Abstract
Yeasts are able to act as biosorbents, as their cell wall includes several components capable of binding organic xenobiotic compounds that can potentially be removed during various fermentation processes. In the present investigation, two novel Saccharomyces cerevisiae strains (LMBF-Y 16 and LMBF-Y-18), previously isolated from grapes, were studied regarding their physiological behavior (dry cell weight—DCW production, substrate uptake, and ethanol and glycerol biosynthesis) during fermentations of grape must, in some cases enriched with commercial glucose and fructose (initial total sugar concentration approximately 150 and 250 g/L, respectively). Myclobutanil (a chiral triazole fungicide broadly used as a protective agent of vine) was also added to the culture media at various concentrations in order to assess the ability of the yeasts to simultaneously perform alcoholic fermentations and detoxify the medium (i.e., to remove the fungicide). In the first set of experiments and for both tested strains, trials were carried out in either 250 mL or 2.0 L agitated shake flasks in either synthetic glucose-based experiments or grape musts. Since the results obtained in the trials where the cultures were placed in 2.0 L flasks with grape musts as substrates were superior in terms of both DCW and ethanol production, these experimental conditions were selected for the subsequent studies. Both strains showed high fermentative efficiency, producing high amounts of DCW (9.5–10.5 g/L) in parallel with high ethanol production, which in some cases achieved values very close to the maximum theoretical ethanol production yield (≈0.49 g of ethanol per g of sugar). When using grape must with initial total sugars at approximately 250 g/L (very high gravity fermentation media, close to winemaking conditions), significantly high ethanol quantities (i.e., ranging between 105 and 123 g/L) were produced. Myclobutanil addition slightly negatively affected sugar conversion into ethanol; however, in all cases, ethanol production was very satisfactory. A non-negligible myclobutanil removal during fermentation, which ranged between 5%–27%, as a result of the adsorptive or degradative capacity of the yeast was also reported. The presence of myclobutanil had no effect on DCW production and resulted in no significant differences in the biosynthesis of glycerol. Therefore, these newly isolated yeast strains could be excellent candidates for simultaneous high ethanol production and parallel pesticide removal in a general biorefinery concept demonstrating many environmental benefits.
Highlights
A significant volume of research in industrial biotechnology focuses on the isolation of or genetic manipulation for the construction of novel robust, high-performing microorganisms useful for the production of improved products presenting technological interest [1]
Glycerol, and glucose and fructose were quantified through high-performance liquid chromatography (HPLC) in a Waters Association 600E apparatus equipped with a RI detector (Waters 410, Midland, ON, Canada)
In order to confirm that the two isolates of S. cerevisiae corresponded to two different strains, amplification of the interdelta region was performed by using the primers delta 12-delta 21 [43]
Summary
A significant volume of research in industrial biotechnology focuses on the isolation of or genetic manipulation for the construction of novel robust, high-performing microorganisms useful for the production of improved products presenting technological interest [1]. Research activities concerning bioethanol production have focused on the process optimization, bioprocess modeling, and the development of novel fermentation strategies and configurations (e.g., simultaneous saccharification and fermentation, consolidated bioprocess, etc.) [4,5,8,9,10] To this end, the establishment of fermentation when very high initial quantities of sugars (i.e., ≥250 g/L) are employed as substrate (so-called “very-high-gravity fermentation”), as well as the accomplishment of fermentation under non-aseptic conditions, has recently gained significant attention [5,11,12]. Both these strategies may offer great savings in process water and energy requirements, and when appropriate microbial strains are found for these purposes, no major alterations to the production lines of existing bioethanol production plants are required [11,13]
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