Page 167 - Analytical method for food addtives

112 Analytical methods for food additives

preservation of the sample and to prevent condensation of moisture on the inside
surface of the container.

Preparation of test sample: dried milk products
Transfer the test sample into an airtight sample container of capacity about twice
the volume of the test sample. Close the container immediately. Mix the test sample
thoroughly by repeatedly shaking and inverting the container.

Preparation of test sample: liquid milk products
When testing a sample from non-skimmed milk products adjust the temperature of
the test sample to between 35 and 40 ºC in the water bath. Mix the test sample
thoroughly, but gently, by repeatedly inverting the sample bottle without causing
frothing or churning, and cool quickly to approximately 20 ºC.
Preparation of the reduction column
Weigh about 10 g cadmium granules in a glass beaker. Add about 25 mL of
hydrochloric acid and wash the cadmium granules by swirling the glass beaker for
several minutes. Decant the hydrochloric acid and subsequently wash the cad-
mium granules with water until the water is pH neutral. Check the pH with
indicator paper. Transfer about 50 mL copper sulphate solution to the washed
cadmium granules in the glass beaker. Allow the obtained mixture to stand for 5
min while regularly swirling the beaker. Decant the copper sulphate solution and
wash the cadmium granules 10 times with water. Keep the cadmium granules
immersed in water.
Connect the glass funnels with the tube, fold in a U-form and fill almost
completely with water. By means of a spatula transfer small portions of the
cadmium granules through the funnel into the tube. Tap the tube to pack the
column. After the column has been filled almost completely, transfer a glass wool
plug with a length of about 1 cm into both ends of the tube. Before use, run the
nitrate solution with the highest nitrate concentration through the column for 1 h.

Checking the reducing capacity of the column
Using the analyser measure subsequently the absorbance of two cups with the
–
sodium nitrate standard solution of 1.2 mg NO per mL, two cups with water and
3
–
two cups with a sodium nitrate standard solution of 0.89 mg NO per mL.
3

average absorption of the nitrate standard solution
The rate –––––––––––––––––––––––––––––––––––––––– must be 0.95
average absorption of the nitrite standard solution

When the ratio is lower than 0.95, a new column should be prepared.
Preparation of test portion
• Cheese: Weigh, to the nearest 0.005 g, 2.500 g of test sample in a glass beaker.
Add 50 mL of ammonium and sodium solution III at about 50 ºC and suspend