Taken from one of my notes (at work):
Every concentrated flavor is a mixture of raw materials, and every flavor blend can act differently. For example, flavors that have a vanilla characteristic are going to have slightly different storage capabilities than fruit flavors. Here’s the reason: vanilla and caramel flavors are mostly made of large molecules like vanillin, ethyl vanillin, etc.; we call them compounds in a finished flavoring. They are the 2 that really deserve the royal treatment as far as storage. I keep mine in a zip lock bag and then brown bag the zip lock.
Time is not kind
Whenever you open a bottle, its the lightest and smallest molecules that escape and reach your nose quickly. When you open a bottle over and over again more and more proportion of these lighter molecules leave the bottle eventually the character of the flavor will be changed. Some might have noticed the flavors steep just like our liquids. This doesn’t mean the flavors spoiled, it’s just different. One piece of advice from me is this. Please transfer the flavor to smaller bottles if you are going to store a flavor for a long period of time. This way will you will not have to open over and over again.
When a flavor is warm, like if its a hot day or stuck in transport from a mail-order online. If you open the bottle even more of the volatile molecules will escape than if the flavor was cool. This is true for all liquids. When liquids are heated the molecules are much more easily converted to their gaseous state. This is also the reason why you should not be “top off” on creating your juices. They depend on the gases to help steep into your base mix.
HDPE plastic is very resistant to interaction with the flavors.
However, even with HDPE plastic, I really would not recommend storing them for longer than a month or so. It is much better to store things long-term in glass, or PET bottles. It is not a good idea at all to store the flavors with the plastic eyedropper caps on the bottles. The rubber that is used with the eyedropper is extremely soft and interacts with the flavoring. Some Flavors can appear to eat into the rubber material. If you buy your flavors in larger quantities, glass is the best way to go. At the least, use the PET bottles, as they are almost equal to quality glass bottles.
I hope this helps in understanding how to manage your flavors, and as usual, any questions, just ask!