Page 171 - Analytical method for food addtives

116 Analytical methods for food additives

Test portion
Weigh, to the nearest 0.1 mg, 2.5 g of the test sample into a centrifuge tube of
50 mL.

Extraction
Add 24 mL of extraction buffer, preheated to a temperature of 50–55 ºC, to the test
portion. Mix with the homogeniser for about 3 min until the test portion is well
suspended. Centrifuge at c. 1500 g for 5 min. Place the centrifuge tube in a mixture
of water and ice for 15 min. Use a pipette to withdraw the de-fatted suspension
from underneath the fat layer in the centrifuge tube and to fill the cups of the FIA,
for both the nitrite and nitrate determination, with the obtained suspension.

Determination of nitrite
Install the manifold according to the scheme in Fig. 1 (see BS EN ISO 14673-
3:2001). Use the pump tubings for the determination of nitrite while leaving out the
cadmium reduction column. Connect the bottles with both reagent solutions R1
and R2 and carrier solution. Start the pumps to flush the system for 5 min to 10 min.
Load the FIA-program for nitrite. Run the nitrite standard solutions to calibrate the
system, followed by the test portion suspensions. Check the calibration, both at the
end of a series and after each group of 10 samples, by analysing the nitrite
calibration standard solution of 0.10 mg nitrite ion per litre.

Calculation of the nitrite content
Calculate the nitrite content of the sample W , using the following equation:
NI

25
W = ––– × C [9.10]
NI M Ni

where:
–
W is the nitrite content of the sample, in milligrams of nitrite ion (NO ) per
NI 2
kilogram
C is the numerical value of the concentration read from the calibration graph,
Ni
corresponding to the measured absorbance of the test portion suspension, in
micrograms of nitrite ion per litre
m is the mass of the test portion, in grams

Expression of results
Express the results to 1 decimal place.

Repeatability
The absolute difference between two single test results, carried out simultaneously
or in rapid succession by the same operator under the same conditions on identical
test material, will be greater than 0.5 mg/kg in not more than 95 % of cases.