A recent email from my local resource network contained an interview with a long-time proponent of the Brix scale and the use of refractometers. The interviewee also mentiond the work of Drs. Reams, Albrecht, Andersen and Skow. [You can google those names and get quite a lot of information on their work so I won't summarize.] The Brix scale is a measure of sugar content and what some people are now doing is to use this as a proxy for nutrient-density.
I have been aware of this line of thinking for several years, but it has always seemed too simplistic. The interview I read hasn't changed my mind any. It still smacks of pseudo-science, especially as the Brix reading can be altered by adding sugar to the solution or diluting the solution itself. However, the interview did make a comment about categorizing grapes via the Brix reading while they were green and that was rather astute. These grapes can then be sorted into more valuable wine grapes and less valuable concentrate grapes at harvest. This is similar to what the fieldmen in Washington are doing when they use a refractometer to help time the apple harvest so they will store well in the packing shed.
That said, I have been planning on buying a refractometer for several years, since I should have one for my apples once my trees get more mature and produce more. Then I can test some plants and see for myself whether Brix readings correlate well with taste and nutrition.
This also raises the larger question of subjectivity vs. objectivity. Subjectivity is just fine and people who are doing experiments that work DO NOT have to rise to a quantitative level that appears to be scientific. Simply saying the Brix scale is correlated with taste is quite enough. A researcher doesn't have to postulate that insects see something that we are measuring on an abstract numerical scale. [The example in the interview was that an insect sees weakness in a plant as a measurable wavelength on the electromagnetic spectrum and attacks it. We can see the same thing on the Brix scale. This is laughable.] He just has to say it works. In point of fact, it is better to just say, "I feel better because I feed my soil and the plants feed me." One of the changes that are happening all around us is that the pseudo-scientific method used by economists and bankers who want to enhance their prestige by appearing to be objective is already in the toilet. It then becomes an opportunity to just start talking subjectively like the First Nations elders (or like us dirty hippies used to talk 40 years ago). I am a scientist and I say science is overrated.
[As another sidebar, the creationists used to use the term "scientific creationism." This was a political move to take advantage of the patina of objective scientific method in order to enhance credibility. It didn't take too long for real scientists to point out that there is no way to refute, or test, creationism, so it is not amenable to the scientific method. The creationists are now having more luck getting their message across to high schools across the country since they accepted their basis in subjectivity.]