Think twice: Is it ok to leave water in a reservoir overnight for coffee?

Many people leave water in coffee makers for extended periods of time

When it comes to brewing coffee, things are getting easier and easier.

At the same time, we care about drinking high quality, delicious coffee. Therefore, we tend to buy nicer coffee machines. Not only do these coffee makers come with the ability to brew excellent cups of coffee with minimal supervision. They offer new features, which make coffee brewing easier, faster and more accessible. One of those features is the ability to fill up water reservoirs to brew coffee QUICKLY at any time we desire.

Larger water reservoirs in many modern coffee makers

One feature coffee machines pride themselves in are large water reservoirs.

• Drip coffee makers typically need larger water reservoirs. They brew whole carafes (typically 8 cups or more), and they just need more water to brew.
• Single cup coffee makers, espresso machines or smaller drip coffee makers may not necessarily need large water tanks. However, their ability to hold more water serves our comfort and affords us unparalleled convenience.

If we brew just a cup or two of coffee every day, should we leave water in the reservoir for coffee? Is the water we leave in the tank for two or three days in a row, or even only overnight, still fine for a cup of coffee?

Benefits of leaving water in coffee makers overnight 🏆

→ #1. Leaving water in a coffee machine’s water reservoir saves time at the critical times of the day

When a coffee maker is prepared and ready to go, it helps save time at the most critical times of the day. Even if it saves me mere 2 minutes, leaving water in the reservoir can help me finish my morning rituals faster, and I can accomplish everything I have to.

Similarly, when I have an unexpected visit from my friends, I am able to serve them delicious coffee quickly and effortlessly. Thus, I can focus on enjoying their company, rather than spend time preparing coffee.

People who like to make everything ready for themselves the day before will most likely fill water reservoirs of their coffee makers a day in advance.

→ #2. Leaving water in a coffee maker’s reservoir affords me convenience

Leaving water in the reservoir is not only a matter of time and efficiency for me, it is also a matter of comfort.

When I wake up in the morning, I tend to be a bit sleepy and distracted. My coordination is bad and my power to focus is weak. Filling water tanks might prove to be a too complicated task for me to do on certain mornings.

→ #3. Leaving water in the coffee maker’s reservoir allows me to brew from (almost) anywhere, anytime ☕

I am not (yet) among the lucky owners of a smart coffee machine. Though, we have tested thoroughly different coffee makers that had smart capabilities and were compatible with smart plugs.

But if you find yourself in this category, you probably leave water in the reservoir of your coffee machine every night. Simply because you can wake up and switch your coffee machine on right from your bed. And when that happens, you need the water reservoir to be filled.

The real problems with leaving water in a coffee maker overnight 🙀

→ #1. Water can turn stale pretty quickly

Leaving water in a coffee maker for an extended period of time may cause water to go stale.

Carbon dioxide causes the pH of water to decrease, which changes the flavor of tap water. Water’s pH decreases whenever air interacts with water. Though those reactions do not occur within seconds, water will have a noticeably different flavor after several hours. This is also true when water stays in a coffee machine overnight.

Therefore, if I don’t want my coffee to have a strangely different flavor, I may want to get into the habit of replacing old water in the water reservoir with fresh water. A rule of thumb when brewing coffee that tastes great is to use fresh water, preferably filtered.

→ #2. Bacteria can form, pretty quickly

When water stands still in a water reservoir, it can become a perfect environment for the growth of bacteria and fungi.

Though coffee machines come with some physical features to minimize the growth of bacteria, they can become a great place for bacteria and fungi to thrive. Coffee machines usually have water reservoirs that are covered with lids, which protects water from dust and flying insects. However, the conditions inside the reservoir might change overnight, especially due to changes in temperature.

While treated water contains chlorine, which manages to take care of microorganisms, when left to stay at room temperature for more than 12 hours, water starts changing its quality and bacteria starts multiplying.

→ #3. Damage to coffee machines due to mineral deposits

Using filtered water to brew coffee is ideal, but some people (including myself) use tap water. The main issue with using tap water is that it might be very rich in minerals like calcium, which can lead to deposits inside the machine.

Emptying the reservoir of water at the end of the day is just one simple thing we can do to increase the longevity of our coffee machines. Hard water, when left for a long time, will, in time, leave calcareous sediments on the walls and on the bottom of a water tank.

→ #4. Contamination

If you decide to leave water in your coffee machine overnight, always remember that once you put water in it, the chemical reactions between water and the material from which a coffee maker is made will commence. The composition of water may change.

Therefore, FDA strongly recommends BPA-free materials in any utensils, kitchen appliances and coffee makers that come in contact with food or water. BPA is a substance that have been shown to cause growth of cancer cells in a human body. A study from 2004 that looked at the impact of several chemicals on human bodies. It found that nearly all the urine samples collected from cancer patients contained traces of BPA.

If water interacts with BPA for long enough, this water can be contaminated with BPA traces, which can have a detrimental effect on human body.

With that said, BPA is only one of the chemicals that have been shown to negatively impact health. Coffee makers are made with plastics that contain other chemicals and materials. Those chemicals have not yet been fully examined.

Owly’s conclusion

So how is it? Is it ok or not to leave water in the coffee machine overnight?

While leaving water in the reservoir overnight should not be a big problem, we do not recommend making the habit out of it. Leaving water in a water tank for too long can create lots of trouble:

• water can get contaminated
• a water tank can become a thriving environment for fungi and bacteria,
• water can become stale (thus, coffee will not taste as great), and
• a coffee machine can get more easily damaged to to mineral deposits from sedimentation.

We strongly recommend using fresh water that comes from safe sources. As a rule of thumb, water once heated should not be re-used. A re-heated water may alter flavor of coffee.

In conclusion, we advise using fresh water to brew coffee as much as possible. Even if water from yesterday looks good to drink, its taste will definitely be different, which will also alter the taste of coffee.

What do you think?

Do you think it is ok to leave water in the coffee maker’s reservoir overnight? Do you refresh water in your reservoir every morning? Have you ever had coffee that was brewed with old water? What did you think?

Let us know in the comments!