- Katharina Butsch
- May 18
- 3 min read
Soft flasks: Water with a hint of plastic flavor.
If you’ve ever used brand-new soft flasks, you probably know the issue: no matter how many times you rinse them, water and even flavored drinks often end up tasting strongly like plastic. With the last flasks I bought, the taste was so intense that I’d rather go without drinking than consume what I had in them. Faced with the choice of either throwing the flasks away or digging into the problem more systematically, I chose the latter. If you're dealing with a similar issue, you might find this summary on how I managed to “save” my soft flasks helpful.
What's that taste?
Depending on the manufacturer, soft flasks are mostly made of thermoplastic polyurethanes (TPU) - a group of various polymers known for being both durable and flexible. However, due to their structure, TPUs are also relatively permeable to small, volatile molecules, which can have a noticeable taste or smell. Quality hydration bladders or flasks are often made from multi-layer systems, where the TPU is lined on the inside with materials like polyethylene, which is odor- and tasteless.
Still, new soft flasks can have a strong “plastic” taste. Studies have shown that many different chemicals can migrate into the water from such drinking systems. [1] These are residues from the manufacturing process and include solvents, additives, unreacted monomers, or byproducts.
Is it a health concern?
Changes in smell and taste caused by plastics are a sign that chemical compounds have leached into the food or drink. However, the intensity of the change doesn’t tell you how much chemicals are in there or whether they pose a health risk. Some substances don’t smell or taste at all, while others are easily noticeable. And not all harmful substances affect taste or smell.
In general, plastics used for drinking systems or bottles need to be “food-safe.” Meaning, by law they have to be made from approved substances, must not release harmful chemicals in dangerous amounts, and should not significantly alter the taste or smell of the beverages. High-quality soft flasks are therefore free of BPA, phthalates, and PVC - substances that have been shown to negatively affect health.
However, many compounds that can leach from plastics, especially those that are just byproducts of production, are poorly understood or not studied at all. Research findings are often inconclusive. Some studies suggest that chemical molecules released from these materials could pose health risks [1], while others find effects only under specific experimental conditions and call for further investigation [2].
So, while a reliable health risk assessment isn’t possible for most of these chemicals and thresholds currently considered safe could change in the future, it’s fair to assume that these substances aren’t doing your body any favors. Especially when the taste or smell is so strong that you don’t even want to drink from the flasks, it’s definitely worth taking steps to deal with the issue.
Cleaning tips before first use
Smelling or tasting chemicals produced during plastic manufacturing are organic molecules, mostly alcohols, aldehydes, or siloxanes. These can be neutralized by treating the flask with a mildly basic solution:
Mix
1 L of warm water
1 tbs of baking soda
Soak the flask in this solution overnight, then rinse it thoroughly before use.
What to avoid:
Don’t use very hot or boiling water! Heat can alter the structure of TPUs, making them harder to clean over time.
Don’t cut the soaking time short. Organic molecules aren’t very water-soluble, and a longer soak allows the solution to better penetrate the material.
In theory, you could also use washing soda to prepare the mix, but its pH is much more alkaline. TPUs are sensitive to strong bases, which can damage both the surface of the material and the seams. So it’s safer to stick with baking soda.
Avoid harsh cleaners or disinfectants. They can degrade the material, making odor or taste issues even worse.
Bonus tip: Once the initial smell and taste are gone, don’t store your soft flasks next to strongly scented items. Since TPUs are permeable to small molecules, they can easily pick up surrounding odors, too.
Literature:
[1] Stevens et al. (2024): Migration of endocrine and metabolism disrupting chemicals from plastic food packaging. DOI: 10.1016/j.envint.2024.108791.
[2] Varalda et al. (2024): Risk assessment of food contact materials.
