The most disappointing bag of coffee I ever bought was from a well-respected specialty coffee company. The bag was being given away in a social media contest where the gorilla mascot on the cover was being named. I submitted the name Magnum - and won.

But the Rwandan coffee wasn’t the paradise of earthy and fruity flavors I expected.

It tasted like potatoes.

We’ve talked a lot about how to taste coffee, but so far we’ve focused on the flavors we love. We haven’t, however, touched on flavor defects.

Flavor Defect - a natural or manmade problem during coffee growing, processing, or roasting that results in a noticeable and distinct flavor imperfection.

You’ll run into these quite often with cheap, low-grade coffee. And, though it’s very rare, even specialty coffee companies send out defected coffee occasionally without realizing it.

Coffee flavor defects are frustrating. They rob you of a rich daily experience that brings you joy. They confuse you and make you wonder what went wrong to make the coffee taste so bad.

By learning about the most common defects, you’ll be able to identify them and inform the coffee company you bought the beans from - no frustration required.

Farming, Processing, And Storage Defects

Most of the magic with our daily brew’s flavor happens on the farm and at the processing station, but farming and processing mistakes happen. Even skilled farmers can make honest mistakes.

Here are some flavor defects that are the result of poor coffee management at the farm.

Over Fermented

All processing methods use fermentation to remove the sticky mucilage and develop the rich, well-rounded flavors we love, though each method approaches it a little differently.

Read: Coffee Bean Processing: The Washed Method

Balanced fermentation can lead to exotic, rich fruity flavors and a crisp wine-like acidity. Think your bright blueberry flavored Ethiopians or richly floral Guatemalans.

However, if fermentation is allowed to continue beyond that sweet spot, the coffee takes on a rotten fruity flavor. Or worse... vinegar. Yuck!

Over fermentation can happen to any bean, but it’s especially common when the cherries are over ripe when they’re harvested.

Musty/Moldy

Coffee beans are porous, which means they let the aromas of things around them seep in. If the beans aren’t dried properly to 10-12% moisture level during the last stage of processing, extra aromas from the dirt and fungi in the air can get stuck in the beans even more easily.

Like your coffee to taste earthy, musty, or moldy? Me either.

Usually, these beans are not dangerous to drink (roasting kills most harmful bacteria) - but they sure won’t taste good.

Read: The Incredible Journey Of The Coffee Bean (Seed To Cup)

Potato Defect

Many coffees from East Africa, including the Rwandan coffee I won by naming the gorilla, are hit with a mysterious potato flavor that we regular folks call the “potato defect”.

Unfortunately, research into the cause of this odd defect is incomplete, though most scientists theorize a mold or insect is responsible.

It’s not easy to detect before brewing, but the aromas and flavors are undeniable once you have a mug in your hand. Just a handful of afflicted beans can impact an entire 12oz bag. And since it only takes a few beans to damage a batch, they can be hard to detect on the farm.

I’ll never forget how awful that Rwandan coffee tasted. Magnum the gorilla betrayed me.

Chemical/Medicinal

This defect can be subtle or intense, but it’s always a cup-killer. Nobody likes the taste of bad alcohol, weird chemicals, or medicine in their coffee - that’s just gross.

There are several sources for this defect, including:

• Storage in plastic bags for too long or at high temperatures
• Industrial substances stored closely to the odor-absorbing coffee beans
• A variety of processing management errors

Read: 3 Reasons Buying Cheap Coffee Is Bad For The World

I’m fairly certain I tasted this defect in a cup of low-grade robusta once before I knew much about coffee (maybe that’s why I didn’t like coffee as a teenager), but thankfully I haven’t encountered it since.

Rubbery

A subtle, yet cup-killing burnt-rubber flavor can be formed when the coffee cherry dries while still attached to the shrub. This one’s usually limited to robusta plants.

Cappy

Refers to coffee that has an unpleasant sharp cheese flavor, often as a result of being stored or shipped with cheese.

Yes, it’s a thing.

Don’t judge me, but I actually do want to taste this one.

Baggy/Past-Crop

When stored for long periods of time, the unroasted beans’ fat soaks up the aromas of the bag - most often with burlap or jute bags. Moisture is also lost over time and a variety of slow chemical reactions take place.

This can cause the old coffee to taste like the bag it was stored in and usually is accompanied by a loss of sweetness and flavor. Thankfully, this defect is not very common anymore thanks to improved storage technology.

Read: Why Fresh Coffee Is The Best Coffee

Roasting Defects

These next few defects are the result of roasting errors. Experienced roasters rarely make these mistakes - if ever - but newer roasters and home roasters often have to learn about them the hard way.

Baked/Stalled

Baked coffee beans are just disappointing in every way. The pleasant flavors are killed entirely, as well as the sugars and acids, leaving you with a dull, lifeless coffee that has notes of oats, grain, and bleh.

This defect can occur in one of two ways.

• The roasting temperature drops. Once the beans are in the roaster and that very first temperature drop is finished, the temperature should only rise. If you lose enough heat or energy to stall that temperature rise or even create a drop, you’ll destroy the acids, sugars, and everything good in the bean.

• You roast for way too long. Most specialty coffee roasters target 10-16 minutes for their roast profiles. If you go longer at a medium-high temperature, you’ll bake the coffee.

Baking coffee is sort of a rite of passage for new roasters, but it’s a pretty easy one to grow out of once you have a few dozen batches under your belt.

Read: What Makes Specialty Coffee Special?

Underdeveloped

This used to actually be a big problem in the specialty coffee world. For years, roasters were so afraid of over roasting and burning off all the flavors that they swung too far to the other end of the spectrum and started under roasting them.

The result was underdeveloped coffee, which often has flavor notes of hay and grass. These coffees are also very acidic to the point that it hurts your throat.

Thankfully, specialty roasters have realized that under developing coffee is just as gross as over developing it and experienced roasters are long beyond accidentally under developing.

Overdeveloped

This one is our standard over roasted, burnt coffee.

The beans are black and oily and the coffee tastes like ash, carbon, and raw bitterness. They’re bad in every way, but this has been the norm for centuries.

Thank goodness we’ve moved beyond this (at least, a good portion of us)!

Read: The Difference Between Light, Medium, And Dark Roast Coffee

Scorched/Tipped

These two roasting defects are distinct from one another, but they’re both the result of beans receiving either too much or unbalanced heat.

Scorched coffee is the result of coffee being thrown into the roaster at too high of a temperature and beans literally are scorched at various places on the outside. Tipped beans are beans that have dark marks on the ends of the beans as a result of rapid moisture loss and unbalanced heat on the ends.

Both defects result in a smokey, ashy flavor that tends to overpower the other pleasant flavors of the coffee.

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Whew! Hope I didn’t scare you with these coffee flavor defects. They’re not fun to come across, but now that you know about them, they won’t ruin your day by making you think you’re just a bad coffee brewer. These defects are in the beans - not your skills.

Thankfully, you can avoid these by buying coffee from roasters with years of experience.

Our Chief Brewing Officer, Raj, visited dozens of roasters to find the one that could match our vision for quality (and hatred for flavor defects) - and he found the perfect partner.

Our roaster in San Diego has been sourcing and roasting with excellence for 10+ years. We partner with them to find the best coffee beans in the world that are specialty-grade - which means they have the lowest possible number of defects.

If you’d like to experience coffee that’s rich with vibrant flavors like blueberry, honey, and spice, check out our JavaPresse Coffee Club. We even roast and ship your coffee to you on the same day, which means you get to enjoy the coffee while it’s at peak freshness (and flavor).