Author: Anita Peterson, Ph.D.
The ongoing wildfires have devastated much of California and deeply impacted winegrowing regions and the surrounding communities. These immense, out-of-control fires have caused significant destruction to vineyards, wineries, residences, businesses and restaurants and upended the lives of many since their start in August. The California Department of Forestry and Fire Protection reports that more than 8,300 fires have burned over 4 million acres, setting a shocking and heartbreaking record for the most land burned in a single year in California. While many wineries have suffered losses through direct damage to vineyards and buildings, others are facing the risk of further damage to the 2020 vintage by exposure of grapes to smoke prior to harvest. This exposure can lead to the contamination of grapes and smoke taint in the wine produced from those grapes. Thus, the ultimate impact of the wildfires on this year’s vintage is still unknown. Winemakers as well as analysts in commercial and research laboratories are working tirelessly to analyze samples for smoke taint and nevertheless produce a quality vintage. As the wildfire situation continues, so does the investigation into improving analytical methods and treatments for the assessment and amelioration of smoke-tainted wines.
Smoke-tainted wines are characterized by smoky, burnt, and medicinal odors caused by the presence of volatile phenols from wood smoke. These compounds can permeate grape skins and rapidly react with grape sugars to produce the non-volatile glycoside-bound forms of the phenols. Non-volatile glycosides are hydrolyzed by enzymes during fermentation or by acid during storage, gradually releasing the volatile phenols into wine. Smoke taint is currently assessed by the sensory evaluation of juice and wine and the quantification of volatile phenols, typically guaiacol and 4-methylguaiacol (creosol). Several others, however, can also contribute to undesirable smoky and medicinal odors in wine, including 4-ethyl guaiacol, meta- and para-cresol, syringol and 4-methylsyringol. The compounds that are present due to smoke depend on the source of the fire (brush vs hard wood fires). These compounds are naturally present in wine grapes, but smoke-contaminated grapes will contain excessive amounts in comparison.
The determination of both free and bound volatile phenols in grapes and wine is necessary to assess the potential for smoke taint, as many glycoside-bound forms are released after wine has been finished and bottled. Wine and grape samples are acidified (adjustment to pH 1.5 by addition of a strong acid) and heated in order to hydrolyze glycosides and free volatile phenols to be extracted, identified, and quantified by GC-MS (Noestheden, Thiessen, Dennis, Tiet, and Zandberg, 2017). The isolation/extraction of the hydrolyzed, free volatile phenols from grape juice and wine matrices is most efficiently performed by solvent-less methods including stir bar sorptive extraction (SBSE – sold as GERSTEL Twister®) and thin film solid phase microextraction (TF-SPME). Sequential SBSE (Seq-SBSE), which utilizes two GERSTEL Twisters (one stirring and one suspended with a magnetic clip) for extraction, has been shown to be a simple, rapid, and sensitive method for the analysis of smoke-exposed wine for free and bound volatile phenols (TechNote 315 ).
TF-SPME has proven to be a great compliment to SBSE and Seq-SBSE for free volatile phenols and other polar compounds. TF-SPME is a novel sampling device that employs a carbon mesh sheet impregnated with a sorptive phase. The larger surface area and sorbent volume of the device as well as the wide polarity range of the sorptive phase enable fast extraction and improved analyte recovery compared to SPME and in some cases SBSE. The TF-SPME device can be used in headspace or immersive mode and in both cases, the sample is agitated by stirring. Analytes are thermally desorbed from the extraction device (TF-SPME/Twister) using the GERSTEL Thermal Desorption Unit (TDU 2), which provides a short, valve-less flow path to the GERSTEL Cooled Injection System (CIS). The CIS is employed as a cryotrap to focus analytes prior to rapid heating and release onto the GC column. TF-SPME has proven to be an excellent extraction technique and superior to traditional SPME, evidenced by the analysis of various types of food (Application Note 202) and beverages (Application Note 200) using TF-SPME devices with a divinylbenzene/polydimethylsiloxane coating (DVB/PDMS).
Ray Marsili of the Marsili Consulting Group has performed extensive analysis of smoke-tainted wines by SBSE and immersive TF-SPME in conjunction with the GERSTEL TDU 2 and GC-TOFMS (Leco BT-TOFMS). The benefits and applications of TF-SPME are well demonstrated in his on demand webinar through SeparationScience. He has evaluated the advantages of employing a Twister stir bar to agitate samples for TF-SPME, followed by desorption of both devices together. Through an increase in extraction capacity and polarity range, this method resulted in ‘1.8 times the sensitivity of Seq-SBSE’ for smoke taint compounds. He notes that because volatile phenols are polar, difficult to extract from complex wine matrices, and present at low levels, HS-SPME lacks the extraction capacity to provide the required limits of detection for a wide range of volatile phenols. The analysis of smoke-tainted wine and grapes for volatile phenols by TF-SPME followed by TD GC-MS is by far the most sensitive and efficient method available.