26 May / June 2016
the five target components: acetaldehyde, dimethyl sulphide, 2,3-butanedione, 2,3-pentandione, and t-2-nonenal. The detection limit goal was 40 parts per billion (ppb) for acetaldehyde and 5.0 ppb for the remaining targets, which was made at the request of several breweries. The standards were analysed in Simultaneous Full Scan (FS) and Single Ion Monitoring (SIM) acquisition modes. An example of the SIM chromatogram of Diacetyl at the 5.0 ppb concentration is displayed in Figure 1.
The calibration data for all five compounds are shown in Table 4. Considering the extremely low levels in a highly complex matrix, it can be seen that these data demonstrate acceptable linearity. In addition, it should be pointed out that these results were acquired using a small turbomolecular pump; it can be assumed enhancements can be achieved using the recommended larger turbomolecular pump. The signal to noise for Dimethyl Sulphide at the 5 ppb concentration was 80,000 to 1, and the 1000 ppb concentration saturated; therefore, the results from full scan acquisition are used for this target.
Characterisation of Beer
One of the benefits of mass spectrometry is that it enables the identification of a large suite of volatile flavour compounds in beer, without having to change or use another detector. Figure 2 is an example of such a characterisation that was carried out on American pale ale, while Figure 3 shows a comparison of the flavour compound responses found in an alternative beer product. Note: For comparison purposes, three different batches of the pale ale are shown (red, yellow, light blue) with the other beer product (grey).
Monitoring the Fermentation Process
This approach also provides the ability to obtain analytical data during the entire fermentation process.
For this part of the investigation, an experimental batch of American pale ale was brewed and fermentation initiated. A sample was analysed every eight hours for the eight days of the fermentation process. Specific gravity is typically used as an indicator of the fermentation progress and is shown for this beer in Figure 5. It can be clearly seen that the final gravity of this beer was 1.012, which was achieved in about 100 hours.
Figure 3: Comparison of chromatographic responses for a suite of volatile flavour compounds between two brands of beer (data courtesy of the Long Trail Brewing Company, Vermont). Note: The competitive beer sample is shown in grey
To get a better understanding of the characterisation of the favour compounds during the fermentation process, a comparison was made of the flavour profiles of the same beer from five different fermentation trials. This comparison is exemplified in Figure 4.
The concentrations of key components in the beer were also monitored during the fermentation process. The profiles of two key flavour ‘defects’, diacetyl and dimethyl sulphide, are shown in Figures 6 and 7, respectively. It can be seen that the concentration of the diacetyl was reduced to a negligible amount in about 80 hours
while the dimethyl sulphide only took about 30 hours to get down to trace levels. This is an analytical approach of monitoring when the brewing process has been completed. Using an analytical technique to determine when the brew is ready can provide enhanced productivity of up to 40%.
Figure 2: Typical chromatographic profile of volatile flavour compounds in an American pale ale
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