Page 134 - Analytical method for food addtives

84 Analytical methods for food additives

Analysis of foods and beverages with sulphite (total) content ≥≥ ≥≥ ≥5 mg/kg
using flow injection analysis method 12

Scope
Flow injection analysis method for determination of ≥5 µg/g total SO in shrimp,
2
potatoes, pineapple and white wine.
Results of the interlaboratory study supporting the acceptance of the method:
S = 23.5; S = 35.1; RSD = 7.5 %; RSD = 14.0 %
r R r R

Principle
Sulphite is determined by flow injection analysis (FIA) using reaction with
malachite green. In the FIA system, test solution is first reacted with NaOH to
liberate aldehyde-bound sulphite. Then test stream is acidified to produce SO gas,
2
which diffuses across Teflon membrane in gas diffusion cell into flowing stream
of malachite green, which is discoloured. Degree of discolouration of malachite
green is proportional to amount of sulphite in test solution.
Procedure

Test portion preparation
(a) Solid foods – weigh and transfer to blender representative test portion. Add
weight of sodium tetrachloromercurate (TCM) solution, at 3–30 times that of
test portion. Use amount of reagent that will result in SO concentration in final
2
extract that is within range of standards and also result in semiliquid slurry in
blender. Blend until slurry is homogeneous (c. 2 min), and centrifuge.
(b) White wine – weigh representative test portion (1–2 g) into tared 50 mL
disposable tube. Add TCM solution at level 19 times weight of test portion,
cap tube and mix well. Centrifuge if solution is turbid.

Determination
(a) System start-up – Begin pumping 2 donor reagents, sulphuric acid and sodium
hydroxide, and recipient reagents, malachite green solution(2) and phosphate
buffer reagent, through pump tubes specified in Fig. 990.29. Once flow is
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established and system has equilibrated (c. 10–20 min), zero detector and
monitor A at 615 nm with detector set at 0.1 AUFS and recorder at c. 0.5 cm/
min. Base-line noise (peak to peak) should be <3 mm; if it is not, determine
source of excessive noise and make necessary corrections. (Note: Some
detectors are prone to trapping air bubbles that must be purged from flow cell.)
If detector has variable electronic filter, this can be adjusted to c. 1–5 s to
reduce baseline noise. Check flow rates of donor and recipient reagents to be
sure that they meet specifications in Fig. 990.29.
(b) Test sample analysis – Repeatedly inject 10 µg/mL sulphite working standard
condition, until peak height is stable and recorder deflection is c. 60–90 % full