I'm still very much a newbie, and simply don't have enough experience yet to talk about. But I've been reading voraciously of (mainly) this forum for a few months now, so maybe some theoretical points wrt steeping that I have pondered and picked up may be helpful:
IF* you consider steeping an important part/tool of mixing there seems to be two distinct schools of thought:
- Time
- Air
Within these two schools there is then some variety in how much people utilize heat, either directly (microwave) or indirectly (hot water bath).
From what I've read and thought I would say that the "Air-school", ie cap-off, are probably people who are very sensitive to certain smells (for instance alcohol), and hence absolutely need a steep that evaporates away some stuff. They may also really like the smell/taste/feel of some oxidized compounds (might be nic, might be something totally different, might vary between flavours).
The "Time-school" otoh are either not very sensitive to some smells/tastes (alcohol again the first that come to mind, and of course flavours that need alcohol as a medium), or like them. They are probably more sensitive to a blend being as evenly blended as possible (see below, heat). OR they may be sensitive to, or dislike, flavour notes that occurr with oxidation.
And then we have the thing that both Air and Time people have in common: what is it that happens when flavours sit together for a prolonged time? Something obviously does, but is it a chemical change (liquids that change colour when in a closed bottle would indicate this), or just a really really thorough blending, evening out of all the molecules so they are evenly suspended?
We don't know - a good chemist who is also a professional flavourist or perfumer might have an idea here, but I think for most of us it is just trial and error and guesswork based on a) experience and b) guesses according to what we know about a flavourants volatility.
Now, what about heat?
When applying heat to something cap-on we just make the liquid flow better => mix more thoroughly. This is easy to see if you use a glass cylinder and measure up several very thick things in it, they will want to layer and not budge and blend. Heat it up and shake it - voila!
But there's another factor here too: if we are applying heat with the cap removed, we are also letting things evaporate. My guess is that alcohol and water are the two that goes first - so flavour molecules that are more attached to alc and water than to VG or PG also preferentially leave your mix, leaving a different ratio of flavours than what you added originally.
Personally, theoretically, I don't think there's one ultimate right way to approach steeping. I think what one decides to do must be based on a) hunch, and b) convenience.
When it comes to hunches, sniffing and tasting your newly mixed liquid might give you an idea - "hmm, there's this something I don't like, maybe it will evaporate away if I leave it open/maybe it will blend in if I leave it closed".
Convenience also is not to be underestimated: do you have the space to open-steep things? Do you have the time to mix things that require a long steep?
Finally a few guesses based on what I've read:
- Fruity flavours require very little steep.
This may be because they are often not overly complex, and it may also have something to do with the actual chemistry of these molecules - most of them might be of the type that quickly and readily "takes to" PG/VG.
- Tobaccos are often said to require long steeps, and many swear by at least some open-cap time.
There might be several reasons for this, the first that comes to mind is that there are several "perfume-y" notes in many tobacco-flavourings (both natural and absolute-based), and some people are very very sensitive to these.
Another reason might be that tobaccos are inherently pretty complex tastes/smells. I've been mixing mostly tobaccos, and it is *insane* how many of the notes in these (both artificial and also absolutes and extracts) are also ingredients in cosmetics!
Whether you are working from artifical flavourings or absolutes (or your own extracts) you are already dealing with pretty complex smells/tastes even before combining them.
- Bakery and custard type flavours I have absolutely no experience with, but given that these are flavours that try to emulate things that happen in cooking processes, my guess would be that these could go either way in how much steeping they need. They might be a mix of several discrete notes that need steeping, or they might be "ready" as they are and not change much. Since most people seem to mix these with other types of flavour, it is almost impossible to say whether it is the mix that need steeping, or the bakery/custard flavour on its own.
*That is a pretty big if: there are people who just don't care about steeping. Some say it makes no difference, others say that they mix so frequently that it doesn't matter - better to mix for consumtion within the 1-3 days.
