and this process is usually done by hand.  After the washing process, when the parchment coffees are still very wet it is also apparently possible to see the damage and discard the beans at that point.  However if a bean has been missed and is dried then it is undetectable until ground.

The potato defect is a problem in several countries, but has become more of a talking point as Rwanda has risen to prominence as producing country capable of stellar coffees, but also a producing country that has problems with potato.  I don’t want to rattle on too much about it – I highly recommend reading this thread if you want to know more about the potato defect.

The real point of this thread is to talk about the weird psychological effect that potato had on me, and ask if anyone else suffers this.  Once you smell potato a few times in a coffee you are using you become hyper sensitive and a bit paranoid.  Cautiously sniffing for even the faintest sign of potato.  The Nyamagabe lot we used in Stephen’s WBC espresso suffered from potato, and we went to ludicrous efforts to prevent it¹

It snuck up again in Copenhagen – during the Cupping competition I was stood near the prep area, next to Edwin from Finca Vista Hermosa, and we both caught the distinct potato peelings smell in the air.  That cup was pretty easy to pick out!

After the lot of Nyamagabe was finished I thought I would be able to relax, but still the smell haunted me.  Every now and again I’d find myself sitting bolt upright in the roastery, sniffing the air thinking “Is that potato?? IS THAT POTATO??” before remembering that it couldn’t be, and that I was loosing my mind.

Am I the only one?  Does anyone else who has experienced potato a bit get somewhat paranoid about tuberous smells?  Am I making quite a public fool of myself?  You decide…..

1. We packed all the Nyamagabe component seperately in 500g bags that were sealed.  Before use we’d carefully open each bag and elimate any that showed even the slightest waft of potato before blending with the El Bosque.  Regardless of this – the final practise shot on stage before the finals there was a clear and terrifying *poof* of potato flavour that filled the stage.  We hoped lightening didn’t strike twice!

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).

A coffee bean after pulping, still coated in the mucilage

The mucilage layer is primarily carbohydrates – a variety of simple and complex, long chain sugars² , and is between 0.5mm to 2mm thick³. 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 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 acidic⁵”. 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 fermentation⁶ 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 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

I’ve been thinking about doing a green bean gallery for a while, and when I had a little spare time this evening I thought I’d have a go at it. Right from the off I should make it clear that this is not my area of expertise, and certainly down in the second half I might get the order a bit wrong, but it should still be of interest.

This post is mostly pictures. For some people this will all be very familiar, for some perhaps very new. I think the diversity in how green coffee looks is so fascinating that it deserved a post. If you are reading this in a feed reader it won’t be nearly as much fun. If you are reading it here you can either click each picture as we go or you can click on one and cycle through them as they pop up – they are all labelled. For now I just want a gallery, I will try not to rant too much about my personal opinions on certain coffees….

So, from the very beginning…..

PART ONE:

The Cherry and the Parchment:

I broke open one of them, and scraped off a little parchment from the corner so you can see all the layers properly. I wish I had some fresh cherries to photograph…

Natural, Pulped Natural and Washed:

For me it is interesting to see how the colour changes across the processing methods (though these coffees are not all from the same farm or region, but hopefully they are “typical” enough to be benchmarks)

Kenyan Peaberry, Harrar Longberry and Sulawesi Kalosi:

I thought it would be interesting to have the slightly orange/yellow tinged Harrar next to the swampy green of the Kalosi.

The evil aged coffees – Monsooned Malabar and Old Brown Java:

I find it odd that the two aged coffees seem to have gone polar directions from their original colours, the Malabar fading away and the OBJ developing that disconcerting brown colour.

Supercritical CO2 Decaf (Colombian):

I’d like to find some more methods of decaf to photograph, and when I do I will add them in here.

Unwashed and Washed Robusta:

The washed robusta is a really clean prep and is a good robusta, even if it isn’t my kind of coffee.

Defected Maragogype and Triage Coffee:

I took my SCAE Barista Level 2 (though I never got round to paying for it, which means I don’t officially have it!) and one of the questions was about Triage coffee. At that time I had no idea what the term meant, and had to ask Alf Kramer who explained that it is pretty much the sweepings that no one would ever admit to buying but some people clearly do….

PART TWO:

The next part is something might be of interest to quite a lot of folks. All of the next coffee is from the same mill in Kenya, and we go from AA all the way down to the sweepings. Because grading is partially based on size, sometimes distinctions may not be very clear from the photos.

Kenyan AA, Kenyan AB and Kenyan C

Kenyan PB, Kenyan TT, Kenyan T

Kenyan MH, Kenyan ML, Kenyan Madres/Elephant Ears

Kenyan E (Large screen, fat beans)

Just to be clear the E isn’t the lowest grade – I just couldn’t figure out where to put it. I will stick this post in the Articles section and in time (I hope) keep adding to it.

Comments and suggestions are very welcome….