Non-invasive headspace sorptive extraction for monitoring volatile compounds production by saccharomyces and non-saccharomyces strains throughout alcoholic fermentation

Abstract

Wine is a solution containing abundant volatile compounds which contribute to their aroma. Many of them are produced by yeast as metabolism by-products. Different yeast strains produce different volatile profiles. The possibility of studying the evolution of volatile compounds during fermentation, using sampling methods that not alter the volume of fermentation media, is of great interest. In spite of this, non-invasive methods to monitoring the evolution of volatile profile during fermentation have been seldom used. The goals of this work were to use by first time the headspace sorptive extraction (HSSE) as non-invasive method to monitor the evolution of volatile profiles throughout alcoholic fermentation and to study the changes on volatile profiles produced by Saccharomyces cerevisiae and Lachancea thermotolerans during fermentation of a must with high sugar content. Fermentations were carried out by autochthonous yeast isolated from previous laboratory-scale fermentations with sun-dried Pedro Ximénez must: one Saccharomyces cerevisiae strain and one Lachancea thermotolerans strain. Fermentations were performed at 22 ºC in 500 mL Erlenmeyer flasks containing 350 mL of sun-dried Pedro Ximénez must that were inoculated at a density of approximately 5.5×106 cell/mL. The online sampling was performed by HSSE with PDMS Twisters. Twisters were maintained in the headspace at 2.5 cm above the liquid surface during 2 h at 22 ºC of temperature. A total of six extractions were accomplished for each fermentation assay in the following manner: Before inoculation, every 24 h after inoculation (24, 48 and 72) and at 144 and 192 h after inoculation. A total of 141 volatile compounds throughout fermentations could be monitored by HSSE. 84 of them were positively identified and 28 tentatively identified (TI). The primary difference between the two yeast strains was the different rate of production of ethyl esters. The total content of acetals increased along fermentation. This increase was higher when the process was carried out by S. cerevisiae strain. With respect to acids, the overall balance was an increase of them for wines produced by Saccharomyces strain and a decrease for wines produced by non-Saccharomyces one. An important increase in alcohols was observed, having the same rate of ethanol production both yeasts. However, the global increases of alcohols were significant higher when the fermentation was carried out by non-Saccharomyces strain. The overall content of acetic esters was significant higher for Saccharomyces strain in all the stages.