Page 163 - Analytical method for food addtives

108 Analytical methods for food additives

column, completely fill the reservoir with water. Run the complete content of the
reservoir through the column at maximum flow rate. Collect nearly 100 mL of the
eluate.
Remove the 100 mL volumetric flask. Dilute its contents to the 100 mL mark
with water and mix well. Pipette 10 mL of the eluate in another 100 mL volumetric
flask. Dilute with water to obtain a volume of about 60 mL and mix. Proceed as
specified under ‘Colour development and measurement’ below. Calculate the
–
percentage reducing capacity of the column (0.067 µg of NO per mL corresponds
2
to 100 % reducing capacity) from the nitrite content obtained and that determined
from the calibration graph. If the reducing capacity is less than 95 % regenerate the
column as specified.

Regeneration of the column
Regenerate the column at the end of each day of using or, if the check indicates a
loss of efficiency, more frequently. Add about 5 mL of the EDTA solution and
2 mL of hydrochloric acid working solution to 100 mL of water and mix. Run the
thus obtained solution through the column at a flow rate of about 10 mL/min.
When the reservoir is empty, wash the column successively with water, with
hydrochloric acid working solution and with water again. If the efficiency of the
column still is not satisfactory, repeat the procedure specified above.

Preparation of test portion
• Dried milk: Weigh, to the nearest 1 mg, approximately 10 g of the prepared
test sample. Transfer the test portion quantitatively to a 500 mL conical flask.
• Dried whey: Weigh, to the nearest 0.1 mg, approximately 5 g of the prepared
test sample. Transfer the test portion quantitatively to a 500 mL conical flask.
• Caseins: Weigh, to the nearest 0.01g, approximately 10 g of the prepared test
sample. Transfer the test portion quantitatively to a 500 mL conical flask.
• Caseinates: Weigh, to the nearest 0.01 g, approximately 2 g of the prepared
test sample. Transfer the test portion quantitatively to a 500 mL conical flask.
• Cheese: Weigh, to the nearest 0.1 mg, approximately 10 g of the prepared test
sample. Transfer the test portion quantitatively to the glass container of the
laboratory mixer or homogeniser.
• Whey cheese: Weigh, to the nearest 1 mg, approximately 5 g of the prepared
test sample. Transfer the test portion quantitatively to the glass container of the
laboratory mixer or homogeniser.

Extraction and deproteination: dried milk
Add progressively 136 mL of preheated water at 50–55 °C to the test portion.
Disperse the test portion by stirring with a glass rod or by shaking the conical flask.
Add in the following order, swirling thoroughly after each addition, 12 mL of zinc
sulphate solution, 12 mL of potassium hexacyanoferrate (II) solution and 40 mL of
buffer solution and mix. In order to obtain a clear filtrate, leave the mixture in the