Day-old coffee is safe without dairy or sugar though it will be bitter. If you need a caffeine boost, you’ll be fine, but don’t expect it to taste nice!

You can, of course, drink day-old coffee. On a chemical level, the caffeine stays intact. But as soon as brewed coffee cools, it starts to break down, and oils and acids are released—making it taste bitter.

So, your coffee will still have its strength, but not its taste. While generally seen as safe, any sugar or dairy (milk or cream) could lead to a build-up of bacteria.

What Happens to Coffee after a Day?

Coffee loses its quality rather quickly, losing its taste as it cools. As Food Scientist and Chef Melissa Collins of Perfect Brew explains, “Brewed coffee can be left out for about 30 minutes. After that, it’ll start deteriorating in taste due to a process called oxidation.”

This exposure to oxygen breaks down coffee, losing its intensity and taste. This is also why when you buy coffee beans or grounded coffee, it is often vacuum-sealed to prevent oxygen from getting to the coffee and spoiling it.

On a molecular level, the oils in coffee begin to break down and separate. If you’ve ever looked at the top of an old cup of coffee, you can perhaps see the oils swimming around at the top. This is called “Chlorogenic acid.”

Does Old Coffee Lose Caffeine?

Nope, despite the chemical breakdown that begins as soon as it’s brewed, caffeine is a strong chemical compound, and your coffee will not lose its vital caffeine if you leave it for too long. Caffeine is made up of the following formula: C8H10N4O2 = carbon, nitrogen, hydrogen, and oxygen atoms.

Founder and Managing Editor of Craft Coffee Spot, Marko Lazarevic, explains: “Coffee has two molecular parts: volatile compounds and non-volatile compounds. Volatile compounds include all the good stuff in coffee, including flavor notes and aroma. Heat breaks down volatile compounds quickly, and most are gone within an hour, let alone 24 hours.”

Food Scientist and Food Consultant Dr. Bryan Quoc Le adds that “Coffee contains sulfur-based compounds known as thiols, which contribute substantially to the flavor of coffee. These thiols are very volatile and their concentration is relatively minute in coffee despite having an outsized impact on flavor. When coffee is left over, these volatile compounds slowly vaporize and oxidize in the presence of air.”

Does Coffee Get Stronger the Longer It Sits?

By ‘stronger,’ you could mean one of two things—stronger taste or more caffeine.

It will be more bitter, which could give you the perception that the coffee is stronger, but that’s just the flavor changing. Bitter coffee doesn’t mean that it’s any stronger. The amount of caffeine in your coffee will not change or get any stronger if it is left.

Tip: If you’re looking for stronger coffee, you can do two simple things: 1) buy lighter coffee beans instead of dark beans (lighter beans often have more caffeine), and 2) ground your coffee beans finely. The more granular your coffee grounds are, the more caffeine you will get.

Can You Drink Day-Old Refrigerated Coffee?

Yes, it is not a problem to drink refrigerated coffee, though you will probably want to heat it up if you prefer it warm.

Interestingly, refrigerating coffee is the best way to maintain its taste. As Clinical Professor and Director of Dietetics at the University of Georgia and spokesperson for the Academy of Nutrition and Dietetics, Dr. Emma Laing, adds: “Keeping brewed black coffee in the refrigerator for up to 3-4 days is one strategy that can prolong its flavor.”

Laing also says, “storing coffee in a container with a tightened sealed lid will help keep its flavor from waning. Cold brewed coffee can be stored as a concentrate in the refrigerator for up to 7-10 days or for 3-4 days if mixed with water.”

What Does Day-Old Coffee Do to the Body?

Drinking day-old coffee doesn’t affect the body much more than regular, fresh coffee. However, if you’re feeling under the weather, the taste, if very bitter, could make you feel a little sick.

That said, Lazarevic does mention that “there are studies suggesting chlorogenic acids reduce blood sugar and are behind coffee’s growing health benefits.” And so, the remaining acids floating around in day-old coffee might not be so bad.

Is Day-Old Coffee Safe to Drink?

Yes, day-old coffee is safe to drink if it doesn’t contain spoiled milk. Laing advises that “If milk or creamer has already been added, you should discard your coffee after 1-2 hours at room temperature to avoid spoilage.”

Le also explains that “If there’s any sugar or dairy added, microorganisms may have grown and caused it to spoil as well. Some of these could also make you sick, especially if dairy has been added.” So, always consider if there is any milk or sugar already in that coffee.

But if milk is an absolute must for your coffee, it’s not the end of the world. Laing also states, “If you enjoy your coffee with milk, creamer, and/or sweeteners, adding these just before you drink your day-old coffee can improve its taste.”

Another factor to consider is acidity which could be a problem for some people.

Can You Mix Day-Old Coffee with Fresh Coffee?

I do this almost every time I have coffee left over from the day before. It does impact the taste, which is perhaps slightly more bitter. This might not necessarily be because the taste has been impacted, but because there is more coffee.

I don’t do this with coffee with milk or cream. I use a percolator, and if I make too much coffee, I let the leftover coffee cool, pour it into a mug, and place it in the fridge. When I mix the coffee in the morning, that’s when I add the milk and sugar.

Can You Reheat Day-Old Coffee?

Yup, you can easily reheat day-old coffee, but don’t expect it to taste the same. Because of this, many coffee connoisseurs wouldn’t even think of looking at day-old coffee

What Happens When You Reheat Coffee?

Your coffee will not lose any of its strength, but reheating coffee will not fix its taste. It will only make it warmer and increase its bitterness.

If my coffee had milk in it, I would only reheat it if it’s a couple of hours old. If it’s a coffee with milk and it’s older than 24 hours, I will abandon it.

Does Microwaving Coffee Lose Caffeine?

Microwaving coffee does not rid it of any of its vital caffeine unless you zap it to an ungodly heat as part of some strange experiment.

Should I Throw Out Day-Old Coffee?

Throwing out day-old coffee is a matter of preference. Some would rather not take the risk or just dislike the taste, for others pouring it down the drain is just a waste.

The question comes down to how busy are you? And will reusing day-old coffee make you feel sick?

If you’re super busy to the point that you can’t even spare a few minutes to make a new batch, then reheating day-old coffee isn’t such a bad idea—it will save you a few precious minutes but at the cost of a bad taste.

If you drink coffee for the energy and care little for taste, this isn’t such a bad trade-off, but if you want to enjoy that coffee, a new batch is a better idea.

However, let’s say you’re a little hungover and your stomach isn’t feeling its strongest. In such a case, you may want to make yourself a new pot of coffee.

What to Do with Old Coffee?

There’s a lot you can do with old coffee if you’re not interested in drinking it. It can be added to a variety of meals, particularly deserts. Sheela Prakash of Kitchn lists seven great ideas for leftover coffee:

  1. Add a kick to oatmeal
  2. Make ice cream
  3. Make ice cubes
  4. Add it to marinade
  5. Turn hot chocolate into a mocha
  6. Add it to baked goods
  7. Make tiramisu

The Verdict: Is Day-Old Coffee Still Good?

Day-old coffee is still good if you’re looking for a good caffeine pick-me-up, but not necessarily in terms of taste. While Day-old coffee is not necessarily dangerous, it is probably best to discard it if it had milk and sugar added as it could become spoiled and contain bacteria.

To summarize:

  • Can you drink day-old coffee with cream? If your day-old coffee has cream or any other dairy, it’s best to throw it away.
  • Is drinking old coffee bad for you? Old coffee is not particularly bad for you if milk and sugar are absent, though the bitterness may be too much for some.
  • Is it ok to drink coffee left out overnight? Refrigerating coffee will slow down disintegration and reduce bitterness—so if you can put it in the fridge and don’t leave it on the kitchen table.

How long can black coffee sit out at room temperature? Approximately 24 hours is the longest you’d probably want to leave black coffee at room temperature, though most would recommend drinking it within 30 minutes to four hours.

To destroy caffeine, you would have to heat it over 350°F (176°C), according to an article by Zac Cadwalader of Sprudge. So, unless you’re using a monster of a microwave, you probably aren’t going to hit that temperature.

Microwaving coffee can make your coffee taste worse. If your coffee is already at the point where you’re looking to reheat it, that means its chemical components have already started to disintegrate, and sticking that coffee into the microwave can further that disintegration and make it taste worse.

That said, microwaving coffee can supposedly kill any bacteria that may have accumulated.

How Many Times Can I Reheat My Coffee?

Theoretically, you can reheat your coffee as many times as you’d like but do note that every time you do, you are creating a chemical reaction in that mug that will speed up its disintegration, and it will only taste more bitter.

Your coffee will not get any fresher. It’s not like pressing a reset button every time you reheat coffee; it just warms it up and will not get any stronger.

If you find yourself reheating the same cup over and over, maybe you don’t really need it right now. And if that’s because the taste is foul, maybe you should just get rid of it.