SOP: Determination of Furfural

GIFSQ — Global Institute for Food Safety & Quality

Principle

Furfural forms from ascorbic acid and pentose sugars; it can polymerize or combine with amino compounds and so drives nonenzymatic browning in stored fruit and vegetable products. Measuring furfural is therefore of value in storage studies. It is measured either by its ultraviolet absorption maximum at 277 nm (alcohol concentration has no effect on the determination) or by its colour reaction with aniline and hydrochloric acid measured at 520 nm (reproducible results only at constant alcohol content).

Apparatus required

• Steam distillation assembly
• Spectrophotometer or colorimeter (readings at 277 nm and 520 nm)
• 50-ml graduated cylinders
• 100-ml volumetric flask

Reagents

a. Furfural standard solution: redistil furfural at atmospheric pressure and collect the fraction boiling at 161.2 °C (uncorrected). Weigh 1 ml of the redistilled furfural into a 100-ml volumetric flask and make up to the mark with alcohol. When required, dilute 5 ml to 500 ml with 50% alcohol (concentration approximately 116 mg per litre).

b. 50% ethyl alcohol.

c. Colourless aniline.

d. Hydrochloric acid, sp. gr. 1.125.

Procedure

Sample extract and distillation

• Dry material: grind the dry material; to 50 g of it add 250 ml of 50% ethanol, extract by steeping, and filter.
• Liquid samples: if the solid content is 25 g per 100 ml or less, add 12.5 to 25 ml of water; if the solid content is more than 25 g per 100 ml, add 5 ml of water for each 10 g of solid matter present. Add sufficient alcohol to raise the concentration to 50%, mix, and filter. Pipette 100–200 ml of the filtrate.

↓

Steam-distil slowly, collecting a volume of distillate equal to the volume taken for distillation

Spectrophotometric method

Dilute the distillate with known volumes of 50% alcohol and determine the absorbance at 277 nm

↓

Determine the absorbance of standard furfural solutions containing 0, 1, 2, 3, 4 and 5 mg of furfural per litre; plot the calibration curve of concentration against absorbance and read the concentration in the sample from the curve

Colorimetric method

Pipette aliquots (0, 0.5, 1, 2, 3, 4 and 5 ml) of the standard (116 mg per litre) furfural solution into 50-ml graduated cylinders and make up to the mark with 50% ethyl alcohol; add 2 ml of colourless aniline and 0.5 ml of HCl (sp. gr. 1.125); keep for 15 minutes and measure the colour at 520 nm

↓

Dilute an aliquot of the sample distillate to 50 ml with 50% ethyl alcohol, develop and measure the colour exactly as for the standards

Calculation

Furfural, mg% = (mg of furfural × dilution × 100) / (weight or volume of sample × 1000)

Precautions

• In the colorimetric method keep the alcohol content constant — reproducible results are obtained only then.
• In the ultraviolet method alcohol concentration has no effect on the determination.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Schoeneman, R. L., J. Ass. Off. Agric. Chem., 43, 657 (1960) — as cited in the book.

Stillings, R. A. and Browning, B. L., Ind. Eng. Chem., Anal. Ed., 12, 499 (1940) — as cited in the book.