Page 157 - Analytical method for food addtives

102 Analytical methods for food additives

dissolve any lumps formed with a homogeniser.
(Note: In order to become familiar with the various shades of colour in adjusting
the pH while learning the method, titrate a few sample extracts using the indicator
and then measure the pH. For uncooked meat products the pH should not exceed
8.5 since otherwise it may not be possible to clarify the solutions by filtering after
adding the Carrez reagent. In the case of simmered or boiled meat products, the
danger of this happening is much less and the pH may be allowed to rise to about
9.5. If a pH meter is used, adjust the pH value to between 8.0 and 8.5.)
In the case of boiled sausages, the (instantaneous) colour change to bluish-
green when adjusting the pH is readily detected. On the other hand, this change is
usually sluggish in the case of extracts of uncooked sausages and the shade of
colour is greyish-green. In the case of boiled sausages containing blood, a similar
shade of colour is usually observed. Even in the case of extracts having a strong
natural colour, the colour change can be recognised very readily with a little
practice despite the fact that the shade of colour is frequently not the theoretically
expected one because the colour of the extract is superimposed on that of the
indicator.
In the case of extracts of uncooked sausages, a reversal of the colour change is
sometimes observed after a certain time. In such cases, readjust the pH by adding
a little sodium hydroxide solution.
Cool to room temperature and transfer the contents of the flask quantitatively to
a 200 mL volumetric flask and add 4 mL each of Carrez solutions No. 1 and No. 2
shaking after each addition. Then dilute to the mark with water, mix thoroughly
and filter through a fluted filter paper, discarding the first 20 mL of the filtrate. The
clear residual filtrate is used for the determination (sample solution).

Calibration graph for the nitrite content
Mix 2.0 mL of water for the blank and 2.0 mL of each of the standard sodium
nitrite solutions with 1.0 mL of water and 3.0 mL of the colour reagent mixture in
a test tube, shake and store the solution in the dark at room temperature. After 30
min, measure the absorbance values of each solution at wavelength of 540 nm in
a spectrometer against water. Plot the absorbance values obtained for the four
sodium nitrite solutions against the corresponding absolute amounts of nitrite ions
(in 200 mL solution), respectively, in milligrams of nitrite ion.

Calibration graph for the nitrate content
Introduce 0.2 mL of the NADPH solution, 2.0 mL of water for the blank or 2.0 mL
of each of the standard potassium nitrate solutions respectively and 0.8 mL of
nitrate reductase buffer solution into a test tube, mix thoroughly and allow to stand
for 1 h at room temperature. Then add 3.0 mL of the colour reagent mixture, shake
and store the solution in the dark at room temperature. After 30 min, measure the
absorbance values of each solution at a wavelength of 540 nm in a spectrometer
against water. Plot the absorbance values obtained for the four potassium nitrate
standard solutions against the corresponding absolute amounts of total nitrate/