Transfer kinetics of labeled aroma compounds from liquid media into coffee beans during simulated wet processing conditions.

The transfer kinetics of three labelled compounds (butanal, 2-phenyethanol, isoamyl acetate) was studied from a liquid medium into the coffee beans during simulated wet processing using four media (M) (M1: contained dehulled beans, M2: contained demucilaginated beans, M3: contained depulped beans, M4: contained depulped beans with yeast). Trials were carried out at 25 °C, under agitation and for five time periods (0, 6, 12, 24 and 48 h), and then the labelled volatiles were analyzed by SPME-GC-MS. The three labelled molecules were transferred into the coffee beans with different mass transfer rates; reaching at 12hrs in the M4, 0.2 ± 0.03, 11.2 ± 0.66 and 1.3 ± 0.04 µg/g of coffee respectively for butanal, 2-phenyethanol and isoamyl acetate. The parchment resistance significantly affected the mass transfer of the 2-phenylethanol. Butanal and isoamyl acetate underwent metabolic reactions, which decreased their amount in the coffee beans. Furthermore, an interaction between molecules and the yeast was observed and decreased significantly the butanal's transfer.

Effect of grape must polyphenols on yeast metabolism during alcoholic fermentation.

In red winemaking, polyphenols from grape berry pericarp and seed are extracted during fermentation and their interactions with yeast have been widely demonstrated. However, information concerning the impact of extracted polyphenols on yeast metabolism during fermentation is missing. The aim of this study was to further explore interactions between yeasts and polyphenols and to identify their effects on yeast metabolism and fermentation kinetics. This impact was studied in synthetic musts for four commercial Saccharomyces cerevisiae wine strains, using polyphenols purified from a thermovinification must, in both stressed (phytosterol deficient medium) and non-stressed conditions. Interactions between grape polyphenols and yeast cells were substantiated from the early stage of fermentation by means of epifluorescence and confocal microscopy. If these interactions were limited to yeast cell walls in non-stressed conditions, the passage of polyphenols through yeast envelope and their accumulation in the intracellular space of living cells was shown in phytosterol-deficient medium. Whatever the conditions used (stressed and non-stressed conditions) and for all strains, the presence of polyphenols led to a significant decrease of cell growth (50%), CO2 production rate (60 to 80%) and nitrogen consumption (3 to 4 times less), resulting in increased fermentation lengths. The perturbation of yeast growth and metabolism due to polyphenol compounds was likely mostly linked to their interactions with the yeast plasma membrane. From the mid-stationary phase to the end of the fermentation, an adaptive response was exhibited by yeast, resulting in lower mortality. This work evidenced a strong impact of polyphenols on yeast fermentative capacity and highlighted the importance of a better knowledge of the mechanisms involved to improve the management of fermentations in the context of red winemaking.

Interactions of grape tannins and wine polyphenols with a yeast protein extract, mannoproteins and ß-glucan.

At present, there is a great interest in enology for yeast derived products to replace aging on lees in winemaking or as an alternative for wine fining. These are yeast protein extracts (YPE), cell walls and mannoproteins. Our aim was to further understand the mechanisms that drive interactions between these components and red wine polyphenols. To this end, interactions between grape skin tannins or wine polyphenols or tannins and a YPE, a mannoprotein fraction and a ß-glucan were monitored by binding experiments, ITC and DLS. Depending on the tannin structure, a different affinity between the polyphenols and the YPE was observed, as well as differences in the stability of the aggregates. This was attributed to the mean degree of polymerization of tannins in the polyphenol fractions and to chemical changes that occur during winemaking. Much lower affinities were found between polyphenols and polysaccharides, with different behaviors between mannoproteins and ß-glucans.

Sorption of grape proanthocyanidins and wine polyphenols by yeasts, inactivated yeasts, and yeast cell walls.

Inactivated yeast fractions (IYFs) can be used in enology to improve the stability and mouthfeel of red wines. However, information concerning the mechanisms involved and the impact of the IYF characteristics is scarce. Adsorption isotherms were used to investigate interactions between grape proanthocyanidin fractions (PAs) or wine polyphenols (WP) and a commercial yeast strain (Y), the inactivated yeast (IY), the yeast submitted to autolyzis and inactivation (A-IY), and the cell walls obtained by mechanical disruption (CW). High affinity isotherms and high adsorption capacities were observed for grape PAs and whole cells (Y, IY, and A-IY). Affinity and adsorbed amount were lower with wine PAs, due to chemical changes occurring during winemaking. By contrast to whole cells, grape PAs and WP adsorption on CW remained very low. This raises the issue of the part played by cell walls in the interactions between yeast and proanthocyanidins and suggests the passage of the latter through the wall pores and their interaction with the plasma membrane.

Effect of a fungal chitosan preparation on Brettanomyces bruxellensis, a wine contaminant.

AIM: To investigate the action mechanisms of a specific fungal origin chitosan preparation on Brettanomyces bruxellensis. METHODS AND RESULTS: Different approaches in a wine-model synthetic medium were carried out: optical and electronic microscopy, flow cytometry, ATP flow measurements and zeta potential characterization. The inactivation effect was confirmed. Moreover, fungal origin chitosan induced both physical and biological effects on B. bruxellensis cells. Physical effect led to aggregation of cells with chitosan likely due to charge interactions. At the same time, a biological effect induced a leakage of ATP and thus a viability loss of B. bruxellensis cells. CONCLUSIONS: The antimicrobial action mode of chitosan against B. bruxellensis is not a simple mechanism but the result of several mechanisms acting together. SIGNIFICANCE AND IMPACT OF THE STUDY: Brettanomyces bruxellensis, a yeast responsible for the production of undesirable aromatic compounds (volatile phenols), is a permanent threat to wine quality. Today, different means are implemented to fight against B. bruxellensis, but are not always sufficient. The chitosan of fungal origin is introduced as a new tool to control B. bruxellensis in winemaking and has poorly been studied before for this application.