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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. (Presumably the flavor extracted is the form of dissolved solids, no?)
  • A proof hydrometer, though I'm unsure if we can reasonably expect proof to change much over the process.
  • Arduino biometric sensors 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?

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    $\begingroup$ It’s really really hard because flavour, as perceived by humans, can originate from all kinds of molecules that generally have little in common (except for at least one carbon atom). Each of these methods suffer from you not knowing what the actual substance is, that you are isolating, and that creates a taste. NMR or IR (wait, did I actually just suggest IR?) may be the most promising attempts that help in identifying in some mediocre way, but they too work best on pure compounds, not really on mixtures. $\endgroup$ – Jan Dec 30 '15 at 22:14
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    $\begingroup$ Many of the compounds are volatile, so I'd suggest looking at what's in the headspace gas (GC comes to mind, but I'm not sure what the DIY/maker equivalent looks like, if there is one) of whatever vessel you're storing your solutions in. You'll have less water in the gas phase (and you'll know your solvent signature), so maybe @Jan's dreaded IR could be used without water dominating the spectrum where you want to look. $\endgroup$ – Todd Minehardt Dec 30 '15 at 23:54
  • $\begingroup$ Thanks @Jan, that confirms my intuition about the difficulty. I'll read up on IR. $\endgroup$ – Sean Easter Dec 31 '15 at 19:53
  • $\begingroup$ @ToddMinehardt Thanks, I hadn't considered headspace gas. A quick search suggests the DIY options for GC are pretty sparse, but I'll keep digging. $\endgroup$ – Sean Easter Dec 31 '15 at 20:03
  • $\begingroup$ I don't think you understand the enormity of the question. Look at all the hoopla about coffee. How to choose the right beans, how to roast the beans, how to grind the beans, how fresh the grind/roast must be, and finally how to brew the grind - all to get a cup of coffee. What you have to realize is that "coffee" is not A chemical, but a myriad of chemicals that are affected by all the choices made in producing the cup of coffee. $\endgroup$ – MaxW Jan 1 '16 at 20:32

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