New Precursor of 3-Mercaptohexan-1-ol in Grape Juice: Thiol-Forming Potential and Kinetics during Early Stages of Must Fermentation

Abstract

Two volatile thiols, 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), are key aroma impact compounds in many young white wines, especially of the variety Sauvignon blanc (SB). Although great effort has been invested to identify their precursors in recent years, the origin of the majority of 3MH and 3MHA generated during wine fermentation still cannot be explained. Here we demonstrate that supplying an external source of hydrogen sulfide to grape juice hugely increases its thiol-forming potential. We further describe the discovery of (E)-2-hexen-1-ol as an additional new thiol precursor and demonstrate that it possesses, together with (E)-2-hexenal, an immense thiol-forming potential during fermentation. Both C6-compounds are extremely rapidly metabolized by yeast during the first hours after inoculation, even under commercial conditions, and can be interconverted during this phase depending on their initial concentration in the grape juice. Spiking grape juice with additional acetaldehyde greatly enhanced the (E)-2-hexen-1-ol to (E)-2-hexenal conversion rate. Delaying the metabolization of the two unsaturated C6-thiol precursors by yeast, at the same time as increasing hydrogen sulfide production early in fermentation, opens up a great opportunity to tap into this enormous potential 3MH and 3MHA source in grape juice and extends the possibility of thiol production to other non-grape-based alcoholic beverages as well.

Review of Aroma Formation through Metabolic Pathways of Saccharomyces cerevisiae in Beverage Fermentations

“…Harsch et al (2013) identified (E)-2-hexenal and its corresponding alcohol, (E)-2-hexen-1-ol, as precursors to the formation of 3MH (Harsch et al 2013). This work established the potential for enhancing 3MH and 3MHA in the presence of a sulfur donor and (E)-2-hexenal and (E)-2-hexen-1-ol, opening the door to the possibility of using yeast to enhance production of positive aroma thiols (Figure 3) (Harsch et al 2013).…”

“…For example, free terpenoid compounds in grapes impart specific aroma characteristics and vary depending on the grape varietal (Conde et al 2007). Muscat grapes have the highest Figure 3 The predicted pathway leading to the production of the aroma thiols 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexyl acetate (3-MHA) (adapted from Harsch et al 2013). When the six-carbon (C6) precursors, (E)-trans-2-hexen-1-ol and (E)-trans-2-hexanal have a relative ratio greater than one and a sulfur donor is present, the reaction is driven to produce 3-MH and 3-MHA.…”

Sauvignon Blanc Wine: Contribution of Ageing and Oxygen on Aromatic and Non-aromatic Compounds and Sensory Composition : A Review

“…Other than release from precursors, the biogenesis of volatile thiols also has been proposed. The direct addition of H 2 S or another sulphur donor to conjugated carbonyl compounds such as (E)-2-hexenal and mesityl oxide, followed by a reduction step, has been shown to contribute significantly to the pool of volatile thiols in Sauvignon blanc wines (Harsch et al, 2013). The formation of the volatile thiols is not fully understood, since the main precursor has yet to be elucidated and the production of these volatile thiols needs further investigation.…”

“…Practices such as mechanical harvesting, skin contact, pressure during pressing and oxygen exposure could significantly increase the precursor content in the juice and influence the volatile thiol concentration in the corresponding wines (Maggu et al, 2007;Patel et al, 2010;Roland et al, 2010b;Capone & Jeffery, 2011). The liberation of the volatile thiols from the respective precursors depends on conditions such as yeast strain, nitrogen availability and fermentation temperature (Murat et al, 2001a;Masneuf-Pomarède et al, 2006;Swiegers et al, 2006;Subileau et al, 2008b;Anfang et al, 2009;Harsch et al, 2013). More detail regarding the influence of these processes on the volatile thiol concentrations can be obtained from a review article published in 2012 (Coetzee & Du Toit, 2012).…”

Effect of Skin Contact Before and During Alcoholic Fermentation on the Chemical and Sensory Profile of South African Chenin Blanc White Wines

“…S-3-(hexan-1-ol)-L-cysteine (Cys-3MH) (Darriet et al, 1995), as well as glutathionylated S-3-(hexan-1-ol)-glutathione (Coetzee & Du Toit, 2012). Another source of volatile thiols is from the conjugation of (E)-2-hexenal and certain sulphur compounds (Harsch et al, 2013). Research by Tominaga et al (1998) showed that the amplification of varietal aromas during fermentation occurs by the action of yeast carbon-sulphur lyases through an enzymatic mechanism that releases the volatile thiols.…”