SOP: Quantitative Determination of Sulphide Stain in Canned Foods

GIFSQ — Global Institute for Food Safety & Quality

Principle

Sulphide staining hits canned foods rich in sulphur compounds — vegetables and meats — and can follow residual sulphur dioxide. High-temperature processing frees the sulphur as volatile sulphides that react with tin or exposed iron to discolour the metal. Here the stain is driven off as hydrogen sulphide with hydrochloric acid, trapped in ammoniacal cadmium chloride, and the sulphide measured by iodine titration.

Apparatus required

• 250-ml conical flask fitted with a ground glass stopper, and having a thistle funnel and a delivery tube

Reagents

a. Cadmium chloride solution: dissolve 10 g of cadmium chloride, CdCl₂·2.5H₂O, in 250 ml of water, add 100 ml of ammonia (sp. gr. 0.91), make up to 1 litre with water and filter.

b. 0.01 N sodium thiosulphate solution.

c. 0.01 N iodine solution.

d. Starch indicator.

e. Dilute HCl, 1 + 4 and 1 + 1.

Procedure

Cut a known area of the tinplate from the test can and place inside the flask; fix the stopper

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Dip the end of the delivery tube in 50 ml of cadmium chloride solution contained in a tube

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Add 30 ml of dilute HCl (1 + 4) into the conical flask through the funnel

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Heat the contents of the flask to boiling and continue distillation for 10 min

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To the cadmium chloride solution containing the distillate, add iodine solution (10 ml or more until there is excess); acidify with 20 ml of dilute HCl (1 + 1), and titrate with 0.01 N sodium thiosulphate solution using starch as indicator

Calculation

1 ml of 0.01 N iodine = 160 μg of sulphide sulphur

μg of sulphide sulphur per 100 sq. cm. = (Titre × 160 × Volume made up × 100) / (ml taken for titration × Area of the tinplate taken for estimation in sq. cm.)

Where,

• Titre = volume of 0.01 N sodium thiosulphate solution used, in ml
• 160 = μg of sulphide sulphur equivalent to 1 ml of 0.01 N iodine (the print reads “160 pg” — carried as 160 μg, the chemically equivalent value; flagged)
• Volume made up = total volume of the cadmium chloride solution containing the distillate
• ml taken for titration = aliquot of the solution taken for the iodine addition and titration
• Area of the tinplate taken for estimation = area of tinplate cut from the test can, in sq. cm.

Reference

Vipson, W. O., M. Mahadeviah, R. V. Gowramma, L. V. L. Sastry & H. C. Bhatnagar, J. Fd Sci. Technol., 7, 97 (1970).