First Evidence of the Production of Odorant Polyfunctional Thiols by Bottle Refermentation

J. Am. Soc. Brew. Chem. 71(1):15-22, 2013 Bottle refermentation, which confers effervescence and resistance against infection and oxidation to beers, has also long been known to affect the fruity character imparted by esters. Yet it is recognized to improve the flavor perception, first by reducing stale aldehydes (trans-2-nonenal, 3-methylthiopropionaldehyde, etc.) to low-odorant alcohols, and also by bringing new pleasant odors. In this work, the polyfunctional thiol contents of a beer subjected or not to bottle refermentation were compared. A trained panel detected a strong organoleptic impact of bottle refermentation. Specific pHMB thiol extraction was applied and the extracts were analyzed by GC-MS, GC-PFPD, and GC-olfactometry (AEDA). Many sulfanylalkylalcohols, sulfanylalkylacetates, and sulfanylalkylcarbonyls were shown to be produced during the refermentation process, especially after 3 weeks. Among them, the hoppy 1-sulfanyl-3-methyl-2-butene was still perceived at the sniffing port after diluting the extract by a factor of 32,768. The major thiol, 2-sulfanylethyl acetate, reached 10 mu g/L. As shown by spiking deuterated cysteine before bottle refermentation, the Ehrlich pathway revealed still efficient in the bottle. Most of the other identified polyfunctional thiols shared a common beta-sulfanyl structure, which lead us to suspect that hop cysteine adducts might be hydrolyzed by yeast-derived lyases. The spiking of 5 and 10 mg/L of S-3-(1-hydroxyhexyl)cysteine confirmed the ability of yeast to release free thiols through bottle refermentation. Therefore, a better control of the refermentation process requires both an excellent control of yeast (Ehrlich pathway and beta-lyase activity) and strict selection of the hop variety (level of cysteine adducts).