Page 128 - Analytical method for food addtives

78 Analytical methods for food additives

sulphite by oxidation or reaction with components in food. Since sulphites are
reactive with air and food matrices and often lack stability, portions are fortified
with a stable source of sulphite, not sodium sulphite or similar salts. Sodium
hydroxymethylsulphonate (HMS), which is a bisulphite addition product of for-
maldehyde and which is structurally similar to some combined forms of sulphite in
foods, is useful for preparing stable fortified test materials.)
For analysis, 50 g of prepared sample of sulphite-free food are transferred to the
2
flask (Fig. 1 ). An aliquot portion of aqueous solution of HMS sodium salt is
added. The solution is analysed immediately.
HMS recoveries of more than 80 % from food matrices fortified at 10 mg/kg are
recommended to ensure accurate analytical data.

Sample preparation
Solid sample:
Transfer 50 g of food, or a quantity that contains 500 µg to 1500 µg of sulphur
dioxide, to a food processor or blender. Add 100 ml of the ethanol/water mixture
and briefly grind the mixture. Continue the grinding or blending only until the food
is chopped into pieces small enough to pass through the ground glass joint of the
flask.

Liquid samples:
Mix 50 g of test sample, or quantity that contains 500 µg of sulphur dioxide, with
100 mL of the ethanol/water mixture.

System preparation
2
Use the distillation apparatus assembled as shown in Fig. 1, put the flask in the
heating mantle and add 400 mL of water to the flask. Close the stopcock of the
funnel and add 90 mL of hydrochloric acid solution to the funnel. Begin nitrogen
flow at 200 mL/min ± 10 mL/min and initiate the condenser coolant flow. Add
30 mL of 3 % hydrogen peroxide solution to the vessel. After 15 min, the apparatus
and water will be thoroughly deoxygenated and the prepared test portion may be
introduced into the apparatus.

Sample introduction and distillation
Remove the dropping funnel and quantitatively transfer the test portion in aqueous
ethanol to the flask. Wipe the tapered joint clean with a laboratory tissue, quickly
apply stopcock grease to the outer joint of the funnel, and return the funnel to the
flask. Examine each joint to be sure that it is sealed.
Use a rubber bulb equipped with a valve to apply head pressure above the
hydrochloric acid solution in the funnel. Open the stopcock of the funnel and let
hydrochloric acid solution flow into the flask. Continue to maintain sufficient
pressure above the hydrochloric acid solution to force the solution into the flask.
The stopcock may be closed, if necessary, to pump up pressure above the acid, and
then opened again. Close the stopcock before the last 2 to 3 mL drain out of the
funnel to guard against the escape of sulphur dioxide into the funnel.