Stress tolerance and growth physiology of yeast strains from the Brazilian fuel ethanol industry

Abstract

Improved biofuels production requires a better understanding of industrial microorganisms. Some wild Saccharomyces cerevisiae strains, isolated from the fuel ethanol industry in Brazil, present exceptional fermentation performance, persistence and prevalence in the harsh industrial environment. Nevertheless, their physiology has not yet been systematically investigated. Here we present a first systematic evaluation of the widely used industrial strains PE-2, CAT-1, BG-1 and JP1, in terms of their tolerance towards process-related stressors. We also analyzed their growth physiology under heat stress. These strains were evaluated in parallel to laboratory and baker's strains. Whereas the industrial strains performed in general better than the laboratory strains under ethanol or acetic acid stresses and on industrial media, high sugar stress was tolerated equally by all strains. Heat and low pH stresses clearly distinguished fuel ethanol strains from the others, indicating that these conditions might be the ones that mostly exert selective pressure on cells in the industrial environment. During shake-flask cultivations using a synthetic medium at 37 °C, industrial strains presented higher ethanol yields on glucose than the laboratory strains, indicating that they could have been selected for this trait-a response to energy-demanding fermentation conditions. These results might be useful to guide future improvements of large-scale fuel ethanol production via engineering of stress tolerance traits in other strains, and eventually also for promoting the use of these fuel ethanol strains in different industrial bioprocesses.

Physiology of the fuel ethanol strainSaccharomyces cerevisiaePE-2 at low pH indicates a context-dependent performance relevant for industrial applications

“…Previous experiments on exponentially growing shake‐flask cultures of PE‐2 and CEN.PK113‐7D, in which both strains exhibited a maximum specific growth rate of 0.34 h −1 , showed a higher biomass yield, ethanol yield and glycerol yield on glucose for PE‐2 than CEN.PK113‐7D (Della‐Bianca & Gombert, ). These differences are most likely due to different cultivation conditions.…”

“…In a previous study on acid tolerance of strains PE‐2 and CEN.PK113‐7D, growth was assessed on YPD agar plates at pH 2.5. Under these conditions, CEN.PK113‐7D failed to grow, but PE‐2 did grow (Della‐Bianca & Gombert, ). To investigate whether and how pH tolerance might be affected by medium composition, anaerobic batch bioreactor cultures were grown on YPD medium at pH 2.7.…”

“…These results corroborate our previous observation of the different tolerances of strains PE‐2 and CEN.PK113‐7D to low pH on YPD plates (Della‐Bianca & Gombert, ). The impact of medium composition on pH tolerance demonstrated by our experiments underlines that, in studying characteristics of industrial micro‐organisms, growth conditions and medium composition should be as close as possible to those in the industrial process.…”

“…Nonetheless, baker's strains are still used at the start of the sugarcane crushing season (alone or in combination with a small proportion of some selected fuel ethanol strain), due to their much lower cost and higher availability compared with the fuel ethanol strains. We previously screened several Brazilian fuel ethanol yeast strains for tolerance towards a number of stress factors using agar‐plate assays (Della‐Bianca & Gombert, ). These experiments indicated that wild fuel ethanol yeast strains were more tolerant to low pH than two laboratory yeast strains and a baker's yeast strain.…”

Nisin incorporation enhances the inactivation of lactic acid bacteria during the acid wash step of bioethanol production from sugarcane juice

“…Our stock sugarcane juice (SCJ) had 12·5% carbohydrate. SCJ was sterilized by centrifugation and filtration on a NalgeneTM Rapid‐FlowTM sterile disposable bottle top filters (SFCA membrane, 50 mm diameter and 0·2 μ m pore size) (Thermo Fisher Scientific, Waltham, MA) according to the methods of Della‐Bianca and Gombert (). For fermentations, SCJ was diluted to 6% carbohydrate with sterile water, supplemented with 6 g l −1 yeast extract, and pH adjusted to 5·5 with HCl resulting in YSC‐6 (Daum ).…”