Coffee ranges from heavenly nectar to gutter water. How can it be so inconsistent? What makes a good bean, and what makes a bad brew? Well, here’s the short answer to what makes coffee taste bad:

Many factors matter: water temperature, bean quality, grind level, and brew time. Your equipment should be clean, beans fresh, and the right time/temp for brewing. This will yield the best results.

There are two different kinds of bad: one is in the bean, which can have a variety of causes; the other is from problems in the brewing process. 

If you prefer visual representations that get to the point (and gloss over details), skip to the very bottom!

What makes a “Good Bean”?

1. Freshness

This is the most obvious factor when it comes to the quality of a coffee: fresh ingredients! However, as soon as the beans are roasted, they start losing flavor and freshness.

Every moment beans are sitting out, they’re emitting carbon dioxide (CO2). As they let the gas out, oxygen can get in and start oxidizing the tasty oils inside the beans. This can cause them to go rancid, which, while safe to drink, will taste like trash.

Some believe that beans sitting open for 24h actually lose 10% of their shelf life. That means in ~10 days of sitting out, they’re as stale as can be.

The Fix: Toss out old beans, don’t buy beans in bulk, and make sure you keep your beans in an air-tight container at room temperature outside of direct light, optimally with a date of purchase/roasting on it. The best time is between 4 days and 2 weeks old (some say day 10 is peak flavor). 

As well, only grind what you need for maximum freshness. 

Finally, avoid keeping your beans in the fridge or freezer, as the condensation will make them age even faster! Dry, air-tight, and room temperature is best!

2. Roast

If you’re like me, you may think that the roast is only an issue if you roast them yourself (I have; it was a misadventure). But, in truth, even industrial roasters have difficulty with consistency. 

You see, beans need to constantly be jostled around to ensure they don’t burn. An analysis of 15 different roasters found that the sugar and protein contents of blends were inconsistent, meaning that the beans were not uniformly roasted. It’s a tricky business!

The Fix: depends on where the bad beans came from. If you roasted them yourself, try again and be extra careful with each step and consider cleaning your machine beforehand. If you bought it from a large roaster, try a different roast batch – which can be as simple as buying another bag of the same blend on a different day.

3. Quality

This is much harder to discern from simply looking at the bean or origin. 

For those of you who are new to coffee, there are two main bean varieties: robusta and arabica. Arabica is considered sweeter, more expensive, and more desirable. 

Robusta is cheaper, easier to produce, and has nearly twice the caffeine – but its flavor profile is considered flatter and less appealing.

The Fix: consider where it’s grown or how much the beans cost. Generally, I’m not a fan of the “expensive = quality” shorthand, but in the case of premium goods like coffee, the more valid shorthand is “inexpensive = bad”. From there, you’re going to have to experiment with the brand, blends, and bean mixtures.

4. Personal Preference

As just mentioned, many coffee snobs think that robusta is a waste of time. Personally, I don’t mind it and actually like it in certain forms (like Vietnamese coffee). No such thing as a bad flavor, just bad uses. 

But here’s what the other side says about them:

In the Tea & Coffee Trade Journal, a tirade was written about robusta:

I have discouraged the acceptance of [robusta] because… to my mind, the species is incompatible with the spirit of virtue that our coffee should represent to the world. The gospel according to David Weinstein …. calls for uncompromisable coffees. [Robusta] is a compromise. No coffee man adds [robusta] to a blend based on the taste qualities it brings to the table.

See? Personal preference. The speaker clearly never heard of a Vietnamese coffee.

The Fix: Unfortunately, there’s no accounting for taste.

What makes a Bad Brew from a Good bean?

1. Grind Size

This post talks about the varieties of grinds and what they’re good for, but they have select grind levels for various kinds of brewing methods. In general, the rule of thumb is that it needs to be consistent.

If it’s inconsistent, it’ll extract the flavors at varying rates; some will pull out bitter flavors while others may pull out sour flavors. The easiest thing to do is to match the grind coarseness to your brewing method.

TL;DR: The longer it’s going to be steeped, the coarser you want it. The quicker it’s made, the finer it should be. So, coarse for french press; superfine for espresso.

2. Bad Water

This factor will be overlooked by most people – I know that I did.

When it comes to water, there are two main facets that will affect the coffee it produces.

• Temperature
  • Many people erroneously believe that boiling water (100ºC/212ºF) is optimal for making coffee, pouring it straight from a boiling kettle onto the beans. Not so.
  • Why? Because if it’s too hot, it will destroy elements that are needed for the perfect cup, namely volatile oils. 
  • A few degrees seems to make all the difference: you want the water to be as close to 205ºF (98ºC) as you can get.
  • The Fix: you’re best off buying a thermometer and measuring as you go. Science!
  • IMPORTANT UPDATE: As it turns out, even if you heat the brewing container, the beans and the pouring of the boiling liquid seem to bring it down enough to bring it within acceptable temperature ranges. TL;DR: You don’t need to worry about this at all.
• Water quality
  • This one is so variable that it could easily throw off the process. Do you have hard water? Chlorine? Other random particles? Any or all of that could throw off the chemistry when brewing. 
  • The Fix: filter your water (cold) before boiling.

For a much more thorough breakdown, check out this monster post.

3. Timing was off (over/under extraction)

Only about 60% of the beans’ dissolvable parts (solutes) are tasty. Luckily, that flavorful portion is the quickest part to dissolve. This means it’s a balancing act between too short and too long.

• Too little time, and you’ll get a thin, watery, coffee-like substance.
• Too long, and it’ll be a bitter, acrid brew.

Timing Guidelines:

• Espresso should only be roughly 30 seconds.
• Drip pots are somewhat variable, between 5-10 minutes
• French press takes between 2-4 minutes of steeping before pouring

The Fix: Experimentation. Unfortunately, it’s all determined by brewing method and grind size. I cover some specifics in this post.

4. The Brew has Expired

Old coffee is bad coffee, but how long does a brew last once it’s sitting in your cup?

• There’s some disagreement, but here’s what I could find:
  • Espresso: Until the Crema (the foam on top) dissipates
  • Other Brews: the general consensus seems to be that coffee is still good until it cools. So, roughly 30min in an open cup, or up to 4 hours in a sealed container.
  • Note: The oils in coffee start to turn rancid after 4 hours in an open cup (couldn’t find time frame for a thermos)

5. Coffee’s too Weak

This one’s pretty easy to fix. If you use too little grinds or too much water, then the brew will come out thin and weak.

The Golden Rule:

• 1-2 Tbsp of ground coffee for every 6oz water. (If you want to be more precise, they say 60g for 1L)
• That’s a lot of variance, but experiment and see where you like it best.

6. Dirty Equipment

When was the last time you cleaned your coffee maker, including the water basin? Every part that water touches needs to be periodically cleansed. 

I use a vinegar soak, running it through the machine, and then running water through it several times to remove any leftover vinegar.

The Fix: clean your coffee maker once a month.

7. Old Equipment

There’s no avoiding the ravages of time, unfortunately. Your ol’ reliable coffee maker will eventually wear down and break over time. 

It could be that the rubber becomes sticky, the plastic degrades, or the metal rusts – regardless of what it is, if you’re doing everything right and you get bad results, it may be because your machine is past its prime.

The Fix: investigate any visible signs of breakdown and try to fix it, or get new equipment.

8. Wrong Equipment

We’ve all been there: you want to try making something, but your equipment isn’t perfectly suited for it.

It’s not a big issue, but the results may be subpar when compared to the real, dedicated equipment that was designed for this purpose.

The Fix: Use equipment dedicated for your target beverage.

As well, the cup you use can have an effect. I’ll say this just to be safe: don’t use plastic cups. Hot drinks can increase the amount of plastic breakdown, and they can sometimes melt. And no one should be drinking plastic.

At home, use ceramic or tempered glass cups. 

On the go, stainless steel thermoses are your friend.

9. Mistakes

Due to the fact that we’re not all perfect androids who were built to make coffee in a controlled setting, we will make mistakes.

The Fix: try a few more times with extra attention and see if your results change.

10. Personal Taste

Dude, you’re may just drinking a blend you don’t like.

The Fix: try a different one.

What causes certain flavors?

Bitter

1. Over extracted: try brewing it for a shorter duration
2. Water’s too hot: if you brew with boiling water, it can cause undesirable results either by breaking down good oils or by extracting undesirable elements. Debunked!
3. Stale Beans: if your beans are stale, they can become bitter and/or acidic/metallic tasting

The Fix: Option 1: if you’re stuck with bitter coffee, make the best of it and add some condensed milk to do like the Vietnamese do (Post on Vietnamese coffee). 

Option 2: add cream and sugar. Some also suggest a pinch of salt will help to bring out the better flavors. Experiment!

Sour

Under Extracted: gotta let it brew a bit longer so you can draw out more positive elements. Alternatively, your grind is too coarse for your brew, which requires more time to brew.

The Fix: A finer grind, or longer brew time

Metallic

Surprisingly, this is most often not caused by your machine.

It is probably caused by the quality of water you’re using, especially if you’re using chlorinated or well water.

The Fix: filter your water (cold) before brewing.

Burnt

This can happen if the beans are over-roasted (I’m looking at you Starbucks).

It can also be the result if your coffee’s heating element is too hot.

The Fix: After brewing, keep it as warm as you can without being too hot. Too high heat will destroy the flavor.

Plasticky

Now is the time to interrogate your machine. Chances are very low that it has anything to do with the method or your coffee beans.

The Fix: A deep clean in your machine or an outright replacement is required, especially if the plastic is too degraded.

Watery

Same as the section above (Item 5: Coffee’s too Weak). 

The Fix: Use a finer grind, more coffee grounds, less water, or a longer brew time. As well, ensure your water is hot enough because cold water doesn’t perform as well in shorter durations.

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Tables

The promised chart for those who process information more visually:

Broad Factor	Comment
Beans	Should be freshly ground Should have been roasted within 2 weeks (optimal); within 1 month (or they go stale) Should be stored in an airtight container, outside of light, at room temperature. (Not in the fridge!)
Roast and Quality	Sometimes the roast is inconsistent or off. If this is the issue, just try another bag.
Grind Size	Keep it consistent!
Water	Soft, cold, filtered.
Timing	Coarse grind = long; Fine grind = short.
Expired	Most brewed cups are bad when they get cold.
Weak	Use more coffee, more time, or less water.
Equipment	Clean it! Or replace it! Get the proper equipment!
Human Fallibility	Try, try, again!

Problems and Fixes

Specific Problem	Cause	Fix
Bitter	-Brewed too long -Beans are stale	Use it for Vietnamese coffee; try adding cream, sugar – and maybe a pinch of salt.
Sour	-Too few beans -Too short brew duration -Beans are too coarse for the brew time	Use more beans; Brew Longer; try a finer grind.
Metallic	-Hard/Well Water -Sediment from Boilers	Use softer, filtered, cold water
Burnt	-Roast was too dark -Coffee was left on a heat element after brewing	Buy lighter roasts, don’t reheat
Plastic	-Your machine may be degrading -Your cup is made of plastic	Check your machine and consider upgrading; only use metal, glass, or ceramic cups
Watery	-Too few beans -Too much water -To short brew time -Too coarse grind	Modify those variables and see what helps. 60g coffee per liter of water is recommended. Coarse grinds require longer brewing.

Hopefully, whatever your issues are, this will help you solve them.

If you’ve read through all of this, then congratulations! You should be familiar with how to fix your coffee woes! Congratulations!

If you think I overlooked any, feel free to comment below and let me know!