70 research outputs found
Cellular death of two non-saccharomyces wine-related yeasts during mixed fermentations with saccharomyces cerevisiae
The early death of two non-Saccharomyces wine strains (H. guilliermondii and H. uvarum) during mixed fermentations with S. cerevisiae was
studied under enological growth conditions. Several microvinifications were performed in synthetic grape juice, either with single non-
Saccharomyces or with mixed S. cerevisiae/non-Saccharomyces inocula. In all mixed cultures, non-Saccharomyces yeasts grew together with
S. cerevisiae during the first 1â3 days (depending on the initial inoculum concentration) and then, suddenly, non-Saccharomyces cells began to die
off, regardless of the ethanol concentrations present. Conversely, in both non-Saccharomyces single cultures the number of viable cells remained
high (ranging 107â108 CFU mlâ 1) even when cultures reached significant ethanol concentrations (up to 60â70 g lâ 1). Thus, at least for these yeast
strains, it seems that ethanol is not the main death-inducing factor. Furthermore, mixed cultures performed with different S. cerevisiae/
H. guilliermondii inoculum ratios (3 :1; 1:2; 1: 10; 1 : 100) revealed that H. guilliermondii death increases for higher inoculum ratios. In order to
investigate if the nature of the yeastâyeast interaction was related or not with a cellâcell contact-mediated mechanism, cell-free supernatants
obtained from 3 and 6 day-old mixed cultures were inoculated with H. guilliermondii pure cultures. Under these conditions, cells still died and
much higher death rates were found for the 6 days than for the 3 day-old supernatants. This strongly indicates that one or more toxic compounds
produced by S. cerevisiae triggers the early death of the H. guilliermondii cells in mixed cultures with S. cerevisiae. Finally, although it has not
been yet possible to identify the nature of the toxic compounds involved in this phenomenon we must emphasise that the S. cerevisiae strain used
in the present work is killer sensitive with respect to the classical killer toxins, K1, K2 and K28, whereas the H. guilliermondii and H. uvarum
strains are killer neutral
gasification of lignin rich residues for the production of biofuels via syngas fermentation comparison of gasification technologies
This paper reports the use of lignin-rich residues from second generation bioethanol production, to produce syngas that can be applied in the gas fermentation process. Three gasification technologies at a different scale were considered in this study. Fixed bed updraft gasification of about 30 kg/h solid feed, bubbling fluidized bed gasification of about 0.3 kg/h solid feed and indirect gasification of about 3 kg/h solid feed. Two lignin-rich residues with different properties were tested and the results were evaluated in terms of feedstock pretreatment (grinding, drying and pelleting) and syngas quality requirements for the fermentation process. The molar H 2 /CO ratio (ranging from 0.6 to 1.0) and the tar yield (18â108 g/Nm 3 ) obtained from the three gasification technologies was quite different. For the syngas fermentation process, low H 2 to CO ratio is preferred, as most of the organisms grow better on CO than H 2 . Furthermore, different contents of impurities that can reduce the fermentability of the gas (such as hydrocarbons, HCN, HCl, NH 3 , COS and other organic S- compounds) were detected in the product gas. The concentration of these compounds in the syngas is related to the content of the corresponding compounds in the original feedstock. The different characteristics of the lignin-rich feedstocks are related to the specific pre-treatment technologies for the (hemi)cellulose extraction. By tuning the pre-treatment technology, the properties of the feedstock can be improved, making it a suitable for gasification. Tar and unsaturated hydrocarbon compounds need to be removed to very low levels prior to the fermentation process. As a next step, the combination of the gasification and the appropriate product gas cleaning, with the syngas fermentation process for the production of bio-alcohols will be evaluated and the overall efficiency of the gasification-fermentation process will be assessed. © 201
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