K-FRUC is specific for fructans, including those from chicory, dahlia, jerusalem artichoke, onion, wheat stems and leaves, agave and levan type fructans.
In K-FRUC first the interfering sugars, as sucrose, maltose, maltodextrins and starch, possibly present in the sample are hydrolysed to D-glucose and D-fructose. Then D-glucose and D-fructose are reduced by sodium borohydride to the corresponding sugar alcohols, D-sorbitol and D-mannitol. Native fructans and non-reducing FOS such as Neosugars® are not affected by this reaction (borohydride reduction). Fructan, FOS and borohydride reduced FOS are specifically hydrolysed by ultrapure (recombinant/affinity purified) exo-inulinase and endo-inulinase to D-glucose and D-fructose and measured using the PAHBAH reducing sugar method. The above procedure gives accurate measurement of fructan and non-hydrolysed FOS (e.g. Neosugars®). However, FOS produced by acid or enzymic hydrolysis of fructan (e.g. Raftilose P-95® -partially hydrolysed chicory inulin) will be underestimated by 15-20 % depending on the degree of hydrolysis of the original fructan. This underestimation is due to the reduction of terminal reducing sugars in the oligosaccharides to sugar alcohols, which are not measured in the current assay. If samples of these FOS are available, the degree of underestimation is readily determined. Please see the Appendix on pg. 10 of the K-FRUC Data Booklet.
K-FRUCHK is specific for fructans, including those from chicory, dahlia, jerusalem artichoke, onion, wheat stems and leaves and agave. It does not measure levan type fructans.
In K-FRUCHK first the interfering sugars, as sucrose, maltose, maltodextrins and starch, possibly present in the sample are hydrolysed to D-glucose and D-fructose. Then Fructan and FOS are specifically hydrolysed by ultrapure (recombinant/affinity purified) exo-inulinase and endo-inulinase to D-glucose and D-fructose. The concentration of fructan and FOS (as D-glucose and D-fructose) is measured with a hexokinase/phosphoglucose isomerase/glucose 6—phosphate dehydrogenase system. The amount of NADPH formed in this reaction is stoichiometric with the amount of D-glucose plus D-fructose. It is the NADPH which is measured by the increase in absorbance at 340 nm. Fructan content is determined by subtracting absorbance values from sample blank from those for the sample. This assay is linear over the range of 4 to 80 μg of D-glucose or D-fructose per assay. Therefore if the amount of D-glucose and D-fructose delivered by hydrolysis of interfering sugars (e.g. sucrose, maltose, maltodextrins and starch) is high in comparison to fructan content, it may give very high background and make it impossible to measure fructan or may lead to big errors in the determination.
In both methods galactosyl-sucrose oligosaccharides, if present, must be enzymatically removed. Please see the controls and precautions sections in the data booklets.
In general, for the measurement of FOS we recommend using K-FRUCHK, assuming that interfering substances (e.g. sucrose, maltose, maltodextrins and starch) are present at lower concentrations than fructan.