Can You Reuse Coffee Grounds?

If you are a regular drinker of coffee, there is a good chance that you’re spending a decent portion of your budget on coffee beans or coffee grounds. On top of that, going to the coffee shop to get a new pack of coffee can be a nuisance at times.

Since coffee grounds aren’t something that disappears after usage, you might be thinking to yourself if you could be putting those grounds into something useful. While it’s known that used coffee grounds can be reused in other areas such as gardening, the biggest question is whether it’s possible to reuse coffee grounds to make another cup of coffee or not.

Technically speaking, coffee grounds aren’t completely extracted during the brewing process anyway, meaning that there should be a portion of coffee components that are ready for extraction during the second brewing cycle.  

Sounds good on paper, but brewing coffee is a very intricate process with plenty of factors involved, which makes the situation not as straightforward. From caffeine content to flavor, there are plenty of things that will be affected when coffee grounds are reused for another brew. For this reason, it’s almost a sure thing that your coffee drinking experience will be suboptimal when you reuse coffee grounds for a second cup.

The problems don’t end even if we put the unpleasant coffee experience aside for a second. Since coffee grounds are exposed to water during the first brew, they are now a breeding ground for all kinds of bacteria. With each passing moment, there is a greater chance that your used coffee grounds will become contaminated, meaning that re-brewing with them could prove to be dangerous.

While it’s possible to reuse coffee grounds for another cup of coffee if done carefully, it’s not worth the risks associated with it for a suboptimal cup of coffee with unattractive flavor. A better idea is to repurpose your used coffee grounds by using them in different ways, such as fertilizing your plants.

Problems with Re-brewing

When it comes to brewing the perfect coffee, there are a lot of things that we have to do correctly.

Using the right ratio of coffee to water, the right brewing time, the right brewing temperature, the right grind size, and fresh coffee beans are all conditions that must be satisfied to brew the perfect cup. Not satisfying even one of these conditions can suddenly turn your coffee into an undrinkable mess, leaving you to wonder what went wrong.

While we can check most of these boxes even when we are re-brewing coffee grounds, there is one thing that is a deal-breaker, which is using fresh coffee. Once coffee grounds are used, they aren’t fresh anymore due to most of the components needed for a fresh cup of coffee being extracted already.

The extraction of coffee components follow a certain order where sour flavors and high acidity comes first, sugars and coffee oils come second, and the bitter parts that are unwanted in coffee come last. It is for this reason that under-extraction or over-extraction can easily ruin a cup of coffee.

When coffee grounds are used for a re-brew, the only thing left to be extracted is bitterness. The flavors that make coffee taste and smell good are gone, meaning that it shouldn’t be much of a surprise when the end product tastes nothing like coffee.

How Re-brewing Impacts Flavor and Mouthfeel

To understand how re-brewing impacts flavor and mouthfeel, let’s first dive deeper into coffee extraction. 

Coffee Extraction

During the process of extraction, coffee components are extracted in an order determined by the solubility of its components. As the solubility of a compound increases, the time it takes to dissolve it shortens. 

If we quickly break it down, the order in which coffee components are extracted would be acids, oils, sugars, and plant fibers.

First up are the acids. Acids are the first compounds to be extracted due to being highly soluble.   These are the components that add the bright, fruity taste of the coffee. Coffee that has only been extracted up to this point usually ends up being too sour and has no body.

Coffee oils come right after acids. They are responsible for adding body to your cup, which makes your coffee have an enjoyable mouthfeel. If you only extracted your coffee up until this point, it’s still under-extracted due to a lack of sugars.

Ideally, the brewing process should be over right after sugars are extracted. As expected, sugars are responsible for adding sweetness, which balances the acidity of your coffee. With the addition of sugars, the flavor profıle of your coffee is balanced, meaning that it’s ready.

In the case of over-extraction, the brewing process doesn’t end when sugars are extracted. At this point, the plant fibers start being extracted, slowly causing the flavor of the coffee to become bitter and dry. Since plant fibers add nothing of value to a cup of coffee, we don’t want them to be extracted at all.

Extraction of Used Coffee Grounds

Now that we know how coffee extraction fully works, let’s use this information to understand the extraction process of used coffee grounds.

Using coffee grounds for the first time ends up with most of the required components being extracted, meaning that there isn’t much left to work with for the second brew. Alongside this, coffee grounds are already exposed to moisture from the first brew, which will accelerate the rate of extraction of unwanted components such as plant fibers.

Putting the same grounds through the brewing process again will immediately cause the plant fibers to be extracted, meaning that you will end up with a bitter liquid that doesn’t resemble coffee. 

How Re-Brewing Impacts Caffeine Content

Caffeine is extracted from the coffee grounds the same way the other components are and is considered to be extracted relatively early in the brewing process. While there is some caffeine left within the grounds, there is a good chance that it’s not enough.

Let’s do a quick calculation to see how much caffeine is left in used coffee grounds to verify our claim. A gram of fresh coffee beans contains between 10 and 12 milligrams of caffeine[1], while used coffee grounds contain between 3.5 and 8 milligrams of caffeine per gram[2]. By looking at these figures, we can deduce that roughly half of the caffeine content is gone after the first brew.

While one might think that since half of the caffeine is gone with the first brew, we can extract the other half when we brew again, and that would be perfect. Unfortunately, that’s not exactly the case. In reality, we would only be able to extract 50 percent of the 50 percent that remains at best, meaning that our cup would contain roughly 25 percent of the caffeine found in fresh beans. The change in the chemistry of coffee grounds after they have been used will also not be in our favor, meaning that the real amount of caffeine extracted will be less than our assumptions.

Considering this, our second cup of coffee would only have half as much caffeine as our first cup in the best case. With a reduced amount of caffeine alongside unpleasant taste, there isn’t much of a reason to re-brew with used coffee grounds.

Risks Associated with Used Coffee Grounds

When it comes to keeping coffee fresh, moisture is the biggest enemy. Exposure to moisture can destroy coffee like nothing else, and could even cause coffee to be dangerous to consume. While other factors could also cause coffee to lose freshness, the damage caused by these factors doesn’t come close to the damage that is caused by moisture.

When coffee gets moist, it opens up for the growth of bacteria and fungi. Within hours, wet coffee grounds can start harboring these harmful organisms, which can cause your coffee to develop mold. These factors will cause the taste of your coffee to be even more unpleasant, and also not be safe to drink.

Since it may not be apparent at first look that your coffee has gone bad, you shouldn’t automatically assume that used coffee grounds are safe to use. Even without visual differences, your coffee might have already started gathering harmful bacteria.

For this reason, it’s a very bad idea to reuse coffee grounds. Even if you don’t mind the unpleasant taste and lack of caffeine, there are actual dangers that are attached to reusing coffee beans for another cup, which should be enough of a reason to steer clear.

Effects of Reusing Coffee Grounds on Different Brewing Methods

We have concluded that reusing coffee grounds for another brew is not a good idea for various reasons. With a big part of the problem being related to the extraction process, let’s evaluate how different brewing methods could change things as different methods have different ways of extracting coffee.

Reusing Coffee Grounds for Espresso

Since espresso is a very concentrated form of coffee, it’s not the best idea to reuse coffee grounds when it comes to brewing espresso. Especially when consumed as a shot, espresso needs to have a full body and flavor to deliver a pleasurable coffee experience. Alongside the lack of flavor and body, you won’t be able to get crema with used coffee grounds either, which is an important part of espresso that would be missing. For these reasons, it’s highly recommended to use a fresh batch of coffee grounds to brew espresso.

If you insist on reusing your coffee grounds for espresso, refrain from tamping the grounds again as tamping grounds that are already wet will practically block the water flow, resulting in burnt espresso.

Reusing Coffee Grounds for Cold Brew

Since over-extraction isn’t a problem with cold brew, there is some value in reusing your coffee grounds. Once again, your second brew will most likely turn out much weaker than the first brew, but at least it will somewhat resemble coffee.

After filtering your initial brew, you can simply add more water to the container that has your coffee grounds to start the brewing process again. This way, you will seamlessly be able to brew with the same coffee grounds for a second time.

Out of all brewing methods, reused coffee grounds are most likely to work the best with cold brew. By avoiding the bitter flavors caused by over-extraction and the downtime where coffee grounds can get infected with bacteria, it should be possible to produce a somewhat decent batch of coffee with no risk.

Reusing Coffee Grounds for French Press / AeroPress / Pour Over / Drip Coffee

While not as bad as espresso, reused coffee grounds used with a french press, AeroPress, pour-over coffee maker, or drip coffee machine will still turn out suboptimal. You’ll notice that coffee has a color and consistency that’s watery, and it will taste pretty bad. Over-extraction is a problem that still exists unless coffee grounds are dried first.

Drying Coffee Grounds for Reuse

Regardless of what you’re planning to reuse your coffee grounds for, it’s a good idea to dry them after the first brew.

While you will still end up with a weak cup of coffee if you reuse them for brewing, you will at least be avoiding the over-extraction problem caused by coffee grounds being moist. This way, it may be possible to brew a cup of coffee that isn’t completely bitter and undrinkable.

If you’re planning to use your coffee grounds for other things than re-brewing, drying them will serve the purpose of preventing them from being infected with bacteria and allow you to store them safely for future use.

You can follow two different methods to dry your used coffee grounds, which are sun drying and oven drying.

Sun Drying

Sun drying is a simple and no-risk method of drying your coffee grounds. All you need is a wide container such as an oven tray, some baking paper, and sun.

  1. Cover your oven tray with baking paper.
  2. Place the coffee grounds on the oven tray, making sure that they are spread as a thin layer.
  3. Place your oven tray somewhere it will receive direct sunlight. If you couldn’t spread the grounds evenly in a single layer, make sure to shuffle them around to ensure that the grounds in the bottom also dry. Change the baking paper if it gets wet.

By following this method, you should be able to dry your coffee grounds in 2-3 days. As long as you ensure that your grounds are getting sunlight, you don’t have to watch over the grounds as they dry.

Oven Drying

Oven drying is a faster drying method compared to sun drying, but it will require attention to prevent the coffee grounds from burning. For this method, you will need an oven tray, baking paper, and an oven.

  1. Place the baking paper on your oven tray.
  2. Spread the coffee grounds on the oven tray, ensuring that it’s a thin layer.
  3. Preheat your oven to 200 degrees Fahrenheit. (93 degrees Celsius)
  4. Place the tray in the oven, and bake for 10 minutes.
  5. Flip the grounds after 10 minutes, and repeat every 10 minutes until the grounds are completely dry. It will take around 20 to 30 minutes on average for the grounds to dry.
  6. Remove the grounds from the oven, and let them cool down. Make sure to check that they are completely dry before storing them.

We recommend oven drying over sun drying as it’s faster and more effective. You can make sure that your coffee grounds don’t end up being burnt by setting timers with 10-minute intervals.

Do not raise the temperature over the recommended amount as you will burn the grounds if you do so.

Creative Ways to Reuse Coffee Grounds

We have established that reusing coffee grounds for another cup of coffee isn’t the greatest idea. Luckily, there are plenty of creative ways to make use of your coffee grounds around the house. You can even save some money by reusing your coffee grounds in places where you would otherwise have to buy extra products.

Fertilizer

Coffee grounds contain plenty of nutrients that are good for plants, making them a good fertilizer. They also attract worms that are good for your plants and reduce the presence of heavy metals in the soil.

You can simply sprinkle the coffee grounds on the soil for them to do their magic.

Composting

If you have a compost bin, you can add your used coffee grounds to it. Coffee grounds will help to speed up the composting process as they contain acid which helps the decomposition process.

Odor Removal

Since coffee grounds are very good at absorbing odors, you can use them to mask bad smells. Simply place your used coffee grounds in a small cloth bag, and put it where you wish.

You can use them practically anywhere where a bad smell is a problem, such as a fridge.

Conclusion

There is a lot to consider when it comes to reusing coffee grounds for another brew, but it’s far from a good idea most of the time. In the best-case scenario, you end up with a low-quality cup of coffee, and in the worst-case scenario, there are dangers involved.

On the other hand, reusing coffee grounds for different purposes can prove to be extremely beneficial and save you a decent amount of money in the long run.

Have a great day!

References

  1. Evaluation of Spent Coffee Obtained from the Most Common Coffeemakers as a Source of Hydrophilic Bioactive Compounds
  2. Variation in Caffeine Concentration in Single Coffee Beans