Page 124 - Analytical method for food addtives

74 Analytical methods for food additives

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vapour phase Fourier transform infrared spectrometry (FTIR). Recently a
comparison of five methods has been published for the analysis of total SO in
2
22
23
grape juice. Classical methods, i.e. gravimetric and titration 24–26 are still used by
public analysts. A summary of these is given in Table 8.1, together with the
matrices for which the methods are applicable. If statistical parameters for these
methods were available they have been summarised in Table 8.2. Many of these
methods are AOAC Official Methods of Analysis and have been accepted as
European Standards.
The optimised Monier–Williams method for the analysis of sulphites in foods
is applicable to the determination of ≥ 10 ppm (µg/g) sulphites in foods and has
2
been collaboratively tested. The method measures free sulphite plus reproducible
portion of bound sulphites (such as carbonyl addition products) in foods. The test
portion is heated with a refluxing solution of hydrochloric acid to convert sulphite
to sulphur dioxide. A stream of nitrogen is introduced below the surface of the
refluxing solution to sweep sulphur dioxide through a water-cooled condenser
and, via a bubbler attached to the condenser, into the hydrogen peroxide solution,
where sulphur dioxide is oxidised to sulphuric acid. The generated sulphuric acid
is titrated with standardised sodium hydroxide solution. The sulphite content is
2–5
directly related to the generated sulphuric acid. A summary of the procedure for
this method is given in the Appendix and the performance characteristics are given
in Table 8.3.
The enzymatic method for the determination of sulphite content, expressed as
7
sulphur dioxide, in foodstuffs has been collaboratively tested. Sulphite in liquid
foods or extracts of solid foods is analysed according to the following principle,
where NADH is reduced to nicotinamide adenine dinucleotide:
SO + O + H O sulphite oxidase → SO + H O [8.1]
2–
2–
3 2 2 4 2 2
H O + NADH + H NADH peroxidase → 2H O + NAD + [8.2]
+
2 2 2
Decrease in NADH is measured spectrophotometrically and is proportional to the
concentration of sulphite. A summary of the procedure for this method is given in
the Appendix with a summary of the statistical parameters being given in Table
8.4.
A differential pulse polarographic method for the analysis of sulphites in foods,
applicable to the determination of ≥10 µg total SO /g in shrimp, orange juice, peas,
2
9
dried apricots and dehydrated potatoes has been collaboratively tested. The
method measures sulphur dioxide which is purged with N from acidified test
2
samples, collected in electrolyte-trapping solution and then determined by differ-
8,9
ential pulse polarography. A summary of the procedure for this method is given
in the Appendix and a summary of the statistical parameters in Table 8.5.
An ion exclusion chromatographic (IEC) method for the analysis of sulphites in
foods and beverages, applicable to the determination of SO at≥10 µg/g, has been
2
11
collaboratively tested. The method measures sulphur dioxide which is released
by direct alkali extraction. Diluted test portions of liquids or diluted filtrates of