We’ve recently had the start of our fresh crop Rwandans land at the roastery, two have now made their way onto the offer list, the Shyira & Bumbogo. After getting them profiled & the first production roasts done we left them in the storage bins overnight ready to brew some up first thing in the morning on filter. The bins we use are airtight so after a few hours the gases start to build and usually when you open them, your nose is greeted with the pleasant smell of the roasted coffee, however… After the Bumbogo had sat in its bin overnight we where greeted with the wonderful smell of…. POTATO!! No, nobody left their bag of spuds in the bin with the coffee in fact the smell was actually produced by the coffee itself!
The smell of potato isn’t meant to be the main aroma from Rwandan coffee but it’s a defect, not every bean is going to be affected, so why does the coffee smell of potato? Has it started to rot, did we buy a bad bag? The answer is no! The potato defect has been found as a natural occurrence in certain East and Central African Great Lakes coffees, mostly in Rwanda, Burundi, the Democratic Republic of Congo, and Uganda. It has been less frequently encountered in Tanzania, Zambia and Kenya, as well. The defect is not always apparent in whole bean form but once the coffee is ground it’ll present itself in all its potato glory!
The potato defect has been linked to a native insect called Antestia, after these insects feed on the coffee cherry it leads to a bacterial infection of the seed. Studies have shown that the bacterium, Coffeiphila actually produces a potato like odour. Even though it’s a bacterial infection of the seed it won't make you ill, it’s just an unpleasant taste defect.
Can we get rid of the potato defect??
Farms in affected countries are advancing their measures to try and combat the issue, targeted insecticides being one measure. However in smaller farms this isn’t always a viable option due to the cost.
Antestia bugs thrive in cooler temperatures; Rwanda & Burundi harvest most of their crop in the rainy season where temperature drops below 25 degrees. Post harvest sorting by hand seems to be the primary source of defect reduction, some farmers will float the cherries in tanks allowing less dense beans that have been damaged to be discarded. After pulping and fermenting colour defects become more visible allowing further hand sorting.
What to do if you encounter a potato in your bag???
If you smell a potato in your bag of coffee, keep using it normally until you eventually hit the defect when grinding, discard that dose purge your grinder & carry on enjoying the wonderful coffee that these African countries produce!!