The Rwandan Potato problem

growing, harvesting, processing, cupping, purchasing

The Rwandan Potato problem

Postby James Hoffmann on Thu Jun 28, 2007 8:44 am

I am curious about this and wondered if anyone here had any more information.

As far as I understand it (so this could well be wrong) it is caused by an air-born bacteria that gets into the fruit through damage in the skin. It then causes off flavours similar to raw potato peel in the cup.

Is that wrong? Anyone got any info on how they are trying to fix it?
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Postby Aaron Ultimo on Thu Jun 28, 2007 12:06 pm

I was just having a tasting in our arlington location and tasted this not 15 minutes ago! It's not the first time I've had that experience with some of our coffee from the Koakaka Co-operative, but this was the first time in a while that I smelled and tasted it. It was also the first time I tasted it in our special "reserve" lot of that coffee.
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Postby Peter G on Thu Jun 28, 2007 2:29 pm

James, you are correct: the potato flavor comes from microorganisms that infect the coffee fruit and seed, due to skin damage. This skin damage can happen in a variety of ways, but it is commonly thought that the distinct "potato defect" (which is common also in neighboring Burundi and also occurs in Tanzania, Zambia, and Kenya) is due to damage from an insect called Antestia.

It's very difficult to sort out the "potato" defect, as it is largely invisible. Flotation of cherry before pulping helps, as does densimetric sorting of finished coffee. However, I personally feel that if you taste enough cups of any Rwandan coffee, no matter how good, you will hit the potato defect at some point. It has cropped up even in some of the best Rwandan coffees. Of course, it's far more common in poorly cultivated, sorted, and prepared coffee. When cupping in Rwanda, especially lower quality coffees, it feels like you're in a raw potato soup factory sometimes. Apparently the specific chemical which causes this effect is 2-methoxy-3-isopropyl-pyrazine. So, hold the 2-methoxy-3-isopropyl-pyrazine.

But seriously, this compound is secreted by a number of bacteria which infect fruits. I presume these bacteria are more prominent in the microflora of East Africa, and that's why this problem is more present there, although potato cups can happen from coffees all over the world.

Addressing this problem is a high priority in Rwanda and all over East Africa, and the primary strategies are:
1. Insect Control
2. Careful Picking (no split skins)
3. Pre-pulping floatation tanks
4. Densimetric sorting (Oliver tables)
All have been successful to some degree, and this year I tasted way less potato than I have in years past.

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Postby James Hoffmann on Thu Jun 28, 2007 3:04 pm

Thanks Peter - googling the compound you mentioned turned up this which people might find interesting:

http://www.fao.org/docrep/003/X6939E/X6939e00.htm
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Postby Sean Starke on Fri Jun 29, 2007 5:42 am

That's a very very common defect in Burundi and Rwanda coffees; we fail a lot of them at the Exchange for it.
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Postby Timothy Hill on Fri Jun 29, 2007 9:50 am

When I heard that Rwanda is going to have a "Test" Cup of Excellence, all I could think of is how the potato defect is going to play a part in the cupping. In my opinion if I had to give a number on the frequency of the defect even among the best Rwandan coffees I would say it will show up 1 in 150 cups. (maybe even 1 in 100) with those kind of odds I wonder how many great coffees will be disqualified due to that particular defect. It will be interesting to see how that turns out.
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Postby Timothy Chapdelaine on Tue Aug 07, 2007 1:51 pm

My first post on Coffeed!
I see lots of my good friends here.

I have seen a potato cup in a container that cupped 90+ all year and nary a complaint from a roaster.

This is a snake in the grass defect. It can rear up in coffees that have had all the love they need.

I suspect that the defect or the resultant lightness of beans or the internal issues of the bean. There is a white wormy looking maggot thing that you can squeeze out of the wet bean.

Once this dries into the bean it is tough to detect and some of the sorting methods will not catch it. In particular hand sorting as is common in Africa.

Optical sorting (which may detect color discrepancies inside the bean) is not as prevalent as it should be in East Africa.

My take is that picking and flotation in channels is a good way to prevent this defect. The whole process needs to be right to limit risk of this defect. Seems that nothing is perfect though.

Rock on coffeed.
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Postby SL28ave on Wed Aug 08, 2007 3:18 am

Timothy Chapdelaine wrote:My first post on Coffeed!


Exciting to see you here, Tim!

Would an Oliver Table help at all to separate the infected beans? Do they have Oliver Tables at the Rwandan mills?
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Postby Timothy Chapdelaine on Wed Aug 08, 2007 12:29 pm

Peter,

They do use various types of density sorters and screens on the coffee. Basically a very similar dry mill set up to Kenya.

I am not sure how much of this the oliver/density sorter would get out.
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Postby trish on Thu Aug 09, 2007 9:17 am

I was in Rwanda about a week ago and stopped by a few cupping labs. They showed me their books on all lots that came in for the surrounding washing stations. I saw almost no notes about potato. I asked about it a lot, too.

The coop I bought from last year, (Musenyi, Rusenyi) cupped out great. All lots were "normal"- thier word for specialty or anything above a 83, or "exceptional" - their term for above 88.

I was stoked. They said that they have darned near taken care of all that potato. They acted like, "oh the potato, that was so two years ago!"
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