Abstract
The use of solid starchy waste streams to produce value-added products, such as fuel ethanol, is a priority for the global bio-based economy. Despite technological advances, bioethanol production from starch is still not economically competitive. Large cost-savings can be achieved through process integration (consolidated bioprocessing, CBP) and new amylolytic microbes that are able to directly convert starchy biomass into fuel in a single bioreactor. Firstly, CBP technology requires efficient fermenting yeast strains to be engineered for amylase(s) production. This study addressed the selection of superior yeast strains with high fermentative performances to be used as recipient for future CBP engineering of fungal amylases. Twenty-one newly isolated wild-type Saccharomyces cerevisiae strains were screened at 30 °C in a simultaneous saccharification and fermentation (SSF) set up using starchy substrates at high loading (20% w/v) and the commercial amylases cocktail STARGEN™ 002. The industrial yeast Ethanol Red™ was used as benchmark. A cluster of strains produced ethanol levels (up to 118 g/L) significantly higher than those of Ethanol Red™ (about 109 g/L). In particular, S. cerevisiae L20, selected for a scale-up process into a 1-L bioreactor, confirmed the outstanding performance over the industrial benchmark, producing nearly 101 g/L ethanol instead of 94 g/L. As a result, this strain can be a promising CBP host for heterologous expression of fungal amylases towards the design of novel and efficient starch-to-ethanol routes.
Highlights
In the near future, the non-renewable resources such as crude oil, coal and natural gases, that collectively account for about 82% of global energy needs [1], will no longer be viable
This study looked for novel S. cerevisiae strains with superior fermenting abilities to be used as host strains for heterologous expression of novel fungal hydrolytic enzymes with the final aim of developing efficient consolidated bioprocessing (CBP) yeast
With the final aim of isolating and selecting strains with high fermenting performances from glucose, the winery background was chosen since it is related to ethanol production and yeast are expected to produce and tolerate high alcohol concentrations
Summary
The non-renewable resources such as crude oil, coal and natural gases, that collectively account for about 82% of global energy needs [1], will no longer be viable. The global communities are moving toward the search for reducing fossil oil dependence and long-term sustainable forms of energies such as biofuels [2,3,4,5,6]. Bioethanol has emerged as a clean and eco-friendly fuel that could replace gasoline, both as pure ethanol in specially designed engines, or most widely as blends with fossil gasoline ranging from 5 to 20% ( referred to as E5 or E20) [1,7,8]. The energy equivalent of ethanol as fuel is 66% than petroleum, its combustion is 15% more efficient because of higher oxygen content and less exhaust emissions, such as sulfur and nitrogen oxides, are produced [9]. It has been estimated that the use of 10% ethanol blends could reduce greenhouse gasses emissions by 12–19%.
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