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A key ingredient in beer is hops. In the flavors of many beers, they provide a vital balance for malt. They also help to precipitate proteins etc. during boiling. Hops also have preservative properties, which help keep the beer fresh and free from bacteria.

Hops affect the taste of beer in three ways:

There are many kinds of hops, and a variety of flavors are available. Since the flavor will decrease over time, hops must be stored carefully and used when they are fresh. Therefore, the quality of hops needs to be characterized so that the brewer can develop and deliver the desired product. 

There are many compounds in hops that can affect the flavor, so the aroma characterization of hops is very complicated. The components of typical hops are listed in Table 1, and Table 2 lists some key aroma compounds.

Table 1. Composition of typical hops. Source: PerkinElmer Food Safety and Quality

Table 2. Main hop aroma compounds. Source: PerkinElmer Food Safety and Quality

The traditional method of evaluating the quality of hops is to let an experienced brewer crush some hops with his fingers, and then smell the aroma released to evaluate the hops from the senses. This is valid but not objective, and lacks the quantitative information needed to make the right decision on how to use hops. 

This study outlines a system that can perform objective chemical analysis of hop aromas by using gas chromatography/mass spectrometry, while also providing users with a method to monitor the olfactory sensation of each component eluted from the chromatographic column feature. 

Methods like this enable the user to obtain a more comprehensive characterization of a particular jumping sample. 

The analysis system consists of five main components. 

Static headspace (HS) sampling is very suitable for extracting aroma compounds from hops. As shown in Figure 1, put the weighed hops (particles or leaves) into a glass vial and seal it.

Figure 1. Hops waiting for analysis in the headspace sample bottle. Image source: PerkinElmer Food Safety and Quality

Next, the vial is heated in an oven at a set fixed temperature for a set fixed period of time. The headspace sampling system extracts some vapor from the vial and introduces it into the GC column for separation and analysis. 

This is very convenient, but static headspace injection only provides a portion of the headspace vapor to the GC column, so it is indeed best for high-concentration compounds. 

It is often found that in the analysis of complex samples, the low content of certain components is critical to the overall aroma of the sample. 

The headspace trap system is used to increase the amount of sample introduced into the GC column. Using this technology, most or even the entire headspace vapor passes through the adsorption trap to collect and concentrate VOC. The trap is then rapidly heated, and the desorbed components are transferred to the GC column. 

Using this method, the amount of sample vapor entering the GC column can be increased by up to 100 times. It is very suitable for hop aroma analysis. 

Figures 2 to 4 are simplified representations of the operation of the HS trap-other valves and piping are also needed to ensure that the sample vapor reaches where it should be.

Figure 2. Schematic diagram of the HS trap system, showing the balance vial being pressurized with carrier gas. Image source: PerkinElmer Food Safety and Quality

Figure 3. Schematic diagram of the H2S trap system showing the release of pressurized headspace from the vial into the adsorption trap. Image source: PerkinElmer Food Safety and Quality

Figure 4. Schematic diagram of the HS trap system, showing that the VOC collected in the adsorption trap is thermally desorbed and introduced into the GC column. Image source: PerkinElmer Food Safety and Quality

The principle is very similar to the classic static headspace in essence, but after the vapor pressurization, at the end of the vial equilibration step, it is completely emptied through the adsorption trap. 

In order to effectively exhaust the entire headspace vapor through the adsorption trap, the process can be repeated. Once the trap is loaded, it is quickly heated and the desorbed VOC is transferred to the GC column. 

The workhorse Clarus® 680 GC is an ideal complement to the rest of the system. Since chromatography is not demanding, simple techniques can be used. It is important to have sufficient time between adjacent peaks for olfactory monitoring so that the user can distinguish them from each other. 

Loading as many samples as possible into the chromatographic column without overloading also helps to provide the user's nose with the best opportunity to detect them. For this reason, a long column with a thick stationary phase is used.

 Use a very polar Carbowax® type stationary phase for separation, because many components (ketones, acids, esters, etc.) in hops are very polar. 

Since the column effluent needs to supply the MS and the olfactory port, some form of splitter is required. This should not affect the integrity of the chromatogram in any way. Therefore, it should be highly inert and have a low-volume internal geometry. 

Use make-up gas in the splitter to further stabilize and control the split flow rate. S-SwaferTM is an excellent active spectroscopic device that is very suitable for this purpose. 

The S-Swafer is configured to split the column effluent between the MS detector and the SNFR olfactory port, as shown in Figure 6. The split ratio between the detector and the olfactory port defines the MS and SNFR by selecting the restrictor tube connected between the swap outlet and the olfactory port.

Figure 5. Clarus 680 SQ 8 GC/MS system. Image source: PerkinElmer Food Safety and Quality

Figure 6. S-Swafer configured for use with Clarus SQ 8 GC/MS and SNFR. Image source: PerkinElmer Food Safety and Quality

The Swafer utility software attached to the Swafer system can be used to calculate this split ratio. Figure 7 shows how to use this calculator to determine the working conditions of the S-Swafer for this application.

Figure 7. The Swafer utility software shows the settings used for this hop aroma characterization task. Image source: PerkinElmer Food Safety and Quality

The mass spectrometer is a key part of the aroma characterization system. It is important not only to detect and describe the aroma of the various components eluting from the GC column, but also to determine what these components are and how much they may be contained in the hops. 

For this reason, the Clarus SQ 8 quadrupole mass spectrometer is an ideal choice. It will quickly identify and quantify components using the classical spectra in the provided NIST library. The software can also interact with the olfactory information described later in this research. 

The image of the SNFR attachment is shown in Figure 8. It is connected to the GC through a flexible heating transfer line. The split column effluent flows through the deactivated fused silica tube to the glass nose clamp.

Figure 8. SNFR olfactory port accessory. Image source: PerkinElmer Food Safety and Quality

The user can capture the voice narration through the built-in microphone, and monitor the aroma intensity of the aroma compounds eluted from the GC column by adjusting the joystick.

Table 3. HS trap conditions. Source: PerkinElmer Food Safety and Quality

Table 4. GC conditions. Source: PerkinElmer Food Safety and Quality

Table 5. MS conditions. Source: PerkinElmer Food Safety and Quality

Table 6. Olfactory port conditions. Source: PerkinElmer Food Safety and Quality

Table 7. Wafer conditions. Source: PerkinElmer Food Safety and Quality

Table 8. Example details. Source: PerkinElmer Food Safety and Quality

Figure 9 depicts the total ion chromatogram (TIC) of four typical hops from different countries. A part of Hallertau in Germany is highlighted and expanded in Figure 10.

Figure 9. Typical TIC chromatogram of a four-hop sample. Image source: PerkinElmer Food Safety and Quality

Figure 10. Details highlighted in Figure 9. Image source: PerkinElmer Food Safety and Quality

As shown in Figure 11, the powerful features of MS allow specific peaks to be identified from their mass spectra by searching the NIST library included with the Clarus SQ 8 system.

Figure 11. The mass spectrum of the peak highlighted in Figure 10. Image source: PerkinElmer Food Safety and Quality

Figure 12 shows the results of this search. They strongly indicate that the peak eluting at 36.72 minutes is 3,7-dimethyl-1,6-octadien-3-ol, also known as linalool.

Figure 12. The mass library search results shown in Figure 11. Image source: PerkinElmer Food Safety and Quality

Linalool is an important aroma compound that can provide a delicate floral fragrance to beer. By calibrating the GC/MS with a standard mixture of this compound, the amount of linalool (or any other identified compound) can be quantified. 

The distribution map of hop characteristics can be established by further identifying the chromatographic peaks. Figure 13 shows more peaks identified in the Hallertau chromatogram of Germany shown in Figure 9 earlier.

Figure 13. Typical TIC chromatogram of a four-hop sample. Image source: PerkinElmer Food Safety and Quality

The annotated peaks are mainly fatty acids, indicating the degree of oxidation of hops in this particular sample. The rich myrcene peak is smaller than expected. 

These observations indicate that this sample is quite old (this is true-this is an old sample that is improperly stored). The chromatograms of four additional hop samples are shown in Figure 14.

Figure 14. The TIC chromatogram of a further four-hop sample. Image source: PerkinElmer Food Safety and Quality

Figure 15 shows an example of a skip chromatogram, where audio narration and intensity recording are graphically superimposed. The audio narration is stored in a standard WAV file format and can be played back to the operator from this screen at any point in the displayed chromatogram with a simple mouse click.

Figure 15. An example of a hop chromatogram viewed in TurboMass™ software, with audio narration and aroma intensity superimposed graphically. Image source: PerkinElmer Food Safety and Quality

Narration WAV files can also be played from most media applications, including Microsoft® Media Player, which is included in the Windows® operating system. When recording, audio data can be transcribed into text. 

This function is performed by the Nuance® Dragon® Naturally speak software included in the SNFR product. 

A typical hop analysis report shows the narrative transcribed by the user and the aroma intensity recorded by the joystick, as shown in Table 9. The format of the report is a comma-separated value (CSV) file, suitable for direct import into Microsoft® Excel® or other application software.

Table 9. A typical output report shows the text transcribed from the audio narration and the corresponding aroma intensity data. Source: PerkinElmer Food Safety and Quality

The addition of an olfactory port to the HS GC/MS system expands its application in the aroma characterization of samples such as hops. The ability to directly correlate sensory perceptions with hard-analyzed data provides insights that would otherwise be difficult to obtain. 

Winemakers and researchers involved in the following tasks should be interested in this system:

This information is derived from materials provided by PerkinElmer Food Safety and Quality and has been reviewed and adapted.

For more information on this source, please visit PerkinElmer Food Safety and Quality.

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PerkinElmer food safety and quality. 2021. Use the olfactory port of the HS GC/MS system to characterize the aroma of hop samples. AZoM, viewed on December 6, 2021, https://www.azom.com/article.aspx?ArticleID=20631.

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