Name: Thomas Miller
Title: Professor of entomology
Location: University of California, Riverside

Potato taste is a category of coffee taste, and the name is as close as professional coffee tasters can come to describing it. The term winds up categorizing that batch of coffee as undesirable.

Rwanda started having problems with potato taste about four years ago, after they started concentrating on specialty coffee. The difference between specialty coffee and regular coffee is like night and day. Specialty coffee requires a lot more work, but its price is relatively stable compared with ordinary coffee. That means that coffee represents a whopping percentage of their income. We’re talking millions of dollars that could be lost to potato taste.

I recently joined a small team to give presentations in Rwanda. They set up a tasting for us in a Starbucks, where they have a micro roaster and micro grinder on-site. The taste is very subtle, of course, but to me it had a musty sort of smell that reminded me of old cardboard paper.

We suspect that a microbe is involved, and it might be associated with a group of stinkbugs called antestia. When potato taste comes from a batch, and it can be traced back to an origin, the district is usually infested with antestia. So there’s a loose correlation with the insect. And the antestia bug by itself, regardless of this taste, damages 35 percent of the coffee yield.

My recommendation to Rwanda is to try to reproduce the system. You’ve got the insect, you’ve got the coffee bean, and you put those in a model system and turn the crank. Let’s say you come up with a really bad potato taste, then you can go backward and see where the microbe comes into the equation.

In the meantime, Rwanda can try to deal with the antestia bug because it damages coffee yield anyway. The bug lives on abandoned coffee plantations and in the mulch under banana trees, so if you do some sanitation work to get rid of it, that could help in two ways.

The trouble with the potato taste defect is you can’t see it. It’s not really clear what steps you need to take to prevent it from happening. It is a great big mystery. You’ve got yourself a nice 20 years of work before you figure out what’s causing it. 

This article was published in print as "Coffee Mystery."

9 Comments

1. 1. jcraves 09:18 AM 3/26/12

   I wrote a detailed article about potato taint and potential causes, including Antestia bugs. I wonder about the connection with the coffee bean rot, the bugs, and chemicals responsible for potato taint (http://www.coffeehabitat.com/2012/01/the-potato-taint/). Any thoughts?

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2. 2. Smartse 05:28 PM 4/1/12

   It's great that SA are covering potato taste. The last two sentences are very accurate - this is by no means a new discovery - people have researched it for years. Hopefully with international collaboration the puzzle can be cracked!

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3. 3. dbtinc 08:42 AM 4/13/12

   OMG! Where any federal dollars spent on this? Right up there with tumor genomics!

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4. 4. geojellyroll 10:42 AM 4/13/12

   Now I won't buy coffee from Rwanda...your 'clean up' really means more insecticide spray.

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5. 5. KenMoffitt 05:07 PM 4/13/12

   I would guess geosmin

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6. 6. Steve3 in reply to geojellyroll 05:52 PM 4/13/12

   I doubt that you have ever knowingly bought Rwandan coffee.
   But you're observation is probably correct about the insecticide -unless specialty coffee also means organic.

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7. 7. Smartse 06:34 PM 4/13/12

   NO!
   
   You really should drink Rwandan coffee - a) it tastes amazing and b) doing so really helps people in Rwanda - far better than giving aid.

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8. 8. Quinn the Eskimo 10:01 PM 4/22/12

   Potato flavor? I don't know.
   
   What else I don't know, what cause the charbroiled burnt taste to everything Starbucks sells. They cookin' it over charcoal?

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9. 9. Desert Navy 05:00 PM 5/5/12

   Starbucks' "burnt wood" aroma comes from over-roasting their beans. They roast until the beans are well into the "second crack" phase, which destroys all the varietal characteristics of the coffee and gives it a (strong) uniform flavor. This is because Starbucks makes "coffee-flavored" drinks, NOT coffee. With all the sugar, cinnamons, chocolates, and milks they add for their various concoctions they need very bold coffee for any of its flavor to come through. It also allows them to standardize what customers can expect when they walk into any Starbucks in the world. The McDonaldization of coffee.
   
   They recently released a "blonde" roast of which I actually stopped and purchased a small cup (I haven't patronized Starbucks after about 10 trial cups of coffee years ago.) I found it smokey when it first was handed to me but after it sat for 2-3 minutes the genuine coffee flavors came through (Even coffee I make myself frequently needs to sit a couple of minutes if the brewing water is too hot.) I found the Blonde roast very drinkable and didn't need to add milk or sweetener to cover up any nasty tastes. About a 6-7 out of 10.

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