The basic rules for e-liquid storage come from an understanding of what you should keep your juice away from. Nicotine expert Dr. Jacques Le Houezec points out that light and air will cause nicotine to oxidise, which results in a change of colour (but doesn’t affect its properties). However, he also says: I would be more concerned about the flavours. They certainly degrade faster than nicotine. Before going on to clarify:
It could change the taste, but should not change the effects. Some flavours, particularly tobacco flavours, or chocolate or coffee, can also turn dark with time, probably [due to light and air oxidation]. This should not be a concern, but I would recommend to keep them cool and away from light.
The problem with heat comes down to how chemical interactions work in general:
if something is hotter, its molecules have more energy, and this could be enough to allow them to interact with other ingredients or to break down into smaller components. This wouldn’t be an issue, but the specific compounds create the flavour notes in your e-liquid, so if any of them change, it will alter the taste of your juice.
The reason you’re advised to avoid light and air comes down to the effect of UV light and oxygen on nicotine.
The oxygen reacts with the nicotine to produce cotinine (this “oxidation” process involves the loss of two hydrogen atoms and the gain of one oxygen atom) and the UV light helps this process along by providing energy in a similar fashion (but not in exactly the same way) as heat does. The more the reaction occurs, the less nicotine will be left in your juice, but in most cases the effect won’t be noticeable – unless you’re really careless in your storage. The issue with oxygen can’t be completely avoided, because some will be dissolved in your juice even if you have a full bottle with no “headspace” above it, but the effect will be minimal as long as you keep it away from light.
Does anything change when it comes to long-term storage? Can you keep e-liquid in the fridge or freezer?
The same basic rules apply, but storing your e-liquid long-term does bring with it some potential issues. Firstly, while plastic bottles are absolutely fine for shorter-term storage, they aren’t the ideal solution over the long-term (more than a couple of months): you should use glass bottles instead, at least until they are banned by the EU next year.
This is because the plastic can interact with the e-liquid, and – as explained a moment ago with reference to heat – any chemical changes will alter how your juice tastes. Additionally, although plastics prevent most things from passing through them, some air can get through (in fancier language, they’re “permeable” to some gases), and since air degrades e-liquid, this isn’t a good thing.
How much air can get through depends on the type of plastic – generally speaking, higher-quality plastics are less permeable to air – but glass is a way better approach for both removing the air permeability and minimizing any interactions with the container. Additionally, for long-term storage, using the fridge is ideal. This is for the same reasons we refrigerate or freeze most foods: the “chemistry stuff” is less likely to happen because the molecules have less energy, and bacteria don’t like cold temperatures (although PG is anti-bacterial anyway).
Dr. Jacques Le Houezec also confirmed that freezing or refrigerating e-liquid is absolutely fine. Chances are your e-liquid won’t actually freeze, but when we experimented with one e-liquid there seemed to be some loss of flavour. The only thing to keep in mind is that having your e-liquid in a fridge or freezer will make it thicken up, so you should allow it to warm to room temperature before trying to fill up with it and vape. The e-liquid won’t freeze, since both PG and VG have low freezing points. (While pure VG would freeze at normal freezer temperatures, this would only happen if your juice was over 80 percent VG.)
The other set of considerations when you’re storing e-liquid relate to keeping it away from curious kids and pets. To put it bluntly: nicotine (like many other household goods) is poisonous and there can be tragedies if you aren’t careful with your e-liquid. You might think that your kids are safe anyway because your e-liquid has child-proof caps, but in reality “child-proof” actually means “child-resistant,” and it’s far from a fool-proof solution. A cap can be called child-resistant if most children can’t open it if left for a short period of time.
In other words: if you leave your kid with something in a child-resistant bottle for more than about five minutes – medicines and e-liquids alike – he or she will stand a good chance of being able to open it, and some kids will open it in a much shorter time. You should never rely on a child-resistant cap to keep your kids safe. The solution here is simple: keep your e-liquid somewhere children or pets can’t get to it. A high cupboard, a lockable box, a high shelf or anywhere out of reach is absolutely fine.