A few days ago I reviewed a Burundi from ROWSTER in Grand Rapids; the package and the barista who sold it to me said that the coffee was intensely fruity and bright with flavor notes of pineapple, strawberry, and coconut. Neither of them even mentioned the thick flavor of split pea soup that I tasted immediately post-brew. I was later informed by the owner of ROWSTER, Kurt, that I was either incredibly lucky or incredibly unlucky to have experienced the infamous “potato defect.”

Kurt writes:

One of the problems with coffees from Rwanda and Burundi has been the potato defect, which is a fairly common problem around the world, but not always called “potato”. The defect is caused by an insect burrowing into the cherry and contaminating it which then produces a normal looking bean which tastes like a raw potato and smells like sticking your nose into a sack of potatoes. It is quite strong. We encounter it probably once in 5-10 pounds. I am told that they have recently discovered the exact cause and are working to eliminate it.

Sure, I was disappointed that this coffee that was sold to me so effectively turned out to have a defect in the cup, but it got me thinking: what are some other common coffee defects that consumers should be on the lookout for?

In today’s Friday Feature, I’m going to address some defects that keep your coffee from tasting as good as it possibly can. By no means is it a comprehensive overview of all the plights and plagues that farmers have to battle with each crop, but it will at least give you a good idea of the incredible amount of things that can go wrong at the farm level, that coffee farmers have to battle to ensure that you get the highest-quality beans for your morning brew or midday pick-me-up.

The Potato Defect

As Kurt writes above, the “potato defect” is a rare happening that occurs in coffees from several different countries, but seem to occur in Rwandas and Burundis most often (other regions that seem to be affected by the defect are Tanzania, Kenya, and Zambia).

The culprit of this defect seems to be a tiny little bug called the antestia. It’s a common belief that these bugs carry the fungus in their DNA; when one burrows its way into a cherry, it spreads its nasty germs like wildfire and infects the cherry. As you can see in the picture to the right, the tunnels that these bugs create are minuscule – very easy to overlook.

However, the entirety of the blame can’t be laid on these bugs because not all of the cherries that become home to the antestia become defected.

So, of course, it’s entirely possible that some of the chemicals that the farmers use can cause the defect. A bug creates a hole in the cherry, chemicals get inside the cherry, and subsequently infect it. Not probable, but possible.

Addressing this problem is a high priority in Rwanda and all over East Africa, and the primary strategies are: insect control, careful picking (no split skins), pre-pulping floatation tanks, and densimetric sorting (Oliver tables). All of these strategies have seen some positive affect, but not total success – while the potato defect isn’t as rampant as it once, it still occurs.

Leaf Rust

The most common enemy of coffee farmers the world over is the dreaded leaf rust.

Leaf rust first struck in Kenya in 1861. It was then reported in Sri Lanka in 1869. By the 1920s it was widely found across much of Africa and Asia. In the last decades of the nineteenth century, coffee rust did serious damage to the coffee plantations of Sri Lanka, Philippines, Java and Malaysia, which led to the collapse of the coffee industry in Lipa, Batangas and almost resulting in the extinction of the Arabica variety. It was most notably reported in Brazil back in 1970, just before it decimated the entire country’s coffee output. Since then, this bacteria has spread like a cancer to every coffee-growing region in the world. Most recently, leaf rust was discovered in Hawaii and has since nearly destroyed Kona coffees.

Leaf rust (or, Hemileia vastatrix) is a fungus that takes its name from the rust-colored lesions that appear on the coffee plant leaves that it infects. Like a stealthy ninja it moves swiftly, first impairing the plant’s photosynthesis, then defoliating it. The result is decreased yield, followed ultimately by the death of the tree. Leaf rust can account for a 20-80% crop loss when it attacks.

Coffee Berry Disease

Imagine yourself as a farmer. You’re walking through your crops, taking stock of your labor – everything up to this point seems to look fine. Then, one day, the trees blossom and, instead of a beautiful, vibrant red cherry, you find that the coffee fruit is a dark, sinister, decrepit black.

You’ve just encountered coffee berry disease.

Coffee berry disease (or, Colletotrichum) invades the main body of the plant but does so without any signs of disease. However, when the plant sets fruit, the fungus becomes agressive and the disease becomes apparent. In many cases, diseases caused by Colletotrichum are known as anthracnose because they turn the fruit black (as pictured to the right).

This is a tremendously crippling disease, with losses of up to 75% of a crop being totally destroyed.

This disease is currently confined to Africa, and some cultivars have proven themselves to be resistant to attack.

Green Coffee Defects

This is an infographic compiled by Colombian Coffee Hub that is incredibly handy. If you find any of these in the package of coffee you bought this morning, you may want to find a new local roaster to frequent:

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