Hach-Lange DR 5000 brewerie analysis, supplementary software LZV570 - Manual (Page 15)

15
Working procedures
2.3 Reducing power (spectrophotometric method)
Reducing power is a measure of the rapidly reducing substances
present in beer. Reductones are found in relatively small amounts
in beer, but are of considerable significance for the chemicophysical
and biological stability of beer, as well as the long-term constancy
of its taste.
Principle
Reductones reduce a certain amount of Tillmann's reagent
(2,6-dichlorphenol-indophenol sodium, DPI) within a given period of
time. The decolouration of the reagent is measured with a
spectrophotometer and calculated.
Measuring range
0–100
Accessories
• Spectrophotometer (520 nm)
• 10 mm rectangular cuvette (OS grade)
• Stopwatch
• Water-jet pump
Reagents
• 2,6-Dichlorphenol-indophenol (0.005M) (DPI solution,
molecular weight of the sodium salt 290.08):
a. Weigh approx. 100 mg DPI into a beaker, add approx.
25 ml H
2
O, and dissolve by heating to about 60 °C.
b. Allow to cool, then rinse into a 50 ml measuring flask, make
up to 50 ml and pass through a tinstrip filter.
c. Introduce 10 ml filtrate, 1 g KI and 2 ml H
2
SO
4
(1+6) into a
150 ml Erlenmeyer flask, titrate with 0.01N sodium
thiosulphate solution until a colour change occurs against
starch paste.
d. Added volume (ml) x 14.5 = mg indicator in 100 ml.
e. Dilute remaining filtrate so that 100 ml contain 145 mg.
f. The solution remains stable for about 1 week if kept at
+4 °C in brown bottles filled to the brim.
• Phosphate-citrate buffer (pH 4.35):
• Dissolve 31.60 g disodium hydrogen phosphate
(Na
2
HPO
4
x 12 H
2
O) and 11.75 g citric acid (C
6
H
8
O
7
x H
2
O) in
H
2
O and dilute to 1 l.
Sample preparation
1. Heat the beer to 20 °C and expel carbon dioxide under a
vacuum (water-jet pump).