Is there a potato in my coffee?

One reason for the successful economic miracle occurring in Rwanda today is its thriving specialty coffee industry that, along with tea, accounts for a large percentage of the country’s exports. Despite this success, however, coffee from Rwanda and neighboring Burundi is plagued by a condition called potato taste defect (PTD) in which a few coffee beans impart an odor and flavor reminiscent of rotten potatoes.

So pernicious is PTD that its occurrence can downgrade the value of the entire crop by a fourth or a third. Worse yet, PTD is only apparent after processing, roasting, grinding and brewing, and can occur long after the coffee has been shipped abroad.

Thought to be caused by chemicals produced by microbes that gain access to the coffee cherries by way of a stink bug called antestia, PTD has gained the attention of an international effort, called the potato taste project, that for two years has sought the cause and cure for the defect

Two undergraduate students at the University of California, Riverside, have played key roles in the potato taste project.

“Lauren Wong and Tony Truong made a key breakthrough discovery that led to our asking one of UC Riverside’s plant pathologists, James Borneman, to do a microbiome of coffee beans in Rwanda,” says Thomas Miller, a professor of entomology at UCR and one of the members of the international team working to mitigate the potential impact of the defect on Rwanda’s specialty coffee industry.

Wong and Truong focused on the microbial difference between the defective coffee beans and the beans that passed the stringent criteria that allows them to be deemed specialty coffee. Wong swiped good and bad raw coffee beans onto culture plates and found a dramatic difference: the good beans produced clean fungal colonies while the bad beans yielded mixed cultures of bacteria and fungi.

“We juxtaposed beans that had passed the stringent criteria against numerous batches of beans that had potato taste defect,” says Wong, who graduated in spring 2013 with a bachelor’s degree in biological sciences and is currently working in Miller’s lab. “When we roasted the beans, we found that all the potato taste defect microbes were killed.”

Truong examined whether the potato taste defect microbes can be manipulated to affect coffee taste.

“My experiment is a stepping stone to finding a solution for potato taste defect,” says Truong, who will graduate with a bachelor’s degree in biochemistry in 2015. “Are bacterial/fungal infections the source of the problem? Can the affected coffee beans be treated to remove the effects of potato taste defect? These are some of the questions I am exploring.”

Neither Wong nor Truong have traveled to Rwanda yet, but are in close communication with researchers there. With its high altitude and volcanic soil, Rwanda is an ideal place to grow specialty coffee. To tackle the antestia-potato taste challenge, Miller and the rest of the international team traveled to Rwanda in early 2012 to join coffee researchers at the National University of Rwanda (NUR).

Then, Miller stayed in Rwanda for two weeks that comprised meetings, workshops and numerous field trips. He has been in nearly daily email contact with Rwanda since.

“That visit helped us all get a better understanding of potato taste and its causes,” he says. “We gathered coffee bean samples for analysis in the United States and began collaborating with Rwandan scientists. We also assisted Rwanda in reaching out and making contact with people willing to help solve the potato taste defect problem.”

The culmination of the first two years of the PTD project will be a coffee summit on 1-2 April 2014 at NUR. The meeting is being organized by NUR and the Global Knowledge Initiative, a Washington DC, non-profit organization dedicated to finding solutions to problems in developing countries.

More information about PTD can be found at www.coffee.ucr.edu.