Degrees Brix is a measure of the soluble solids content of a liquid, determined by refractive index and expressed as the equivalent percentage of sucrose by mass. In a fruit concentrate the soluble solids are mostly natural sugars, with organic acids, soluble pectins and minerals making up the rest.
Why it is the commercial parameter
Concentrate is bought to be diluted. A buyer producing juice or nectar needs to know how many parts of water one part of concentrate will carry, and Brix answers that directly. Divide the concentrate Brix by the reference single strength Brix for the species and you have the concentration factor: the yield of finished juice per tonne of concentrate. That arithmetic is the whole basis of a concentrate comparison.
The single strength reference is not a matter of opinion. Codex standard CXS 247-2005 carries a reference Brix table by species, and Council Directive 2001/112/EC sets minimum Brix values for reconstituted juice in its annexes. Both are the texts a trade dispute is settled against.
Why a bare Brix number is not enough
Refractive index varies with temperature, so a Brix reading is only meaningful against a stated measurement temperature. Organic acids also refract, which inflates the apparent soluble solids in high-acid fruit; whether an acid correction has been applied changes the number materially in sour cherry, blackcurrant and cranberry. ISO 2173 defines the refractometric method and the conditions.
The practical consequence: two offers quoting the same Brix are not necessarily offering the same product. Require the figure with a temperature, a statement on acid correction, and a named method.
What Brix does not tell you
It says nothing about authenticity, colour, aroma, pulp content or pesticide residues. A concentrate can hit its Brix precisely and still fail an authenticity assessment against the AIJN Code of Practice reference ranges, which is why serious buyers pair a Brix specification with a compositional profile. The concentrate sourcing guide covers the full parameter set.