Hach-Lange LZV936 - DR 6000 Brewery Analysis software - Handbuch (Seite 38)

English 38
6. Insert the analysis cuvette with the prepared sample and press
READ.
The result is shown on the display.
Note: In order to analyze further samples, repeat the procedure
from step 6.
Photometric iodine sample
Photometric iodine sample according to the new method (MEBAK from
1993)
Principle
High-molecular dextrines and starches are precipitated through addition
of ethanol to wort and beer, centrifuged out, dissolved in phosphate
buffer and displaced with iodine solution. Depending on molecular
weight and branching factor of the erythrodextrine and starch, a red to
blue color forms, whose intensity is measured photometrically.
Application area
Wort, beer (samples with an iodine value > 0.8 must be diluted.)
0–1 iodine value
Accessories
• Centrifuge
• Centrifuge flasks with ground-in stoppers, 100–110 mL capacity
• Shaking machine
• Pipettes, 0.5 mL, 2 mL, 10 mL, 20 mL, 40 mL
• Spectrophotometer, 578 nm
• 40 mm rectangular cuvette OS
• "Plumper" or plastic spatula
• Reagents
• Ethanol, 95 %
• Iodine solution, 1N (parent solution)
• Iodine solution, 0.02N (prepare fresh daily from the parent solution)
• Phosphate buffer solution, 0.1M, pH 3.5: adjust 0.1M KH
2
PO
4
solution with 0.1M phosphoric acid to pH 3.5
Sample preparation
1. Pipette 10.0 mL centrifuged wort or decarbonated beer into a
centrifuge flask.
2. Add 40.0 mL ethanol and shake mechanically for 10 min.
3. Centrifuge for 5 min at 2500 rpm.
4. Decant the clear phase as carefully and completely as possible.
5. Loosen the residue with 20.0 mL phosphate buffer solution by
mechanically shaking for 10 min.
6. Centrifuge the solution for 5 min at 2500 rpm.
7. Pipette 2 mL of the excess and 8 mL phosphate buffer solution into
a 40 mm rectangular cuvette and measure at 578 nm against
phosphate buffer solution.
8. Add 0.5 mL 0.02N iodine solution, immediately mix the contents
with the "spatula", measure after 30 sec.
9. Iodine blank value
a. Pipette 10 mL phosphate buffer solution and0.5 mL 0.02N
iodine solution into a 40 mm rectangular cuvette and mix.
b. Measure the extinction at 578 nm against phosphate buffer
solution.
Results specifications
Extinction to 2 decimals
Accuracy
Vkr = ± 3 %
Standard values
< 0.3 (wort)
Literature
MEBAK brew-technical analysis methods, 4th edition 2002, Volume II, P.
34 et seq