Determination of fructose and inulin
Fructose, a keto-hexose (called as fruit sugar), is usually accompanied by sucrose in fruits like apple. Honey is a rich source of fructose.
The hydroxymethyl furfural formed from fructose in acid medium reacts with resorcinol to give a red color product.
» Resorcinol reagent: Dissolve 1g resorcinol and 0.25g thiourea in 100mL glacial acetic acid. This solution indefinitely stable in the dark.
» Dilute HCl: Mix five parts of conc. HCl with one part of distilled water.
» Standard fructose solution: Dissolve 50mg of fructose in 50mL water. Dilute 5mL of this stock to 50mL for a working standard.
Inulin is a polymer made of fructose units with b-2-1 linkage. It is found in onion, garlic and in many other plant parts.
Grind the sample and extract in 80% ethanol for six hours to remove free sugars. Dry the sample and take 500mg in a 100mL conical flask. Add 20mL of water and heat it in a water bath at 90°C for 10min. Collect the extract and then add 70mL of water. Replace the flask for another 30min with occasional shaking to dissolve the fructosan, then remove and cool it at room temperature. Combine the extracts and filter the solution if it is not clear and make up to 100mL in a standard flask.
To estimate the inulin content in the extract follow the procedure given for fructose estimation. The amount of inulin is expressed in terms of fructose concentration.
1. Ashwell, G (1957) In: Methods in Enzymol 3 (Eds Colowick, S J and Kaplan, N O) Academic Press New York p 75.
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