SOP: Determination of Acetaldehyde in Frozen Vegetables

GIFSQ — Global Institute for Food Safety & Quality

Principle

Inadequately blanched vegetables develop progressive off-flavours, odours and colours from continued enzyme action; the acetaldehyde content serves as an index of this off-flavour development. The distillate is trapped with excess bisulphite, the remaining bisulphite is consumed with excess iodine, and the leftover iodine is back-titrated with thiosulphate.

Apparatus required

• Steam distilling apparatus: a 500-ml Kjeldahl flask, a spray trap, and a condenser, preferably with all-glass joints (apparatus with rubber stoppers and connections is suitable if satisfactory blanks are obtained; some rubber yields negative blanks)

Reagents

a. Sodium thiosulphate solution, 0.05 N: prepare daily by diluting 0.1 N solution. 1 ml of 0.05 N Na₂S₂O₃ = 1.1 mg of acetaldehyde.

b. Iodine solution, 0.05–0.0515 N.

c. Sodium bisulphite solution, 0.05 N (NaHSO₃) in approximately 10% alcohol: dissolve 2.6 g of sodium bisulphite in 500 ml of water, add 100 ml of alcohol and dilute to 1 litre with water in a volumetric flask. Standardise against the iodine solution, and adjust the concentration to be slightly lower than that of the iodine solution but not lower than 0.0485 N. Prepare fresh daily.

d. Aldehyde-free alcohol: use alcohol containing less than 2 ppm acetaldehyde. Prepare it as follows: add m-phenylenediamine dihydrochloride at the rate of 5 g per litre, allow to stand for 24 hr with frequent shaking (pretreatment with KOH serves no purpose); reflux for at least 8 hr, or longer if necessary, and distil; reject the first 10 ml and the last 5 ml of distillate; store in a dark, cool place in well filled bottles. To 35 ml of alcohol, add 300 ml of freshly boiled and cooled water and determine the acetaldehyde content as in the case of the sample.

e. Starch indicator, 1%.

Sample preparation

Break up the frozen block of vegetable, mix well and grind approximately 150 g.

Procedure

Transfer 50 g of the sample to the Kjeldahl flask with approximately 150 ml of water

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Add 1–2 drops of antifoaming agent and steam distil into a 500-ml conical flask containing 100 ml of freshly boiled chilled water, covering the end of the delivery tube

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Immerse the receiving flask in an ice bath

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Distil approximately 200 ml within 12–15 min

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To the distillate, add 35 ml of aldehyde-free alcohol with a pipette, add 25 ml of sodium bisulphite solution, and allow to stand for 30 min with occasional shaking

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With the help of a pipette, add 25 ml of iodine solution, and titrate the excess iodine with sodium thiosulphate solution using starch as indicator

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Carry out a blank using 300 ml of freshly boiled and cooled water

Calculation

Acetaldehyde, ppm = [(Sample titre − Blank titre) × 1.1 × 1,000] / (Wt of sample)

• Where,
• Sample titre = ml of 0.05 N sodium thiosulphate used for the sample
• Blank titre = ml of 0.05 N sodium thiosulphate used for the blank
• Wt of sample = weight of the sample taken, g (50 g)

Note: the denominator of the printed expression reads as illegible characters in the available copy; “Wt of sample” is carried, flagged for checking against the printed book.

Precautions

• Keep the receiving flask immersed in an ice bath during distillation.
• Use alcohol containing less than 2 ppm acetaldehyde for the aldehyde-free alcohol preparation.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition. Joslyn, M.A., J. Assoc. Offic. Agric. Chem., 40, 338 (1957). Lee, F.A., D. DeFelice, and R.R. Jenkins, Ind. Eng. Chem., Anal. Edn., 14, 240 (1942).