WINETECH Technical Yearbook 2020

Yeast and its ability to release thiols

THIOL PRECURSORS AND YEAST The varietal thiols, 3MH and 4MMP are present in the non-aromatic form in the juice and are formed and/or released from precursors during fermentation. Yeast takes up conjugated precursors from grape juice and then cleaves the precursor, releasing the corresponding free thiol. Non-odourant cysteinylated and glutathionylated precursors, such as Cys-3MH, Cys-Gly-3MH, GSH-3MH, Cys-4MMP, Cys-Gly-4MMP andGSH-4MMP, have been identified as conjugated precursors in the grape must (Tominaga et al ., 1998 & 1995; Des Gachons et al ., 2002; Fedrizzi et al ., 2009). However, studies have also shown that these precursors are not the exclusive source of varietal thiols and othermechanisms and unidentified precursors also contribute significantly to the final content of varietal thiols in wine (Pinu et al ., 2012; Harsch et al ., 2013). Unlike 3MH and 4MMP, 3MHA does not have a conjugated precursor in the must and is formed during the esterification of 3MH. Even though the exact mechanisms for the release and/or formation of these odoriferous thiols are still not entirely elucidated, it is clear that the yeast strain used to perform the fermentation is one of the most important factors affecting the thiol production and the thiol concentration in the final wine (Cordente et al ., 2012). Yeast strains differ significantly in the capability to produce thiols. This

AUGUST 2020

CARIEN COETZEE: Basic Wine, Stellenbosch KEYWORDS: Yeast, thiols, conjugated thiol precursors

Yeast strains vary significantly in their ability to produce varietal thiols. INTRODUCTION The varietal thiols, mainly 3-mercaptohex- an-1-ol (3MH), 3-mercaptohexyl acetate (3MHA) and 4-mercapto-4-methyl-pen- tan-2-one (4MMP) are the three major varietal thiols contributing to tropical fruit aroma wines. These aromatic compounds are considered to be impact compounds for Sauvignon blanc wines, however, elevated levels of thiols have also been found in South African Chenin blanc and Colombar wines.

WINETECH TECHNICAL YEARBOOK 2020 105