The primary thing to understand about aging liquids is ideas of physical changes which have been described, which I will describe as "partitioning effects" - partitioning between solids, partitioning between PG and VG - do not occur. In reality all changes which you observe in taste of a liquid have a common underlying cause - a change in chemical composition of the liquid.
There are several major categories of change which can be applicable in any given liquid system:
1) Loss of components.
2) Degradation or reaction of components.
3) Changes in ionic state, i.e. pH effects.
These may be inter-related, for example, degradation or reaction of a component necessarily means some of that component is also lost. The distinctions should become clearer on further consideration.
1a) Loss of components by deposition. In this case the concentration of a component in a liquid solution is lowered because the component is either adsorbed out of the solution by the container or the component may be only marginally stable in solution and falls out as either a visible or possibly not easily visible film or layer on the container.
This mechanism of composition change is likely to be fairly unimportant as long as you are using decent quality vessels and quality ingredients. The exceptions are certain extracts such as tobacco Absolutes. When making tobacco Absolute stocks from TA and PG or PG/EM, a ring of resin or oily material is typically deposited on the container. You can account for this loss in several ways, I personally allow TA dilutions to stand a few days and then carefully decant the liquid into a new vessel. The resulting clear or nearly clear stock is then stable for a long time.
1b) Loss of components by volatilization. This is a very important mechanism of flavor change. Ethanol is a common ingredient of flavors, and is lost be evaporation over time. Certain top-notes, commonly esters or volatile aldehydes or carbon ring compounds such as terpenes can be fairly volatile, although less so than ethanol. These are lost over time by evaporation as well.
2) Degradation or reaction of components. These phenomenon result in loss of an original component, and commonly the introduction of new components which may or may not also have flavoring characteristics. Reactions can occur between flavoring components. Oxidation can also be important - PG and VG both adsorb considerable amounts of oxygen from air as well as providing a base medium that supports formation of reactive peroxygen compounds.
Oxidation (and probably cross reaction as well) is particularly important when nicotine is added to a flavor liquid. Nicotine base in dilute form is fairly unstable. Darkening of liquids can often be attributed to oxidation of nicotine and other components. Many liquid formulations may be relatively stable in the absence of nicotine. Addition of nicotine in a short time can introduce both oxidation products of the nicotine itself, and from my observations changes in the initial flavoring composition as well.
3) Changes in ionic state i.e. pH effects can have considerable impact on taste. Organic compounds are one of three types:
Acids - certain phenolics and carboxylic acids.
Neutral - ketones, aldehydes, alcohols, cyclic and acyclic carbon compounds.
Bases - amines.
It is already well known that pH itself can have a major impact on the taste of a liquid. Taste can change as salts are formed between acids and bases, and also as ionic state itself changes. An acid which is in a basic environment may be anionic (negative), while in acidic medium it may be protonated and effectively electrically neutral. pH can affect both the flavor and odor imparted by a component as well as affecting it's volatility when vaporizing. Over time the pH of a liquid can also change, usually the result of reactions. Reactions can remove the amine components in a liquid, depending upon the mechanism of xidation this can result in lower pH. Oxidation may also give rise to new acids. Not always, but in more cases than not pH will usually fall not rise over time.
Although we may not have means available to actually determine how all these changes in chemical composition affects taste of a liquid, these are still the major phenomenon which are occurring during aging of a solution.
How does this help you in formulation? Well there is no magic bullet or playbook for an empirical and subjective process. However, rejecting concepts such as the idea that flavorants in PG require any substantial time "to move into" VG right away raises the question is aging even required at all? Some formulations improve upon aging, but many do not, It is certainly not a given that lengthy aging is a requirement or even an improvement.
Also eliminating these fundamentally flawed concepts allows you to easily understand the nature of quick aging processes: all with the exception of seeding involve adding energy, usually heat, but ultrasound is not radically different. Adding thermal energy increases rates of reaction in almost all cases, as well as accelerating the removal of volatile components. The results are not always the same as slow room temperature aging because increasing the temperature may give rise to different reaction pathways which may or may not result in improvement in taste.
Seeding processes are fundamentally different because people generally add more than just a little of the old liquid, so in effect they are adding flavor. I noticed that there was some validity to the concept of seeding in the course of making up liquids I liked using the same unwashed bottle I had made a previous batch in. So I was adding considerable residual flavor. There is also the possibility that seeding is in fact resulting in pH modification.
There is some merit in at least some aging. Many flavorants contain ethanol and other solvents and components that can result in a harsh and/or chemical initial taste. Beyond a little preliminary removal of some low boiling point materials I find that in general prolonged aging does not really improve taste - either a formulation does or just does not taste good and/or eventually with time most formulations in a general category such as "tobacco" end up tasting roughly similar.
What you can do right now to improve your formulations? With just an understanding of the basic processes you can acquire a better empirical understanding of how aging affects your formulations and what you are really trying to achieve. Here is a simple way to begin using this knowledge. Based on the valid phenomena which are occurring, a clearo provides a very useful empirical tool. It is a rapid way to evaluate the effects of both heat and oxidation on your liquids.
Make up a batch of one of your simpler formulations that you feel requires some aging but not a lengthy one. Leave the nicotine out if you use it. Heat in a hot water bath for maybe 15 minutes occasional cracking the top. This will allow some volatiles like ethanol to escape. Allow the liquid to cool and add nicotine if you use it.
Now put about 1 mL in a clearo - I like the metal ones with a glass window, but whatever you prefer. Now give it a good pull. Continue the experiment and make notes of how the flavor profile changes, and evaluate the results in the context of the phenomena that have been described.
What I find with my typical tobacco formulations is usually this pattern:
The initial taste is often a little harsh and/or chemical. These are typically low molecular weight esters, alcohols, ketones etc.
After about maybe 0.1 or so mL, most of these volatiles have been pulled out and a better, rich and complex taste emerges. This is what I am looking for, and something that would be lost if I habitually aged liquids extensively. This often lasts for roughly 0.6-0.7 mL.
The last 0.2 residual liquid usually has diminished taste, and a significant color change is now observed, indicating reactions/oxidation. pH change at this point is likely a factor as well.
Your formulations may vary but this process gives you an idea of what if any pre-aging is likely to be optimal for your tastes. Maybe you need extensive aging, but maybe you just want to lose some of the initial highly volatile components. Maybe you are observing the best taste at the end. This could indicate more extensive aging is required, but you could also experiment and see if addition of a little malic or citric acid gives you a similar taste without prolonged aging.
Try it and see if you find it useful! Also try to describe the tastes you experience. It is quite difficult, but attempting to do so should impress upon you that use of terminology is in fact very important!
There are several major categories of change which can be applicable in any given liquid system:
1) Loss of components.
2) Degradation or reaction of components.
3) Changes in ionic state, i.e. pH effects.
These may be inter-related, for example, degradation or reaction of a component necessarily means some of that component is also lost. The distinctions should become clearer on further consideration.
1a) Loss of components by deposition. In this case the concentration of a component in a liquid solution is lowered because the component is either adsorbed out of the solution by the container or the component may be only marginally stable in solution and falls out as either a visible or possibly not easily visible film or layer on the container.
This mechanism of composition change is likely to be fairly unimportant as long as you are using decent quality vessels and quality ingredients. The exceptions are certain extracts such as tobacco Absolutes. When making tobacco Absolute stocks from TA and PG or PG/EM, a ring of resin or oily material is typically deposited on the container. You can account for this loss in several ways, I personally allow TA dilutions to stand a few days and then carefully decant the liquid into a new vessel. The resulting clear or nearly clear stock is then stable for a long time.
1b) Loss of components by volatilization. This is a very important mechanism of flavor change. Ethanol is a common ingredient of flavors, and is lost be evaporation over time. Certain top-notes, commonly esters or volatile aldehydes or carbon ring compounds such as terpenes can be fairly volatile, although less so than ethanol. These are lost over time by evaporation as well.
2) Degradation or reaction of components. These phenomenon result in loss of an original component, and commonly the introduction of new components which may or may not also have flavoring characteristics. Reactions can occur between flavoring components. Oxidation can also be important - PG and VG both adsorb considerable amounts of oxygen from air as well as providing a base medium that supports formation of reactive peroxygen compounds.
Oxidation (and probably cross reaction as well) is particularly important when nicotine is added to a flavor liquid. Nicotine base in dilute form is fairly unstable. Darkening of liquids can often be attributed to oxidation of nicotine and other components. Many liquid formulations may be relatively stable in the absence of nicotine. Addition of nicotine in a short time can introduce both oxidation products of the nicotine itself, and from my observations changes in the initial flavoring composition as well.
3) Changes in ionic state i.e. pH effects can have considerable impact on taste. Organic compounds are one of three types:
Acids - certain phenolics and carboxylic acids.
Neutral - ketones, aldehydes, alcohols, cyclic and acyclic carbon compounds.
Bases - amines.
It is already well known that pH itself can have a major impact on the taste of a liquid. Taste can change as salts are formed between acids and bases, and also as ionic state itself changes. An acid which is in a basic environment may be anionic (negative), while in acidic medium it may be protonated and effectively electrically neutral. pH can affect both the flavor and odor imparted by a component as well as affecting it's volatility when vaporizing. Over time the pH of a liquid can also change, usually the result of reactions. Reactions can remove the amine components in a liquid, depending upon the mechanism of xidation this can result in lower pH. Oxidation may also give rise to new acids. Not always, but in more cases than not pH will usually fall not rise over time.
Although we may not have means available to actually determine how all these changes in chemical composition affects taste of a liquid, these are still the major phenomenon which are occurring during aging of a solution.
How does this help you in formulation? Well there is no magic bullet or playbook for an empirical and subjective process. However, rejecting concepts such as the idea that flavorants in PG require any substantial time "to move into" VG right away raises the question is aging even required at all? Some formulations improve upon aging, but many do not, It is certainly not a given that lengthy aging is a requirement or even an improvement.
Also eliminating these fundamentally flawed concepts allows you to easily understand the nature of quick aging processes: all with the exception of seeding involve adding energy, usually heat, but ultrasound is not radically different. Adding thermal energy increases rates of reaction in almost all cases, as well as accelerating the removal of volatile components. The results are not always the same as slow room temperature aging because increasing the temperature may give rise to different reaction pathways which may or may not result in improvement in taste.
Seeding processes are fundamentally different because people generally add more than just a little of the old liquid, so in effect they are adding flavor. I noticed that there was some validity to the concept of seeding in the course of making up liquids I liked using the same unwashed bottle I had made a previous batch in. So I was adding considerable residual flavor. There is also the possibility that seeding is in fact resulting in pH modification.
There is some merit in at least some aging. Many flavorants contain ethanol and other solvents and components that can result in a harsh and/or chemical initial taste. Beyond a little preliminary removal of some low boiling point materials I find that in general prolonged aging does not really improve taste - either a formulation does or just does not taste good and/or eventually with time most formulations in a general category such as "tobacco" end up tasting roughly similar.
What you can do right now to improve your formulations? With just an understanding of the basic processes you can acquire a better empirical understanding of how aging affects your formulations and what you are really trying to achieve. Here is a simple way to begin using this knowledge. Based on the valid phenomena which are occurring, a clearo provides a very useful empirical tool. It is a rapid way to evaluate the effects of both heat and oxidation on your liquids.
Make up a batch of one of your simpler formulations that you feel requires some aging but not a lengthy one. Leave the nicotine out if you use it. Heat in a hot water bath for maybe 15 minutes occasional cracking the top. This will allow some volatiles like ethanol to escape. Allow the liquid to cool and add nicotine if you use it.
Now put about 1 mL in a clearo - I like the metal ones with a glass window, but whatever you prefer. Now give it a good pull. Continue the experiment and make notes of how the flavor profile changes, and evaluate the results in the context of the phenomena that have been described.
What I find with my typical tobacco formulations is usually this pattern:
The initial taste is often a little harsh and/or chemical. These are typically low molecular weight esters, alcohols, ketones etc.
After about maybe 0.1 or so mL, most of these volatiles have been pulled out and a better, rich and complex taste emerges. This is what I am looking for, and something that would be lost if I habitually aged liquids extensively. This often lasts for roughly 0.6-0.7 mL.
The last 0.2 residual liquid usually has diminished taste, and a significant color change is now observed, indicating reactions/oxidation. pH change at this point is likely a factor as well.
Your formulations may vary but this process gives you an idea of what if any pre-aging is likely to be optimal for your tastes. Maybe you need extensive aging, but maybe you just want to lose some of the initial highly volatile components. Maybe you are observing the best taste at the end. This could indicate more extensive aging is required, but you could also experiment and see if addition of a little malic or citric acid gives you a similar taste without prolonged aging.
Try it and see if you find it useful! Also try to describe the tastes you experience. It is quite difficult, but attempting to do so should impress upon you that use of terminology is in fact very important!
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