Page 130 - Analytical method for food addtives

80 Analytical methods for food additives

Procedure
Preparation of the sample test solutions
• General
Remove high concentrations of ascorbic acid of more than 100 mg/kg or
100 mg/L of sample (see fruit juices). If the concentration of sulphite in the
sample test solution is higher than 0.3 g/L, dilute the sample test solution prior
to the determination or take a smaller sample volume.

Liquid samples
• White wine, brandy and beer
Analyse white wine and brandy directly. Beer should be filtered to remove
carbon dioxide. It may be necessary to decolourise beer. For the decolourisa-
tion, add not more than about 0.7 g of bentonite to 10 mL of filtered beer in a
50 mL glass beaker, stir the mixture for 2 min using a magnetic stirrer and then
filter the solution into another 50 mL glass beaker.
For the enzymatic determination take 100 µL to 200 µL of wine or 500 µL
of brandy or beer respectively.
• Red wines
Adjust 25 mL of red wine to pH 7.5 to 8.0 with the sodium hydroxide solution
in a beaker. Transfer the solution into a 50 mL volumetric flask and dilute to
the mark with water and mix. It is often necessary to decolourise red wine. This
can be done as described for fruit juices.
• Fruit juices
Centrifuge cloudy juices at approximately 2000 g. Add 5 mL of juice into a
beaker and adjust the pH to 5 to 6 with the sodium hydroxide solution. Remove
ascorbic acid by adding approximately 40 units of ascorbate oxidase in
solution to the juice and leave the sample for 10 min, or remove the ascorbic
units by stirring for 3 min with an ascorbate oxidase spatula. Then adjust the
pH to 7.5 to 8.0 with the sodium hydroxide solution. In the case of coloured
juices, add approximately 0.25 g of polyvinylpolypyrrolidone and stir the
mixture for 1 min. Transfer the mixture to a 10 mL volumetric flask and dilute
with water. Filter the solution and take 200 µL for enzymatic determination.

Solid foodstuffs

Homogenise the sample thoroughly and extract with water at 60 °C for 5 min.
Shake occasionally. Cool the sample to ambient temperature before analysing.
Vary the sample size depending on the amount of sulphite. In the case of e.g. potato
flakes, take 5.0 g of homogenised material into a 50 mL volumetric flask. Add
40 mL of water. Close the flask and extract in a water bath at 60 °C for 5 min. Shake
occasionally. Cool the volumetric flask, either by letting it stand for at least 15 min,
to ambient temperature, or in a water bath of 20 °C, and dilute to the mark with
water (V = 50 mL). If necessary centifruge the solution.
3
The following sample quantities of some other foods are suggested: