Can the concentration of D-gluconic acid in soy sauce be determined using the Gluconate Assay Kit (K-GATE)?

Yes. This kit can be used to rapidly and simply quantify D-gluconic acid in soy sauce, with only the following slight modification to the analysis procedure as described in the booklet:

(a) Sample preparation: Take 2 mL of soy sauce, add 4 mL of Carrez I solution (see page 7 of booklet) and mix.  Then add 4 mL of Carrez II solution (see page 9 of booklet) and mix thoroughly.  Filter the resultant suspension through Whatman No. 1 filter paper without any further additions (in this case, sodium hydroxide is NOT added to neutralise the extract).  The filtrate should be clear and may be slightly yellow / orange in colour (this is normal).

(b) Assay procedure: Use up to 0.5 mL of the filtrate according to the normal assay procedure (see page 5 of booklet) – however, after the addition of 0.02 mL of suspension 3 (6-PGDH), the cuvettes (after mixing) should be incubated for 10 min (e.g. on the bench top or in the spectrophotometer compartment) prior to recording the A1 values.  This is to allow colour complexes time to stabilise, thus enabling the rise in absorbance due to the conversion of D-gluconic acid to be measured accurately.  The speed of the reaction on addition of the gluconate kinase is unaffected by the sample preparation/sample size, and will take just a few minutes to complete as normal. 

Product Page (K-GATE)