Gerstel: Intelligent Automation for GC/MS and LC.MS

New Methods for Beer Flavor and Aroma Analysis

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Craft beers have revitalized the beer industry, to the point that there are now more breweries in the United States than there were before prohibition.  Perhaps the newest trend in brewing is the development of gluten-free beers, which many craft breweries have begun making. The market for gluten–free beers is expected to grow at 40% per year through 2021, so they represent a significant growth opportunity.

There are three main approaches to gluten-free beer production.  Since barley contains gluten, many breweries are using alternative ingredients such as sorghum, buckwheat honey and fruit. Other beer makers are using enzymes to strip the gluten from barley before it is used in the brewing process. German breweries have a particular challenge in making gluten-free beer, as the Reinheitsgebot 700 year old beer purity law requires the use of wheat or barley.  German brewers are using Kebari barley from Australia that has 10,000 times lower gluten than regular barley to reduce the gluten while meeting the standards set out in the purity law.

Whether making traditional or gluten-free beer, brewers must optimize and maintain its flavor. Beer flavor is the result of the interaction of numerous volatile compounds derived from hops, grain (or other ingredients) and yeast during the brewing process.  Identifying these compounds and how they contribute to the quality of beer has proven to be challenging, and the new ingredients being used in some gluten-free beers make the challenge even greater. The sheer number of organic compounds in beer make separation difficult. Some of these compounds can have extremely low odor thresholds and high sensory impact, requiring very sensitive detection methods.

Combining state-of-the-art analytical techniques provides a powerful approach for determination of flavor compounds in complex samples such as beer. These include stir bar sorptive extraction (SBSE), thermal desorption (TD), heart-cutting two dimensional (2D) gas chromatography (GC), and simultaneous detection with mass spectrometry (MS) and olfactometry or selective detectors such as a pulsed flame photometric detector (PFPD).

One key to effective analysis of the flavor components of beer is highly efficient extraction, with a minimum of hands-on effort. Traditional extraction methods used in the brewing industry have been labor-intensive and time-consuming1.  As an alternative, SBSE has gained wide acceptance as a miniaturized, solvent-free extraction method for food and flavor analysis. SBSE is up to 1000 X more sensitive than solid-phase microextraction (SPME), because the stir bar has significantly more sorbent volume. Subsequent thermal desorption of all the extracted compounds from the stir bar enables their determination by gas chromatography (GC).

Extraction by SBSE using the GERSTEL Twister has been used by major brewing companies to identify flavor compounds in beer. Twister is a magnetic stir bar encased in glass and coated with a polydimethylsiloxane (PDMS) sorbent phase of up to 126 μL. The GERSTEL Thermal Desorption Unit (TDU) mounted on the Cooled Injection System (CIS) is then used to desorb the flavor compounds from the SBSE for subsequent determination by GC-MS. Scientists at Asahi Breweries characterizing the contribution of hops to beer flavor used this SBSE-TD-GC approach to compare terpenoid extraction with Twister to conventional extraction with methylene chloride, which identified only three compounds. Using Twister SBSE, the Asahi scientists were able to identify a large number of other terpenoids.

In addition to efficient extraction, resolution of the large number of flavor compounds is a major challenge to separation techniques. The number of known odorants in beer have increased significantly due to technological developments in gas chromatography 2. The technique known as GC-olfactometry (GC-O) combines GC analysis and compound detection with the ability of the operator to actually smell each peak as it elutes, using an Olfactory Detection Port (ODP). Professor Michael Qian at Oregon State University has used olfactometry to understand the contribution that hops make to beer flavor.

Unfortunately, in complex samples such as beer, several compounds can co-elute in one peak, making compound identification of compounds that can impact flavor difficult during GC-O analysis.  This is especially true when larger, odor-inactive peaks co-elute with trace aroma compounds. The co-elution problem can be solved using multi-dimensional GC, as Professor Qian has done with hop aromas. The resolution of complex regions and compound identification can be substantially improved using this heart-cutting 2D GC-MS approach3.   

A dedicated 2D GC-MS system is usually required for conventional heart-cutting, and thus it is not available for routine 1D analysis. The first dimension separation is often monitored using a detector such as a flame ionization detector (FID), which provides only signal intensity without mass spectral information to help guide heart-cutting.  

To address these shortcomings of a dedicated 2D GC-MS system, GERSTEL scientists have developed a novel selectable one-dimensional (1D) or two-dimensional (2D) gas chromatography–mass spectrometry (selectable 1D/2D GC–MS) system with selective detection, using low thermal mass GC (LTM-GC) and Agilent Technologies capillary flow technology3. Selection of 1D GC–MS or 2D GC–MS operation is simple and fast and does not require any instrumental setup change. In order to assure selection of a heart-cut region and correct identification of compounds of interest, this system also provides simultaneous mass spectrometric and olfactometry or element-specific detection for both 1D and 2D separations

This GERSTEL selectable 1D/2D GC–MS approach, including the Olfactory Detection Port, has been used by Scientists at Sapporo Breweries to identify the distinctive aroma profile of a fruity-flavored beer (FFB)2. They identified 53 aroma compounds from the FFB that were associated with fruity, sweet or floral flavors. Other beer types were then analyzed and compared to the FFB to identify which of the 53 aroma compounds were associated specifically with the fruity flavored beer. These were then added to control beer to determine if the fruity flavor could be reconstituted, using a sensory evaluation test. The reconstituted beer and the FFB indeed had similar flavor profiles, proving the utility of the selectable 1D/2D GC–MS approach.

GERSTEL scientists have combined SBSE-TD-2D GC with olfactometry using the Olfactory Detection Port, simultaneous single quadrupole MS and PFPD/nitrogen–phosphorus (NPD) detection, and Agilent MassWorks™ mass spectral calibration software to enable fast and accurate formula identification of flavor compounds in beverages, including beer. With this mass spectral calibration, mass accuracy on the order of 0.00x Da can be achieved, even with the unit mass resolution of the single quadrupole MS, greatly increasing the confidence of compound identification. The potential of this approach using state-of-the-art analytical techniques was illustrated by identifying key aroma substances in beer and coffee.

Beer brewers are continually looking to improve their products, as the trend toward gluten-free beer attests. Many modern analytical techniques are being applied to decode the flavor puzzle of this popular beverage, and GERSTEL is at the cutting-edge of development and utilization of these capabilities.  Our SBSE, thermal desorption, olfactometry, and innovative selectable 1D/2D GC–MS systems provide a complete solution for reproducible and robust separation and identification of very complex aroma mixtures at trace levels. 

References

  1. N. Ochiai, K. Sasamoto, and T. Kishimoto, J Agric Food Chem., 63(30), 6698-706 (2015).
  2. K. Tokita, K. Takazumi, T. Oshima, and T. Shigyo, J Am Soc Brew Chem, 72(2), 154-161 (2014).
  3. N. Ochiai and K. Sasamoto, J Chromatogr A, 1217(17), 2903-2910 (2010).