Identification of cis-2-methyl-4-propyl-1,3-oxathiane as a new volatile sulfur compound (VSC) in wine

Abstract

Despite their trace concentrations, volatile sulfur compounds (VSCs) are an important category of flavour-active compounds that significantly contribute to desirable or undesirable aromas of many foods and beverages. In wines, VSCs in the form of polyfunctional thiols, notably 3-sulfanylhexan-1-ol (3-SH), 3-sulfanylhexyl acetate (3-SHA), and 4-sulfanyl-4-methyl-pentan-2-one (4-MSP), possess extremely low olfactory thresholds (≈ ng/L) and pleasant “tropical aroma” notes. They have received much attention with respect to their sensory contributions, quantitative occurrences, biogenesis, and thiol management through viticulture and winemaking. However, the fate of these potent volatiles are still not fully understood.

In this work [1], the fate of 3-SH in wine was probed based on the potential sensory and chemistry interactions between 3-SH and acetaldehyde, which led to the identification of cis-2-methyl-4-propyl- 1,3-oxathiane (cis-1) in wine for the first time. Subsequently, a stable isotope dilution assay (SIDA) using headspace–solid-phase microextraction with gas chromatography and mass spectrometry (HS–SPME GC– MS) was developed. A range of parameters was optimised, a deuterated internal standard was synthesised, and the method was fully validated and applied to the quantitation of cis-1 in wines. The aroma detection threshold of 1 was also determined.

The existence of cis-1 in Sauvignon blanc wines from a laboratory-scale fermentation trial was revealed by mass spectral comparison to an authentic standard, linear retention indices of naturally present cis- 1 on two GC columns, and co-injection experiments. Challenges were faced when analysing commercial wine samples due to unknown co-eluting interferences in some wines. After employing d4-1 as the internal standard and evaluating additional capillary column phases, a sensitive SIDA HS–SPME GC–MS method was developed and applied to a survey of commercial wines. Interestingly, trans-1 was not detected whereas cis-1 ranged from undetectable to 460 ng/L, which highly correlated (r = 0.72) to the concentrations of 3-SH, determined in the same wines by HPLC–MS/MS after derivatisation. The aroma detection threshold of 1* in a neutral Australian white wine was found to be 7.1 μg/L. Although cis-1 concentrations in the studied wines were below the odour detection threshold of 1, our results suggest a potential link from cis-1 to the potent VSC 3-SH, and more research is required to gain a better understanding of the importance of cis-1 in wine, from both chemistry and sensory perspectives. The identification and method development work will be presented along with additional experiments involving cis-1 in wine.

references:

[1]. Chen, L., D.L. Capone, and D.W. Jeffery, Identification and Quantitative Analysis of 2-Methyl-4-propyl-1,3-oxathiane in Wine. Journal of Agricultural and Food Chemistry, 2018, 66 (41), 10808–10815.

* Using a commercial standard consisting of 85% cis-1 and 15% trans-1.