Editor’s Note: Author and consultant Ray Marisili presented a technical poster at last month’s National Conference of the American Society for Enology and Viticulture detailing a novel stir bar sportive extraction technique using the GERSTEL Twister for the analysis of flavor and aroma of beer and wine. He provides details below:
More than 800 volatiles including alcohols, esters, aldehydes, ketones, carboxylic acids, terpenes, pyrazines, and polyfunctional thiols contribute to the odor of beer and wine. Selecting an extraction technique prior to GC-MS analysis for beer and wine samples has been challenging for such a wide array of analyte types with varying polarities, volatilities, and concentrations ranging from mg/L to ng/L.
Stir bar sorptive extraction (SBSE) with polydimethylsiloxane (PDMS)-coated Twisters was developed by Pat Sandra’s research group in Belgium in 1999. SBSE GC-MS has been shown to offer numerous advantages for flavor analysis including automation, the possibility of using very small sample sizes, solventless extraction, and improved detection limits. In previously published research, a solventless, quantitative, sensitive, and relatively simple analytical method for studying off-flavor in beer was developed based on SBSE with PDMS (1). Heat-abused beer contained higher levels of (E, E)-2,4-dodecadienal, furfural, furyl hydroxymethyl ketone, furfuryl ethyl ether, beta-damascenone, ethyl phenylacetate, and ethyl-3-pyridinecarboxylate. Light-abused beer contained elevated levels of dimethyl disulfide, dimethyl trisulfide, and benzeneacetaldehyde. SBSE has also been successfully applied to wine GC-MS analysis in the past (2).
SBSE with PDMS works best for non-polar compounds; however a technique called sequential SBSE was developed (3) that can enhance the recovery of more polar compounds. Compared to conventional SBSE, sequential SBSE provides more uniform enrichment over the entire polarity range for odor active chemicals. Sequential SBSE consists of SBSE performed sequentially on a liquid sample (e.g., 10 gm sample beer or wine) first without modifier using one stir bar, then on the same sample after addition of 30% NaCl using a second stir bar. The first extraction with unmodified sample is mainly targeting solutes with high Kow (log Kow > 4.0), while the second extraction with modified sample solution (containing 30% NaCl) is targeting solutes with low and medium Kow (log Kow < 4.0). After extraction, the two stir bars are placed in a single glass desorption liner and are simultaneously desorbed. The desorbed compounds are analyzed by thermal desorption and gas chromatography–mass spectrometry (TD-GC–MS).
Recently a multi-SBSE method using an ethyleneglycol-modified Silicone-coated (EG-Sil) Twister and a PDMS Twister was developed to provide uniform enrichment of analytes and increased recovery of these important flavor and aroma compounds (4). I presented a technical poster detailing this method at the National Conference of the American Society for Enology and Viticulture last month, With multi-SBSE, a PDMS Twister is placed in the bottom of the GC vial with the sample of beer or wine (e.g., 10 gm), and the EG-Sil Twister is attached to the inside of the vial submerged in the sample just below the surface of beer with a magnetic clip (GERSTEL Twicester). The sample is stirred for two hours using the PDMS Twister while the EG-Sil Twister remains stationary. Both Twisters are continuously exposed to sample during the two-hour extraction.
Recent studies show that the multi-SBSE with EG-Sil and PDMS Twisters provides higher recoveries of polar compounds in beer and wine than SBSE and even sequential SBSE. Four different SBSE techniques were used to analyze 10 gm samples of Blue Moon beer, a Belgian-style wheat ale. Testing compared four SBSE methods: (1) One PDMS Twister (1 cm x 0.5 mm); (2) two PDMS Twisters; (3) sequential SBSE with two PDMS Twisters; and (4) multi-SBSE with one PDMS Twister and one EG-Sil Twister. Peak areas for method 4 (multi-SBSE) were significantly higher than the other SBSE techniques for fourteen key flavor compounds, including carboxylic acids, linalool, phenylethyl alcohol, o-thymol, p-vinyl guaiacol, and 2-phenylethyl esters (acetate, hexanoate, octanoate, and decanoate). Some of these same chemicals also contribute flavors to wine.
Three different white wines (Pinot Grigio, Chardonnay, and Riesling) were analyzed comparing one PDMS Twister to multi-SBSE using one PDMS Twister and one EG-Sil Twister (5). As in beer analysis, the EG-Sil Twister extracted more of the polar compounds in wine samples. Flavor compounds that were detected with significantly higher recoveries in the white wines included the following: 3-methyl 1-butanol; 1-butanol, 3-methyl-, acetate; phenylethyl alcohol; diethyl succinate; octanoic acid; ionone; succinic acid, ethyl-3-methylbutyl ester; 7-methyl-Z-tetradecen-1-ol acetate; ethyl octanoate; ethyl hexanoate; 3-methylbutyl octanoate; tridecanoic acid, 3-hydroxy-, ethyl ester; hexanoic acid; 2-phenylethyl acetate; 1-hexanol; nonanal; dodecanoic acid; ethyl dodecanoate; decanoic acid; ethyl decanoate; naphthalene, 1,2-dihydro-1,1,6-trimethyl-; cis-oak lactone; δ-dodecalactone; and 1,1,4a-trimethyl-3,4,4a,5,6,7-hexahydro-2(1H)-naphthalenone.
Since its inception in 1999, SBSE has evolved into a superior analytical extraction technique. Development of the EG-Sil polar phase for SBSE has significantly extended application potential of SBSE to include analytes that were difficult to extract at high recoveries with PDMS SBSE. Advances in GERSTEL thermal desorption instrumentation, EG-Sil Twisters, and creative application of Twister technology to a broader spectrum of sample types have made SBSE a powerful analytical tool to assist flavor chemists in off-flavor and malodor elucidation for a wide variety of sample matrices.
Further enhancement in analytical sensitivity, peak identification, reproducibility, etc. can be achieved with the sophisticated peak deconvolution capabilities of the Leco GC time-of-flight mass spectrometry instrument (GC-TOFMS) used in combination with multi-SBSE. As analytical extraction techniques improve in their ability to extract more analytes at higher recoveries, the problem of peak coelution increases dramatically and should not be overlooked. Because of high data acquisition rates (up to 500 complete spectra per second with TOFMS, compared to 10 spectra per second for quadrupoles), TOF detectors are capable of sophisticated peak deconvolution.
To learn more about this novel multi-SBSE method, feel free to contact us.