3. FOOD ChEMISTRy & bIOTEChNOLOGy 3.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Food Chemistry & Biotechnology
P09 MONITORING OF SENSORIALLy ACTIVE
SuLPhuR SubSTANCES IN MALT AND bEER
REnATA MIKULíKOVá, SYLVIE BěLáKOVá,
ZDEněK SVOBODA and SIMOnA MACUCHOVá
Research Institute of Brewing and Malting, Plc., Malting
Institute Brno, Mostecká 7, 614 00 Brno,
mikulikova@brno.beerresearch.cz
Introduction
Recently, a great deal of research effort has been devoted
to sensorially active substances affecting beer quality. Quality
of brewing materials, unhopped wort and hopped wort
production technology, fermentation technology, beer maturation
and beer ageing contribute significantly to analytical
and sensorial beer characteristics.
Heterocyclic and sulphur compounds, some of them with
high sensorial activity even in extremely low concentrations,
belong to sensorially active substances affecting beer quality
principally 1 . Trace amounts of these compounds commonly
detectable in food contribute to their flavor, therefore this effect
can be generally assessed as favourable. In malt and beer,
however, it is true to a limited extend only and the presence
of heterocyclic and sulphur substances is evaluated rather
negatively 2 . Sulphur compounds get into beer either with initial
materials (malt, hop) or they are formed in the course
of chemical or enzymatical reaction during the respective
production phases (mashing, brewing, fermentation, ageing).
Content of sulphur compounds in barley and hop depends not
only on a variety but on a growing locality, course of weather
and growing technology employed as well. In malt, content
of sulphur substances depends first of all on malting technology
and possible contamination with undesirable microorganisms
3 . Most of the sulphur compounds present in barley,
malt and beer are non-volatile substances (amino acids, proteins,
inorganic sulphates). These substances do not directly
account for unfavourable beer flavours and odours but under
certain conditions they may be important precursors of sensorially
active substances. These substances are, in majority of
cases, volatile and their amount is usually less than 1 % of the
total amount of the sulphur containing substances in beer, i.e.
actual amounts of substances responsible for sulphur odours
are extremely low 4 .
Experimental
S e n s o r i c a l l y A c t i v e S u l p h u r
S u b s t a n c e
Following volatile sulphur substances were monitored:
dimethylsulphide [75-18-3], dimethyl disulphide [624-92-0],
dimethyltrisulphide [3658-80-8], carbondisulphide [75-15-0],
ethylsulphide [75-08-1], diethyldisulphide [111-81-6], methionol
[505-10-2], 3-methylthiophen [616-44-4], ethylthioacetate
[625-60-5], 2-methyl-1-buthanthiol [1878-18-8].
s592
S e l e c t i o n o f t h e A n a l y z e d S a m p l e s
Sensorially active sulphur substances were determined
in beers bought in retail stores. Six pale dispensed beers, 4
dark, 6 lager and 3 non alcoholic beers were selected for the
analyses.
The analyzed malt samples were produced from different
barley varieties (Bojos, Jersay, Malz, Prestige, Tolar,
Xanadu), from 3 growing stations (Branišovice, Věrovany,
Vysoká).
D e t e r m i n a t i o n o f V o l a t i l e S u l p h u r
S u b s t a n c e s
Direct analysis of sulphur sensorially active substances
is not easily applicable regarding their very low concentrations
(µg kg –1 ,dm –3 – ng kg –1 ,dm –3 ) in the analyzed matrixes
(malt, beer) 5,6 . Prior to the analysis, the analytes must be
extracted from the matrix and concentrated. The HS-SPME
method was used for extraction and concentration 7,8 . The
CAR/PDMS fibre was chosen as the optimum fibre for this
technique. The gas chromatographic analysis with flame photometric
detection was employed to determinate sensorially
active sulphur substances.
HS-SPME/ GC/ FPD conditions:
Sample: 2 ml of beer (vial 4 ml)
6 ml of malt extract (vial 50 ml)
Sample temperature: 45 °C, 10 min
SPME fiber: 85 μm CAR/PDMS
Extraction: HS-SPME, 30 min, 45 °C
Desorption: 3 min, 250 °C
Chromatographic conditions:
Capillary column: GS-Gaspro (30 m × 0.32 mm)
Thermal program: from 40 °C to 235 °C
Flow of carrying gas He: 1.5 ml min –1
PTV injector: 250 °C, splitless (3 min)
FPD detector: T detector – 150 °C,
T base – 250 °C
Results and Discussion
Two types of fibres (PEG, CAR/PDMS) were tested for
the HS-SPME analysis. Fig. 1 shows higher affinity to the
CAR/PDMS fiber for the analytes studied. This fibre was chosen
for the analysis of sulphur substances, sorption time was
optimized at 45 °C. Optimum sorption time was 30 minutes.
Validation parameters for the relevant analytes are given
in Table I.
Carbon disulphide, methionol, dimethylsulphide,
3-methylthiophen and diethyldisulphide were detected in
beers. Table II shows results of beer sample analyses.
In malt samples only dimethylsulphide was determined.
Results of malt analyses are given in Fig. 2.