The Chemistry of Winemaking: How SPME Can Help

[Daniela-C]

“Wine is one of the most civilized things in the world and one of the most natural things of the world that has been brought to the greatest perfection, and it offers a greater range for enjoyment and appreciation than, possibly, any other purely sensory thing.”
― Ernest Hemingway

Yes, wine is one of the most natural things of the world, brought to perfection by a deep culture, expertise and time, such that skilled winemakers can be seen as magic alchemists. In fact, it is all about chemistry to maintain a critical balance within a complex mixture of organic compounds responsible of the final wine aroma and bouquet, concerted by the high sense of taste and smell of the winemaker.

Can a scientific approach support the instinct and the experience of winemakers? Yes! And chromatography is a powerful tool that can unravel complex mixtures and quantitate even very low amounts of critical compounds which may impact the final quality.

GC-MS analysis of wine headspace is of particular interest to characterize the content profiling of the volatile fraction and make correlation with sensory analysis, with the benefit of requiring minimal sample handling and avoiding the injection of the less-volatile matrix into the GC system.

Solid Phase Micro Extraction (SPME) is a popular technique used, in combination with GC or GCMS, to extract and enrich volatile and semi-volatile compounds from a complex matrix, through a partitioning equilibrium on a sorbent exposed to the sample.

Since its introduction in the early 1990s, the SPME technique has continuously gained in popularity, advantageously replacing liquid-liquid extraction methods, which is more laborious, time-consuming, and uses large amounts of solvents.

SPME is well suited to aqueous matrices and quickly became the method of choice for the analysis of wine aromas, allowing the characterization of key compounds like terpenoids, aliphatic and aromatic alcohols and esters, and provides better understanding of the chemical and biochemical mechanisms underlying wine aroma development.

An interesting webinar titled, “How SPME keeps revolutionizing the analysis of volatile compounds in wine,” has been recently presented by Dr. Céline Franc, Research Engineer at the Institute of Vine and Wine Science and Oenology Research Laboratory, University of Bordeaux in France. Since wine is a highly complex matrix with hundreds of components ranging from traces to g/L with many minor compounds having a high sensory impact, a powerful and selective extraction technique is required as well as a highly sensitive and selective MS detector capable of detecting and quantifying known or unknown wine components.

Dr. Franc combined the highest sorbent capability of the new SPME-arrow technique with the sensitivity of the Thermo Scientific™ TSQ™ 9000 GC-MS/MS. This approach is key for correlation studies between chemical and sensorial analyses, with possible identification of off-flavor markers, maturity markers or aging bouquet markers, as well as markers for exogenous contamination.

Speaking of exogenous contaminants, the use of the HS-SPME technique is also helping several wineries in the US West Coast after the recent wildfires which heavily affected California, Oregon and Washington, for the detection of smoke taint compounds in wine. “Smoke from the Western wildfires has tainted grapes in some of the nation’s most celebrated wine regions with an ashy flavor that could spell disaster for the 2020 vintage” [1].

Smoke-exposed berries show an increase in volatile phenols (i.e. guaiacol, syringol, 4-ethylphenol, etc.), which concentration has been correlated to the intensity of smoky, ashy, burnt meat sensory characteristics in the resulting wines, as negative organoleptic attributes.

It is evident how smoke taint is impacting wine producers in terms of costs and how much the more-frequent wildfires are representing a growing concern for this industry.

The Application Specialist Team in Runcorn, UK, is collaborating with an important wine producer in the US, based in California, to optimize the quantitation of smoke taint compounds by HS-SPME Arrow combined with highly sensitive GC-MS/MS.

Smoke taint compounds are found in both free and glycosylated forms. Acid hydrolysis is required to release the bounded compounds in their free form for GC analysis. For this reason, LC-MS/MS may represent a complementary technique for an easier determination of the glycosidically bound volatile phenols, leaving the GC-MS/MS approach to the determination of the free form.

The use of advanced analytical solutions for determination of free volatile phenols and their glycosides in wine and in smoke-exposed berries before fermentation will be key for wine producers to acquire knowledge on the potential for perceptible smoke taint in the resulting wine, before proceeding with the fermentation process.

I’ll be back on this interesting story of collaboration, so stay tuned to know future developments!