Hach-Lange LZV936 - DR 6000 Brewery Analysis software - Handbuch (Seite 42)

English 42
Procedure for iso-
 and  acids
1. Prepare the blank value and the samples according to the work
procedure.
2. Select STORED PROGRAMS > SELECT BY NUMBER.
3. Enter the number 2013 for the program "Iso-
 and  acids".
Confirm with OK.
4. Press START to start the program.
5. Enter the blank value (see sample preparation) and press ZERO.
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. The prepared blank value can be used for the complete
measuring series.
Vicinal diketones (diacetyl, 2,3-pentanedione)
During yeast metabolism, fermentation causes 2-acetolactate and 2-
acetohydroxy butyrate to emerge. Oxidation causes a conversion into
the vicinal diketones diacetyl (2,3 butanedione) and 2,3-pentanedione.
Diacetyl can however also occur as a characteristic metabolic product of
certain micro-organisms. As soon as the threshold value is exceeded,
the beer obtains a bad aroma.
The photometric determination method in operation control is frequently
preferred over the gas chromatographic methods, as it can be
performed quickly and without great equipment expense. It does not
however permit the preferable differentiation between diacetyl and
pentanedione.
Principle
The basis of the method is the reaction between diacetyl and/or 2,3-
pentanedione and 1,2-phenylenediamine under formation of 2,3-
dimethylquinoxaline, which exhibits a specific absorption at 335 nm.
Application area
Beer
Measurement range
0–1 mg/kg
Accessories
• Appliance for nitrogen determination with heating jacket, macro
execution (for example from Schott). The supplied cooler must
possibly be replaced by a larger one if the distillate is not sufficiently
cooled. Other similar appliances, for example from Büchi, are also
suitable.
• Spectrophotometer, 335 nm
• 20 mm rectangular cuvette QS
Reagents
• Hydrochloric acid, 4N
• 1,2-phenylenediamine, 1 % in 4N hydrochloric acid (prepare
solution fresh on the day of application and store in the dark). 1,2-
phenylenediamine is toxic and an allergen; it must be handled
carefully, work with gloves
• Anti-foam emulsion (free from diketones)
Sample preparation
1. Add 100 g non-decarbonated beer into a pre-heated distillation
appliance.
2. Add a drop of anti-foam emulsion.
3. Control the steam supply so that within 2 min approximately 25 mL
distillate transfers.
4. Catch the distillate in a 25 mL volumetric flask.
5. Pipette 10 mL of the mixed distillate into two 50 mL Erlenmeyer
flasks (main value, blank value).
6. Blank value
• Add 2.5 mL 4N hydrochloric acid.