A Thermo Scientific Spectronic 20D+ digital spectrophotometer (Bremen, Germany) was used to measure the sum of fructose and glucose in column effluents using the Somogy–Nelson method (BeMiller and Low 1998).
In brief, 10-μl portions of effluents were mixed with an alkaline solution of Cu(II) and heated in a boiling water bath to reduce Cu(II) ions to Cu(I) ions.
Then, a solution containing arsenomolybdate reagent, resulted from the reaction of (NH4)6Mo7O24 with Na2HAsO4 in H2SO4, was added to produce an intensive blue polymolybdate complex due to the reaction with Cu(I) ions.
Absorbance of this complex was measured at 520 nm. Concentrations of the sum of fructose and glucose were determined versus simple standard solutions of glucose at concentrations within 1.0–50.0 mg/l.