July 28, 2021

What is potato taste defect & how can coffee producers stop it?

The coffee producing countries around Africa’s Great Lakes are known for producing some of the finest beans in the world. They include Ethiopia, Kenya, Burundi, Rwanda, the DRC, Tanzania, and Uganda.

Despite this, coffee producers in the region struggle with a recurring problem: potato taste defect (PTD). This results in coffee that tastes and smells like raw potato, the flavour of which overpowers other delicate tasting notes.

Furthermore, agriculture is a major source of income in this part of the world. In Rwanda alone, some 90% of the population relies on agriculture as its main source of income, with coffee accounting for 25% of all exports. Despite this, coffee yields in Rwanda and Burundi can be as low as 1.5kg per coffee tree, limiting their income, which is further hindered by any issues with PTD.

So, how does PTD occur and how can it be stopped? To learn more, I spoke to four researchers. Read on to find out what they said.

You may also like our article on coffee defects & how to avoid them.

coffee farm with workers

The origins of PTD

Dr Joseph Bigirimana is a senior scientist at Rwanda Agriculture and Animal Resources Development Board (RAB). He has over 14 years’ experience in coffee research, and his PhD thesis specialised in managing and minimising the occurrence of PTD.

“[PTD] was first detected in the Great Lakes region of Africa, specifically in the eastern part of the [DRC],” Joseph says. “It diminishes the flavour and the perception of quality, reducing its value or causing it to be rejected by consumers.”

However, even though it was first discovered in 1940, research into PTD and why it occurs is relatively new. 

“Scientists have previously associated PTD with [the] antestia bug, but this pest species does not occur only in the Great Lakes region of Africa,” Joseph says. 

Antestia bugs – a genus of shield bugs – can also be found elsewhere in the world, including in Pakistan, India, and Sri Lanka. However, the link between antestia bugs and PTD is mostly observed in the Great Lakes region, despite the fact that the bugs exist in other parts of the world.

Mario Serracin is a plant pathologist and coffee agronomist for Rogers Family Company in Rwanda. He has monitored antestia bugs to analyse how they affect coffee production.

“[PTD] is caused by a bacteria that was not officially identified until French researchers studied antestia bug-damaged coffee beans collected in Burundi,” Mario says.

Ruth Ann Church is the President and founder of Artisan Coffee Imports – a green coffee trader based in Michigan, US, which focuses on Rwandan and Ethiopian coffees.

“It’s actually bacteria that causes the potato taste,” she tells me. “However, this originates in the antestia bug, [so] it’s more like the antestia bug is a transporter.”

Ruth Ann has worked extensively in Kigali, Rwanda with the USAID-funded Africa Great Lakes Coffee programme to reduce incidence of PTD and improve coffee productivity.

shield bugs on coffee fruit

The antestia bug

Susan Jackels was a Professor of Chemistry at Seattle University from 1995 to 2015, and has been a Professor Emerita since 1995. She has written several research papers on PTD, including on its relationship with the antestia bug and how its severity can be analysed in roasted coffee.

“There are two mechanisms, at least, under investigation for the relationship between antestia bug feeding and PTD,” Susan explains. “One is that the bug leaves a hole that can be invaded by a bacterium, which produces the bad smelling pyrazine that causes PTD. 

“The other is that the antestia feeding causes stress on the coffee plant, and the plant then produces the pyrazine in the cherry in response to stress.”

Although they may not be the direct cause of PTD, it’s clear that antestia bugs play a significant role in causing the defect.

“The adult bug is shield-shaped, about 6 to 8mm long and dark brown with orange and white markings,” Joseph tells me, “They hide in berry or flower clusters. Females lay eggs in groups of about 12 on the underside of leaves.”

Antestia bugs have been detected at altitudes ranging from 1,300 to 2,135 m.a.s.l. – an optimal elevation range for the production of arabica coffee.

“It is estimated that the yield loss caused by antestia bugs averages 30% [on infested trees], but it can be as high as 38% if the insect population reaches 15 bugs per tree,” Joseph says. This shows how detrimental the impact of antestia bugs can be for East African coffee farmers, many of whom already suffer from low yields.

Furthermore, research has shown that an increase of just one antestia bug per tree can increase the risk of PTD occurring by as much as 73%. It also estimates that antestia bugs can be found on as many as 98.7% of Rwandan coffee farms, meaning the potential that PTD will occur is extremely high.

Mario explains how the potato aroma and flavour is produced.

“The potato-like odour is caused by the high concentration of bacteria in contaminated beans,” he says. “These bacteria form [a compound called] 2-isopropyl-3-methoxypyrazine (IMP) when beans are heated [between] 60 [and] 200°C.”

This means there is another problem for farmers, traders, roasters, and consumers: PTD is not easily detectable until the coffee has been roasted, at which point the damage is already done.

ripe coffee cherries in hand

Implications for East African coffee production

Although coffees from the African Great Lakes region can achieve cup scores of 90 points or higher, PTD poses a great risk for producers in the area.

According to Ruth Ann, at 2013 Cup of Excellence (CoE) events, 62% of entrants from Burundi and 51% of Rwandan competitors had to withdraw from the competition because of PTD.

PTD has also stifled the development of some countries’ coffee sectors. In 2001, the Rwandan government set out to increase specialty coffee production as part of a strategy to develop the economy.

The construction of washing stations increased by 26% between 2002 and 2012, which helped specialty production grow, but PTD inhibited further development.

“In March 2014, the Rwanda coffee board hosted an international coffee symposium, which was known as the ‘potato taste conference’ by those who attended,” Ruth Ann says. “It was the point at which Rwanda’s coffee board started to take [PTD] seriously.

“The conference was heavily supported by [CoE], and offered US $20,000 as a prize to whoever had the best proposal for new research [on PTD].”

The Alliance for Coffee Excellence (ACE) and Global Knowledge Initiative also offered cash rewards as part of the “Potato Taste Challenge Prize”, a grant to support research into detecting and preventing PTD. 

Joseph was a recipient of the prize, and has since gone on to lead a team in analysing different pest management methods for Rwandan coffee farmers.

Ruth Ann also notes that the use of the word “potato” can be viewed as negative by coffee professionals in East Africa, as it furthers the harmful perceptions of coffees from this region.

“In some of the seminars that we have given in Rwanda, we’ve [received] polite guidance [to] not use the word ‘potato’,” she says. “[Alternative] words that we often use are vegetal, musty, or earthy.”

unwashed potatoes

How can farmers check for PTD & defend against it?

Antestia bugs create lateral holes in coffee cherries, which are visual indications that PTD may have occurred. After pulping and washing, colour defects may also illustrate that the beans should be discarded.

However, Susan adds: “Because of the low concentration of IPMP and the extreme sensitivity of the human nose to this compound, gas chromatography-mass spectrometry is the only method to date to positively identify PTD in green coffee.” This level of complex scientific testing is not accessible for many smallholder farmers.

Ruth Ann adds: “[The] extended supply chain in coffee makes it a real challenge for the producers to correct something that they’re only sometimes finding out is a problem.” 

In her research paper, Ruth Ann states that the average FOB price for Rwandan green coffee between 2015 and 2017 was around US $4 to $5 per kilo. However, PTD can reduce these prices by anywhere from US $0.30 to $2.00.

However, to mitigate this, Mario says farmers can use a number of techniques to reduce the amount of antestia bugs on a farm. 

“We train farmers and researchers to scout and control the antestia bug with integrated pest management [tactics], such as scouting, spot applications of natural insecticides, [and] pheromone traps.

“In the wet mills, we float and separately process floating and [insect-damaged] beans, use yeast to control fermentation, and sort [the] wet and dry beans with great [attention to] detail.”

In a paper which he co-wrote, Joseph details how pruning can also reduce the risk of PTD.

“Pruning helps,” he says. “This is because the antestia bug is drawn to shady, dark areas; coffee trees that have not been well pruned have these dark leafy spaces.

“Researchers at CIRAD observed that coffee farms where the antestia bug is effectively controlled could produce beans with less than 1% [PTD]-contaminated cups, and that with proper processing and sorting, we could significantly reduce the incidence of [PTD] in coffee beans.”

coffee roaster with roasted beans

Managing PTD as a roaster

Ruth Ann says: “Roasters understandably are sometimes hesitant to pay for a Rwandan or Burundi coffee because they could end up with potato taste.

“There’s no way you can erase the taste that a customer has had if they happen to get that potato taste, but we can also join together as an industry and try to mitigate that risk by communicating back to the producers.” 

Research conducted by Counter Culture Coffee found that roughly one occurrence of PTD is common in every 1.55 kg of coffee from this region. However, roasters and green buyers typically sample coffee in batches of 30g to 200g.

Ruth Ann says: “Just because [what] you cupped tasted terrible, it might not be time to throw away the whole bag. You’ve got to open each one and test them.”

Without a more comprehensive protocol for checking for PTD, demand for coffees from this region could plummet.

Ruth Ann says: “I wasn’t hearing from customers when they had potato tastes; they were just telling me the next year that they weren’t going to buy.

“In response, I created a potato taste guarantee for roasters, where they can get two bags replaced [with almost no] questions asked, because I want them to tell me if they [receive] potato taste coffee.” 

Ruth Ann also emphasises how communication along the supply chain is essential in helping to reduce any incidence of PTD.

“I also have contracts with my suppliers [which include] an entire page agreement about if the coffee has potato taste,” she says. “We [provide] a potato taste incident report, confirmation the report has been shared with operations managers [at farm level], and quality control.” 

Ultimately, there is no way to be 100% sure with every bag of green coffee that you buy. But for roasters, grinding coffee in smaller batches, purging grinders when PTD is detected, and not using these coffees for more concentrated beverages – such as espresso or AeroPress – are all ways to mitigate it.

“[Most roasters] understand, as long as it’s not too intense,” Ruth Ann concludes. “They can manage it.”

green coffee beans

Ultimately, the only way to reduce PTD is to keep investing in coffee production in this region of Africa. The more money these farmers receive, the more investment will go into combating PTD. 

Avoiding coffees from these countries does a great disservice to the farmers working hard to grow high-quality beans. As an industry, we need to make greater efforts to support these producers, and keep buying more of their coffee.

Enjoyed this? Then read our roaster’s & coffee buyer’s guide to Rwandan beans.

Photo credits: Mario Serracin, Pixabay

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