Evolution de thiols polyfonctionnels au cours du vieillissement de la bière

The objective of our thesis was to evidence how polyfunctional thiols evolve through beer aging. Thiols were investigated in various fresh beers. By applying pHMB extractions and GC-PFPD analyses, the profiles were shown to be significantly different between beer samples, mainly depending on the hopping procedure and the yeast strain. During beer aging, thiols evolve in different ways. In the early period of storage, both synthesis and degradation were observed. Yet after one year of aging, most compounds revealed to be strongly degraded. In oxidized aged beer, the ribes off-flavor exhaled by 3S3MB-F was shown to occur. This thiol can appear very early, with concentration up to 1230 ppt for a sensorial threshold assessed at 57 ppt. Combinatorial syntheses of 2- and 3-sulfanyl-3-methylbutyl formates and acetates allowed us to evidence how different can be their descriptors. Both formates bring the ribes note, while their acetates were vinegar-like. Another polyfunctional thiol which was often generated at the beginning of aging is 2SE-A. This empyreumatic odorant is usually the major thiol in fresh beer, with concentration up to 10 ppb for a sensorial threshold assessed at 40 ppb. For a same content of cysteine and homocysteine in wort, very different concentrations of 2SE-A can be generated in fresh beer, according to the yeast strain used. When high levels are found in fresh beer, significant degradation can be observed through aging. Commercially unavailable Cys-2SE-A was synthesized for the first time, from acetylation of the S-cysteine conjugate of 2SEol. The low degradation ratios measured by Cys-2SE-A spiking experiments seem sufficient to explain the production of ppb levels of 2SE-A through beer aging. For other minor thiols, a similar pattern was observed through aging. When commercial felinine or synthesized Cys-3SHol were spiked into fresh beer, the corresponding free thiols were generated after 5 days at 60°C (molar conversion ratio of 0.1-0.2%; cysteine adducts recovery factor = 43-84%). As shown by experiments conducted in model aqueous media, chemical degradation of cysteine adducts are most probably favored in a complex medium like beer. A modified Strecker degradation pathway of cysteine adducts in presence of beer dicarbonyls was proposed.