Tracking the Three "Bad" Flavors
Static Headspace Analysis of Beer Volatiles
Gas chromatogram of the components of interest.
by Wynand Diedericks,
W. J. Diedericks S.A. Breweries Ltd., Gauteng (South Africa)
There's more to brewing beer than meets the eye or the
tastebuds, for that matter. The latter are of the brewer's
greatest concern, because volatiles produced in the brewing
process can cause undesirable flavors to varying degrees.
Eliminating or minimizing these flavors is of paramount
importance in efforts to assure a consistently satisfying
experience for the consumer. In the central laboratory of our
brewery, we analyze as many as 4000 samples a year in order to
maintain brand integrity in an environment of multiple brewing
plants.
The headspace technique is particularly appealing for the
analysis of beer volatiles, because sample preparation is minimal
and highresolution gas chromatography makes quantitation
practical.
Pinpointing Disagreeable Flavors
In a recent research project we conducted, we concentrated on three
main categories of undesirable flavors and the volatiles that cause them:
Fourpoint calibration curve on ethyl acetate.
Estery Flavors
- npropanol
- isobutanol
- 2methyl butanol
- 3methyl butanol
"Adhesive" Flavors
- ethyl acetate
Banana Flavor
- isoamyl acetate
The volatile esters and alcohols in the beer were quantitatively
analyzed using an automated static headspace/GC technique. Quantitative
information on the compounds of interest was obtained by comparing their
integrated height to that of an internal standard.
Settings for the headspace autosampler are shown in the tables.
As a general rule, the temperature for the loop and transfer line
should be set 5°C higher than the oven temperature of the
autosampler to avoid condensation.
Preparing Samples and Standards
All samples and standards were made up at a temperature of +/ 4°C.
Ten milliliters of a standard were accurately decanted into a 20mL
sample vial and immediately capped with a septum and crimp cap.
The samples were kept at a temperature of +/ 4°C until just before the
run is started.
The standards are mixtures of the following compounds:
| npropanol |
| ethyl acetate |
isobutanol |
| 3methyl butanol |
2methyl butanol |
| isoamyl acetate |
Ten milliliters of sample at +/ 4°C are transferred to a 20mL headspace vial.
Twentyfive microliters of internal standard (1.6% v/v nbutanol in 99.95%
v/v ethanol) were introduced to the sample and immediately capped
with a septum and crimp cap, with the teflon film facing the
sample to eliminate contamination.
Method Validation
The sample, a typical beer packaged ready for
sale, was injected for a total of 16 runs. Incubation time was 30
minutes at 50°C. The amounts of the components were calculated in
ppm. No runs were excluded from the calculations; every run was
taken into account.
| Component | Mean |
% RSD | Spread |
| nPropanol | 11.8 | 1.61 | 9.7 |
| Ethyl acetate | 18.1 | 6.91 | 4.0 |
| Isobutanol | 9.6 | 0.94 | 0.3 |
| 3Methyl butanol | 44.2 | 1.36 | 1.4 |
| 2Methyl butanol | 19.5 | 1.38 | 1.0 |
| Isoamyl acetate | 1.7 | 8.24 | 0.4 |
Fourpoint calibration curve on n-propanol.
Another batch of sample was spiked with the first calibration
standard to calculate the amount of recovery. Again, incubation
time was 30 minutes at 50°C, and the component amounts were
calculated in ppm. Eleven runs were made. All data was used for
the calculations.
| Component | Mean | Expected | Recovery
% | Difference |
|---|
| n-Propanol | 19.4 | 19.6 | 99.0 | -1.0 |
| Ethyl acetate | 29.8 | 29.3 | 101.7 | 1.7 |
| Isobutanol | 15.1 | 15.6 | 96.8 | -3.2 |
| 3-Methyl butanol | 67.7 | 67.8 | 99.9 | -0.1 |
| 2-Methyl butanol | 31.2 | 31.4 | 99.4 | -0.6 |
| Isoamyl acetate | 3.2 | 3.2 | 100.0 | 0.0 |
We have been using this technique as an important part of our
quality control procedure. It has evolved into an effective
analytical tool we use routinely to ensure consistent flavor and product
integrity of our beer.
| GC Conditions |
|---|
| Column: | Chrompak XL5; 30 m x 0.32 mm x 1.2 µm; film Exell capillary column |
| Carrier: | N2; linear velocity 37.0 cm/s, 2.5 mL/min |
| Inlet Temperature: | 200°C |
| Oven Parameters: | Equilibration 1.00 min;
max. temperature 300°C;
ambient temperature 15°C; oven ON; cryo ON; cryo blast OFF |
| Oven Program: | Initial temperature 12°C for 2.0 min; 7.00°C/min to 150°C;
15.00°C/min to 200°C for 2.00 min |
| Next Run Time: | 27.05 min |
| Headspace Conditions |
|---|
| Temperatures |
| Oven: | 50°C |
| Transfer Line: | 65°C
| | Loop: | 65°C
|
|
| Event Times |
| GC Cycle: | 40.0 min |
| Injection (splitless): | 0.50 min |
| Loop Equilibration: | 0.05 min |
| Loop Fill: | 0.15 min |
| Pressurization: | 0.13 min |
| Vial Equilibration: | 30.0 min |
|
| Pressure |
| Carrier: | 103 Kpa |
| Vial: | 130 Kpa |
For information on the latestmodel
GC and headspace autosampler, click here.
The Equipment Used
HP 7694 Headspace Autosampler
HP 5890 Series II Gas Chromatograph with EPC, FID, and cryogenic cooling
HP GC ChemStation
|
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