What chemical measurements are related to the degree of flavor extracted by alcohol?

A recent project of mine includes making infusions of various ingredients to make liqueurs and bitters. Infusions, I've learned, vary widely in the amount of flavor extracted from an ingredient. Citrus zest, for example, can overbear most other ingredients in ways it wouldn't in similar amounts in other recipes.


To better understand the infusing process, I taste a series of infusions every few days, rating the perceived strength of their aroma and flavor. This is clearly subjective, and I'd like an objective, empirical means to check the amount of flavor compounds extracted from an ingredient over time.


The interest isn't in comparing different infusions, but in comparing a single infusion's strength at different times, and against human ratings. Here are a few questions I'd like to be able to answer:


* Is this infusion done? Has it extracted as much as we can expect it to?
* Has this really changed since yesterday?
* Given this measurement, how strongly would my tasters rate this?


Here are some things I've considered:


* A [refractometer](http://monsterbrew.com/Prod_Refractometer.cfm). (Presumably the flavor extracted is the form of dissolved solids, no?)
* A [proof hydrometer](http://monsterbrew.com/Prod_Proofhydrometer.cfm), though I'm unsure if we can reasonably expect proof to change much over the process.
* [Arduino biometric sensors](https://www.sparkfun.com/categories/146) for things like e.g. pH, though I'm again unsure of the expected relationship.


Are these, or any others, reasonable measures of an infusion's progress?


