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."