Mold in Hydro Flask – Nasty Smell? How to Clean & Prevent

Why Does Mold Grow in Hydro Flasks?

Oops you did it again and left your unwashed hydro flask in your gym bag for a week or even two.

Or perhaps you sat it by the sink intending to clean it but then life happened and before you knew it a few days went by.

Perhaps you have washed it, even multiple times but it still doesn’t smell or taste quite right, even after you have given it a good soaking.

What is going on in there? Could it be mold?

In this guide, we will look at how to save your hydro flask from the dreaded mold and how to prevent this problem in future.

Signs of Mold Growth In your Hydro Flask

Hydro flasks do have the potential to grow mold quite quickly if they are not cleaned and cared for properly.

This is because they provide the ideal dark and damp growing conditions mold loves, especially when we leave sugary or milky drinks inside. Plain water may stay mold free for slightly longer, maybe a few days, but other liquids will quickly turn rancid and then moldy. Mold can also get into various parts, between or under them, and you may not spot the problem until mold starts appearing.

You may notice the following signs that your flask has mold;

  • A strong smell or pungent odour on removing the lid
  • Mold visibly floating in the contents
  • Black spots on the lid, mouthpiece or inside the flask
  • A strange or bad taste when you use the flask

Causes of Mold in Hydro Flask

Most of the time, mold in a hydro flask is caused by insufficient cleaning, particularly when it comes to the rubber seal or ‘gasket’ that is present in the lid. If this is not removed and cleaned periodically, it can develop mold as moisture and germs get trapped between it and the lid itself and mold is able to grow. There may be other areas such as straw lids that develop mold underneath or in crevices.

You can’t usually see bacteria, germs or even the early stages of mold growth so you may be unaware there is a problem.

Is Mold in Hydro Flask Dangerous?

There are many different species of mold with many different colours and textures. Mold and mildew are funguses (neither plant nor animal) and they thrive best in dark, damp and warm conditions. Spores become airborne and can easily spread to other places. They can feed and grow easily in confined places such as a hydro flask with the lid on. In fact, this is a perfect home for mold.

Black mold (Stachybotrys chartarum  ) is one variety of mold that may commonly be found in hydro flasks. There are thought to be more than 100,000 types of mold and some are much more harmful than others. Black mold is one of the most harmful mold species to humans. And, if you aren’t cleaning your hydro flask thoroughly, it is one of the culprits who might just be living in there.

What Happens If You Drink From a Moldy Hydro Flask?

It is never a good idea to ingest any variety of mold as it can be extremely harmful to health.

Prolonged exposure can lead to a variety of health issues, particularly for those who are vulnerable such as the very young, elderly or those who have a weak or compromised immune system. For these people, even mild exposure can cause sickness including sore throats, runny nose, coughs, allergies, respiratory problems, headaches and more.

For otherwise healthy people, the risk is probably fairly small from occasional exposure. However, you wouldn’t want to drink out of a mold infected receptacle as it probably won’t smell or taste very nice. And remember, the problem with mold is that it grows and spreads so you will definitely want to remove it as soon as possible.

Any kind of bacteria or germs could be present on your mouthpiece and you won’t always be able to spot them with your naked eye.

If you have health problems after being exposed to mold, see your Doctor or Health Practitioner.

How to Clean & Get Rid of Mold from Hydro Flask

With a little effort and some household supplies, you should be able to easily remove the mold from your hydro flask and soon have your water tasting great again!

Follow the steps provided below for your situation.

If Your Hydro Flask is Moldy

If your hydro flask or water bottle has actually gone moldy, you will need to do a thorough deep clean and sanitising treatment to ensure you remove all the mold before using it again.

First, give your flask a good rinse out in warm running water. Next, half fill the flask, replace the lid, shake and repeat. Do this several times and empty out.

Now, repeat the process using a cup of white vinegar but this time leave it in there with the lid on for 30 minutes to allow it to soak. Shake every 10 minutes. (you can also leave it in longer or even overnight as vinegar has been shown to effectively kill 82% of mold spores).

Now, make a baking soda paste by adding some baking soda and water in a bowl. Use a bottle or other brush to clean the flask inside with the paste (add the paste directly to the brush). This will cause fizzing when the brush comes into contact with the vinegar you left in there. Use the bubbly solution and your brush to scrub the insides out well.

Leave this solution inside the flask for 15 minutes with the lid on.

Rinse out well with boiling water (be careful not to burn yourself). This helps to sterilise your flask.

Wash your flask again, this time with warm soapy water. Rinse and leave to air dry.

Note- It is not advisable to use vinegar on aluminium flasks or water bottles as the vinegar may react with the aluminium causing unsightly oxidation spots. Baking soda mixed with water is a better option. Leave to soak as above, rinse well and allow to air dry.

How To Clean Mold From Your Hydro Flask Lid

It is not sufficient to simply remove smells from your flask as described above unless you also deep clean the lid. This is because the lid is one of the main places where mold can get in and hide because of parts such as seals, valves, grooves and features such as straws.

You may have tried several methods already to clean your flask and not realised that the lid may be the problem. This is because the lid often has a rubber seal all around it called a ‘gasket’ and if this is not removed and cleaned occasionally, that sneaky mold can develop underneath it. You won’t always see this but you can certainly smell it!

Follow these steps to deep clean your lid as well as your bottle itself.

  • Fill a small bowl with white vinegar.
  • Use a small, flat butter knife or another tool to very carefully prise the rubber seal completely off.
  • Soak this and the rest of the lid in the bowl of vinegar for 30 minutes.
  • Mix some baking soda with water in a small bowl to form a smooth paste.
  • Use a small brush, toothbrush or cloth to apply the paste to the lid, rubber seal and other parts and use the fizzy solution to clean the parts well, making sure to get into all nooks and crannies.
  • Return the parts to the bowl for another 15 minutes to soak.
  • Tip out the vinegar and carefully add boiling water. Swirl the parts around to rinse well. Remove from the bowl.
  • Scrub the parts again, this time in a bowl of warm, soapy water and with a different brush. Make sure you remove all traces of the cleaning solution.
  • Rinse well in running water and leave to air dry fully before reassembling the parts and replacing the rubber gasket.

Cleaning Mold in Hydro Flask Straw

For straw lids, remove the straw and any valve from the mouthpiece completely and follow the steps above.

You may have to pull fairly hard to remove the mouthpiece from the rest of the lid. Hold it firmly and pull it to the side to slide or pop it out. If you do not remove it fully you will be unable to clean it thoroughly and reach the mold that may be growing underneath.

How to Get Rid of a Bad Smell in Your Hydro Flask

You may have caught your flask just in time, before any mold grows but it may still not smell the best, especially if any milky drink like tea or coffee was left inside.

See below for what to do to remove the pongy odour.

Vinegar Soak Method

To get rid of the smell, first wash the flask in warm, soapy water with a bottle brush and leave to air dry with the lid off. Leave it for a few hours as this will get rid of at least some of the smell.

Next, pour some white vinegar into the flask (about half full). Put the lid on and shake well to coat the insides. Leave to sit with the lid on for 30 minutes.

Empty out the vinegar and wash thoroughly again in warm soapy water. Leave to air dry.

Note- It is not advisable to use vinegar on aluminium flasks or water bottles as the vinegar may react with the aluminium causing oxidation spots. Baking soda mixed with water is a better option.

Rice Method

Wash out the flask in warm soapy water and leave to air dry. Add a handful of dry rice, a drop of washing up detergent and a little vinegar or water. Put the lid on and shake well. The dry rice will act as an abrasive cleaner and it will also absorb the smell as it soaks up the water.

Special Tablets Method

You can purchase special tablets for removing the smell from your hydro flask. Simply pop them into the flask, fill it with water and leave to work. (always follow manufacturers instructions). Afterwards, rinse with boiling water and leave to air dry.

Preventing Mold Growth in Your Hydro Flask

Prevent mold by cleaning and drying your hydro flask regularly.

You should aim to deep clean and sanitise your hydro flask at least once a week and rinse it out daily or after every use. Allow to air dry.

Don’t be tempted to ‘top up’ your bottle with fresh water without tipping out the remaining old water and rinsing it out first with fresh water. Bacteria may have gotten inside and you will not be able to see it.

What Not to do With Your HydroFlask

Avoid the following when cleaning and looking after your hydro flask.

  • Do not put in the dishwasher as this can damage the vacuum sealed insulation and the finish on the outer surface. Although some types of flask may be put in the dishwasher, if they are moldy it is best you wash and fully treat this first by hand as described above.
  • Do not use harsh abrasives or cleaners as these can scratch off the outer surface and damage the flask inside.
  • Do not fully immerse the flask when soaking as this can also damage the outer surface. Instead soak only the inside area by filling with warm soapy water and leaving it to stand.

So there you have it! Follow this guide to get many years of mold-free service from your hydro flask or water bottle.


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