Galactomannan is hydrolysed by β-mannanase and α-galactosidase to produce α-D-galactose.
Galactose mutarotase causes the subsequent interconversion of α-D-galactose to β-D-galactose.
β-D-galactose is hydrolysed by β-galactose dehydrogenase to produce D-galactonic acid, with the conversion of NAD to NADH. The amount of NADH formed can be determined by an increase in absorbance at 340 nm and is stoichiometric with the amount of galactose present.
Once the Galactose:Mannose ratio of the sample is known, the measured galactose content can then be used to determine the amount of galactomannan in the sample.
These calculations can be found on page 7 and 8 of the assay protocol, or can be simplified by using the corresponding megacalc.
Galactose mutarotase is included in bottle 5 with β-galactose dehydrogenase.