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.

Principle The hydroxymethyl furfural formed from fructose in acid medium reacts with resorcinol to give a red color product.

Materials

• 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.

Procedure

1. To 2mL of the solution containing 20 to 80mg of fructose add 1mL of resorcinol reagent.
2. Then add 7mL of dilute hydrochloric acid.
3. Pipette out 0.2, 0.4, 0.6, 0.8 and 1mL of the working standard and make up the volume to 2mL with water. Add 1mL of resorcinol reagent and 7mL of dilute HCl as above.
4. Set a blank along with the working standard.
5. Heat all the tubes in a water bath at 80°C for exactly 10min.
6. Remove and cool the tubes by immersing in tap water for 5min.
7. Read the color at 520nm within 30min.
8. Draw the standard graph and calculate the amount of fructose present in the sample using the standard graph.

INULIN 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.

Sample Extraction 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.

Reading

1. Ashwell, G (1957) In: Methods in Enzymol 3 (Eds Colowick, S J and Kaplan, N O) Academic Press New York p 75.