I received this from a coffee newsletter. The summary of this study from the email:
The tl;dr of the paper is this: Measuring brewed coffee using refractometers has limited use, because you only know the total extraction and not the quality of the extraction. This paper argues that by passing a voltage through the liquid coffee, you can create a kind of profile. Once you’ve established a profile of a good-tasting extraction, you can then measure the quality of the coffee by seeing whether or not it closely matches your desired voltage profile.