Hach-Lange DR 5000 brewerie analysis, supplementary software LZV570 - Manual (Page 42)

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Working procedures
2.11 Vicinal diketones (diacetyl, 2,3-pentanedione)
The metabolic processes of yeast produce 2-acetolactate and
2-acetohydroxibutyrate during fermentation. These are oxidised to
the vicinal diketones diacetyl (2,3-butanedione) and
2,3-pentanedione. Diacetyl can, however, also occur as a
characteristic metabolic product of certain microorganisms. When
the threshold value is exceeded, the beer acquires an off-flavour.
The photometric determination method is often used in preference
to the gas chromatographic method in operational checks, because
it can be carried out quickly and without the need for expensive
apparatus. Unfortunately it is not capable of differentiating between
diacetyl and pentanedione.
Principle
The basis of the method is the reaction of diacetyl or
2,3-pentanedione with 1,2-phenylenediamine to form
2,3-dimethylquinoxaline, which exhibits specific absorbance at
335 nm.
Fields of application
Beer
Measuring range
0–1 mg/kg
Accessories
• Macro version of apparatus for nitrogen determination, with
heating jacket (for example from Schott). The accompanying
cooler may need to be replaced by a larger one if the distillate
is not cooled sufficiently. Other, similar units (for example from
Büchi) are equally suitable.
• Spectrophotometer (335 nm)
• 20 mm rectangular cuvettes (QS grade)
Reagents
• Hydrochloric acid (4N)
• 1,2-Phenylenediamine (1% in 4N hydrochloric acid). Prepare
the solution freshly on the day when it is needed, and keep it is
a dark place. 1,2-Phenylenediamine is toxic and allergenic;
handle it carefully, and wear gloves while working.
• Antifoam emulsion (free of diketones
Sample preparation
1. Introduce 100 g beer, from which the carbon dioxide has not
been removed, into a preheated distillation apparatus.
2. Add one drop of antifoam emulsion.
3. Regulate the steam supply so that about 25 ml distillate are
obtained in 2 min.
4. Collect the distillate in a 25 ml measuring flask.