Enzymatic determination of total D-gluconic acid in honey

The total D-gluconic acid (free D-gluconic acid and D-glucono-delta-lactone) content of 20 Galician (northwestern Spain) honeys has been determined using an enzymatic method. To the author's knowledge, this is the first time this method has been applied to honey. The solution of honey was adjusted to pH 10.5 with 0.1 N KOH; 10 min at room temperature was allowed for hydrolysis of D-glucono-delta-lactone to D-gluconic acid. After this, it was necessary to adjust to test pH (7.8) with 0.1 N HCl. With these conditions, there were no observed interference effects. The enzymatic determination was measured spectrophotometrically at 340 nm, using gluconate kinase and B-phosphogluconate dehydrogenase. The method combines precision (% CV was 0.30, at worst), good recovery (99.8%), simplicity, and low cost because this cost was reduced by 60% using a microtest. The total D-gluconic acid content of the honeys analyzed ranges between 3.91 and 11.71 g/kg (mean = 7.37 g/kg), which is in keeping with value ranges obtained by other authors using HPLC methods.