Enzymatic determination of inulin in food and dietary supplements

An enzymatic method for the determination of the oligofructoside inulin based on the enzymatic hydrolysis of inulin by inulinase was developed. Fructose formed in this reaction was phosphorylated by adenosine-5'-triphosphate and hexokinase, and isomerized to glucose-6-phosphate by phosphoglucose isomerase. Glucose-6-phosphate was finally oxidized to gluconate-6-phosphate by glucose-6-phosphate dehydrogenase. Reduced nicotinamide adenine dinucleotide phosphate formed in this sequence of reactions is determined spectrophotometrically by its absorbance at 340 nm. The absorbance is directly proportional to the initial amount of inulin. Precision, accuracy, repeatability and recovery rates of inulin following standard addition to sample solutions were evaluated. All steps of the enzymatic analysis were carried out on an automatic centrifugal analyzer. This, together with rapid and simple sample preparation, makes the method suitable for the routine determination of inulin in food surveilance and for quality control purposes.