Page 129 - Analytical method for food addtives

E220–8: Sulphites 79

Apply power to the heating mantle. Use a power setting that causes 80–90
drops/min of condensate to return to the flask from the condenser. Let the contents
of the flask boil for 105 min, and then remove the vessel.

Determination and calculation
Titration
Immediately titrate (burette) the contents of the vessel with sodium hydroxide
standard solution to a yellow end-point that persists longer than 20 s. Calculate the
mass fraction, W, of sulphite, round the result to a whole number and express the
sulphite content as sulphur dioxide in milligrams per kilogram, using equation
[8.3]:

32.03 × V × 1000 × N
W = ––––––––––––––––– [8.3]
m

where
32.03 is the milliequivalent weight of SO in grams per mole
2,
N is the molarity of the sodium hydroxide standard solution, in mole per litre
V is the volume of sodium hydroxide standard solution with N = 0.010 mol/
L required to reach the end point in millilitres
1000 is the factor to convert milliequivalents to microequivalents
m is the amount of test portion introduced into the round-bottomed flask in
grams
Blank determination
Determine blank on reagents by titration and, if necessary, correct results accord-
ingly. (For further details see BS EN 1988–1:1998.) 2

Analysis of foodstuffs using enzymatic method 6

Scope
Enzymatic method for determination of sulphite content, expressed as sulphur
dioxide. Other sulphur-containing substances such as sulphate, sulphide or thio-
sulphate do not interfere with the determination. Carbonyl-sulphite complexes
react as free sulphites. Isothiocyanates, occurring in, for example, mustard,
interfere with the determination. The method is not applicable to cabbages, dried
garlic, dried onions, ginger, leeks and soy proteins.

Principle
Sulphite oxidised to sulphate in the presence of sulphite oxidase with the liberation
of hydrogen peroxide at the same time. Reduction of hydrogen peroxide and
+
conversion of NADH to NAD in the presence of NADH peroxidase. Conversion
+
+
of NADH to NAD is determined spectrometrically at 340 nm and is proportional
to the concentration of sulphite.