How battle is waged upon Potato Taste Defect (PTD)
If you’ve not had the pleasure of tasting it in a brew, Potato Taste Defect, or PTD, is a coffee quality defect that makes coffee smell and taste just like raw potato… delicious.
It is found in coffee from The Great Lakes region of Africa – most commonly, Rwanda, Burundi, Western Uganda, and the Democratic Republic of Congo. For some roasters, the likelihood that they will encounter this defect is enough to steer them away from purchasing coffee from this region. But how concerned should you really be about PTD?
Picking cherry, Nyamasheke, Rwanda
What Causes Potato Taste Defect?
Research indicates that pyrazines are responsible for the odour and flavour of PTD, specifically 2-isopropyl-3-methoxypyrazine (IMP) and 3-isobutyl-2-methoxypyrazine (IBMP). Pyrazines are organic compounds known for producing strong grassy and peppery odours. They are found in consumables such as wine, meat products, and vegetables but are also used as additives in cereal and perfume.
The presence of IMP and IBMP in coffee cherries has been linked to the antestia bug – a pest native to Africa that feeds on the leaves and fruit of coffee trees. Some researchers have theorised that coffee trees produce pyrazines as a stress response to the antestia bug’s feeding habits. However, other researchers claim that the damage caused by the antestia bug enables bacteria to enter the coffee cherry, and these bacteria lead to the production of IMP and IBMP.
Thankfully, whether pyrazines produced in this case by stress or bacteria, they are perfectly safe to ingest. So, you don’t need to be concerned about what impact drinking a coffee with PTD might have on your health. It just won’t be the tastiest coffee you’ve ever had.
Minimising Potato Taste Defect at the Farm
Over the last decade or so, great effort has been made to reduce the occurrence of PTD starting at the farm. Most of this effort is targeted towards reducing the antestia bug population, since the antestia bug appears to be the main culprit behind PTD. Studies have shown that the most effective way to do this is through integrated pest management.
1. Regularly pruning coffee trees – since the antestia bug prefers to live in a shady environment, managing overgrowth and weeds is an easy way to discourage the antestia bug from setting up shop.
2. Targeted use of pyrethrum-based insecticides – this helps to control the antestia bug population. Often farmers in a region will coordinate their applications so that the bugs don’t just migrate from one farm to the next. Pyrethrum-based insecticide is the most effective against antestia, and because it is made of dried chrysanthemum flower which makes it compatible with organic certification regulations.
Of course, good farm practices such as regular mulching, fertiliser application, and weed maintenance help to keep the trees healthy and improve coffee quality. As does removing all remaining coffee cherries from the trees at the end of the harvest. Cherry removal also has the added benefit of discouraging pests from inhabiting the trees by removing a potential food source.
Minimising Potato Taste Defect at the Washing Station and Mill
Once coffee has been harvested, minimising the occurrence of PTD comes down to identifying and removing defected beans. This is most commonly done by hand sorting and removing any cherries with tears or abrasions. But can also be achieved by floating coffee cherries in water to separate low density (damaged) cherries. At the mill, this is assisted by using technology such as colour sorters, density sorters or UV light. Usually, a combination of methods is used.
Our friends at Muraho Trading Co, for example, hand sort at every stage of processing, but also use colour and density sorters. They will then cup every coffee multiple times to check that specialty grade lots have little to no defect. And yet, even when all the right processes are followed, it is possible that a bean with PTD will still end up in a sack of coffee.
Minimising Potato Taste Defect in the Cup
Thankfully, one bad bean does not ruin the bunch. If you get hit with a waft of potato when roasting or brewing your coffee, it’s likely that there are just one or two defective beans in the mix that are responsible.
If you’re a roaster and you’re sampling a selection of coffees from a region where PTD is known to occur, try to increase the number of cups for each lot that you’re sampling. This will give you a more accurate representation of what a sack of this coffee is like. We wager that you won’t come across PTD as often as you might expect, and if you cup more samples of each lot, you won’t miss out on buying a delicious coffee based on one potatoey cup.
If you’re brewing, make sure to grind your coffee in small doses and smell each dose before combining. This will minimise the amount of coffee that you need to discard if you do come across PTD. And if you do identify PTD in one dose, dispose of it then purge the grinder with coffee from another origin that is unlikely to have PTD. This will ensure that subsequent doses don’t get tainted with potato grinds.
Ultimately, however, we need to approach PTD with understanding. Extraordinary effort has been made in places like Rwanda and Burundi to reduce the occurrence of PTD, and while this effort has been largely successful it may not be possible to eradicate this defect altogether. But by slurping a few extra cups of samples and grinding in smaller doses, you can continue supporting producers in this region and drinking high quality, tasty coffee.
How concerned should you be about PTD?
We think, not very.
Murakoze Cyane to Muraho Trading Co. for sharing their knowledge with us for this article!