On Permies.com there is a post, Brix Testing, which references Brix Readings, and What They Tell Us. The source of that article is Biologic Ionization as Applied to Farming and Soil Management.
Brix has been used for a long time in the wine industry to measure ripeness. But Brix has been linked to nutrient density. Nutrient density in foods is considered to be important to overall good health although there is some debate about definition.
Going back to the Permies.com post,
Crops with higher refractive index will have a higher sugar content, higher protein content, higher mineral content and a greater specific gravity or density. This adds up to a sweeter tasting, more mineral nutritious feed with lower nitrates and water content and better storage attributes.
If one accepts that the wine industry knows what it is doing when it uses Brix to measure sugar content in grapes in order to make business decisions about harvest time, then yes, brix measures sugar content. However, extending high brix to indicate “more mineral nutritious food” aka nutrient dense food seems to be a stretch since there doesn’t seem to be anything that confirms that. In fact, when the question is asked of a Brix-nutrient dense proponent, Could someone explain whether it is known that higher Brix fruits/vegetables have higher mineral and protein contents, or is it only known that they have higher sugar/flavor components?, the answer given is “I’m unaware of any charts or tables that bluntly state something along the lines of “6 Brix tomatoes have 49mg of this, 75mg of that, and 100mg of the other.”
In fact, most of the High Brix=High Nutrient Density commentary leads to Carey Reams: it is known that he created a bombshell in the early 1970’s when he, refractometer in hand, walked into the office of ACRES USA and placed a simple chart on the editor’s desk. That chart correlated brix numbers with four general quality levels for most fruits and vegetables. Copied innumerous times, it has made its way around the world over and over. The Carey Reams story is told in many places so I’ll only link to one here since it includes a link with more Reams history.
A bit of digging leads to some interesting information: Critique and study of all the works I was able to gather generated by Carey Reams but nothing on the Brix/nutrient dense connection.
But there is this analysis – Brix and Nutrient Density of raw data from a competition sponsored by International Ag Labs in 2013 to see who could grow the best butternut squash. Unfortunately the data no longer exists at IAL but the winner’s data does exist and the competition data exists elsewhere. The blogger concludes that [t]here does seem to be a correlation between Brix and mineral nutrients but others have questions –
Unfortunately, I was not able to find the explanation of the nutrient density standard that was used to rank the samples. However, the results show some interesting things:
The brix reading does not correlate with protein content.
The brix reading does not correlate with calcium content.
The brix reading does not seem to correlate with any other mineral content
But there is information that comes close to linking Brix to nutrient density. This article from Boreal Agrominerals who sell Spanish River Carbonatite, a glacial deposit rock dust high in trace micronutrients such as zinc, copper, sulfur, iron and magnesium. The application of SRC to asparagus fields resulted in higher yields and higher Brix. There is no indication of mineral content. In a second trial with field tomatoes, there is an increase in mineral content but no Bris measures. They dance around the edges but never get to the heart.
Perhaps it is possible to see if there is a correlation between Brix and nutrient density without resorting to expensive lab testing. If we use a mix of peatmoss and perlite in pots and have one pot with nothing but the mix, a second pot with the mix and an N-P-K formula, a third pot with a rock dust such as Spanish River Carbonatite, and a fourth pot with the same amount of N-P-K as the second pot plus the same amount of Spanish River Carbonatite as the third pot. The seed used would all be from the same fruit. Taking Brix analysis of leaf tissue of each of the four plants at the same time and of ripe fruit at the same time should yield different Brix readings and confirm whether or not those plants with access to more nutrients have higher Brix readings. Obviously, this does not test for mineral content but knowing that a plant with higher Brix readings came from a plant with greater access to minerals would be going in the right direction.