Grade blending

Light transmittance doesn’t average when two syrups are mixed. Absorbance does: it is the negative logarithm of transmittance, and a blend’s absorbance is each syrup’s absorbance weighted by its share of the volume (the Beer–Lambert law). The calculator converts both readings to absorbance, finds the share of each syrup that gives the target, and converts back to transmittance.

That is why the North American Maple Syrup Producers Manual notes that a little dark syrup darkens light syrup a lot, and it takes more light syrup than you might expect to bring dark syrup back up. Blending 78% and 20% syrup to reach 55% takes about 74 parts light to 26 parts dark; a straight average of the readings says 60 to 40, which lands near 45%, in the Dark class.

Real syrups don’t follow the law exactly, so the calculator takes an optional reading of a measured half-and-half blend, the input the Cornell Maple Program’s grade blending calculator asks for, and bends the curve to pass through it. The manual’s advice still applies: blend small trial amounts first, and taste the result to make sure the flavor fits the color class.

Grade A color classes are by transmittance at 560 nm: Golden, delicate taste at 75% and above; Amber, rich taste from 50 to under 75; Dark, robust taste from 25 to under 50; Very dark, strong taste under 25. Photometers commonly differ by about 2% between readings, so aim inside the class rather than at its edge.