Review
Bioenergy: Sustainable fuels from biomass by yeast and fungal whole-cell biocatalysts

https://doi.org/10.1016/j.bej.2008.11.016Get rights and content

Abstract

The dependency on depleting natural resources is a challenge for energy security that can be potentially answered by bioenergy. Bioenergy is derived from starchy and lignocellulosic biomass in the form of bioethanol or from vegetable oils in the form of biodiesel fuel. The acid and enzymatic methods have been developed for the hydrolysis of biomass and for transesterification of plant oils. However, acid hydrolysis results in the production of unnatural compounds which have adverse effects on yeast fermentation. Recent advancements in the yeast cell surface engineering developed strategies to genetically immobilize amylolytic, cellulolytic and xylanolytic enzymes on yeast cell surface for the production of fuel ethanol from biomass. Whereas in the case of biodiesel fuel production, alkali catalysis gives high levels of conversion in short reaction times. But complexity in the separation of produced biodiesel fuel from glycerol by-product led to intensive research on lipase enzyme and immobilized whole-cell biocatalysts. This system facilitates the easy separation of glycerol with the advantage of cost effectiveness. This review gives an insight in to the recent technological developments in the production of bioenergy, i.e., bioethanol and biodiesel fuel using surface engineered yeast and whole-cell biocatalysts.

Introduction

Bioenergy represents the utilization of biomass as starting material for the production of sustainable fuels and chemicals [1]. Environmental concerns and the depletion of oil reserves have resulted in governmental actions and incentives to establish greater energy independence by promoting research on environmentally friendly and sustainable biofuels like bioethanol and biodiesel. Ethanol is the most widely used biofuel either as a fuel or as a gasoline blends. The world ethanol production has reached about 51,000 million litres, being USA and Brazil the first producers [2]. In average 73% of produced ethanol worldwide corresponds to fuel ethanol, 17% to beverage ethanol and 10% to industrial ethanol [3].

The fuel ethanol can be obtained directly from sucrose or from starchy and lignocellulosic biomass. The complexity of the process depends on the type of feedstock. The spectrum of designed and implemented technologies ranging from simple conversion of sugars by fermentation, to the multi-stage conversion of lignocellulosic biomass to ethanol [3]. Among the new research trends in this field, process integration by means of developing yeast strain has the ability to do simultaneous saccharification and fermentation is the key factor for reducing costs in ethanol production. In the recent days, considerable research has been carried out on the cell surface engineering of Saccharomyces cerevisiae for the production of bioethanol from various biomass resources [4], [5], [6], [7].

A number of studies investigated promising methods that make use of triglycerides as an alternative fuel for diesel engines [8], [9], [10], [11], [12], [13]. However, the direct use of vegetable oils or oil blends is generally considered impractical because of the high viscosity, acid composition and free fatty acid content of the triglycerides. To address these issues, transesterification, also called alcoholysis, has been employed in order to reduce viscosity and to improve the physical properties of biodiesel fuel [11]. Transesterification involves the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except that the alcohol is employed as an acyl acceptor.

Although alkali and acid catalyzed transesterification promotes high conversion rates of triglycerides to their corresponding alkyl esters within short reaction times, the process has several drawbacks, including energy intensiveness, difficulty of glycerol recovery, removal of the alkaline catalyst from the product and treatment of the highly alkaline waste water. The related enzymatic process offers several advantages over these conventional processes, especially with regard to ease of separating the glycerol by-product without any complex operation steps. A number of research efforts exploit the enzymatic process with the aim to improve biodiesel fuel production. Even though preparations of lipases that were immobilized on acrylic resins have shown to offer better conversion rates together with ease to recycle the biocatalyst, the high cost of lipase enzyme makes this process cost-prohibitive for most commercial applications. Several studies have reported the utilization of microorganisms, such as yeast and fungi as whole-cell biocatalysts in attempts to improve the cost effectiveness of bioconversion processes [14], [15], [16]. Among the established whole-cell biocatalyst systems, filamentous fungi have arisen as the most robust whole-cell biocatalyst for industrial applications, since they can be spontaneously immobilized on to the biomass support particles (BSPs) during batch cultivation and no further purification of enzyme is necessary.

This paper attempts to present a comprehensive idea on the developing technologies for bioenergy production which includes cell surface engineering of yeast for the production of bioethanol from various biomass resources and the enzymatic and whole-cell catalyzed transesterification of plant oils to biodiesel fuel.

Section snippets

Bioethanol

Main feed stock for ethanol production is sugar cane in form of either cane juice or molasses (by-product of sugar mills) and starchy biomass. About 79% of the ethanol in Brazil is produced from sugar cane juice and the remaining part from cane molasses [17]. The most widely used microorganism for ethanol fermentation is S. cerevisiae due to its potential ability to hydrolyze cane sucrose in to fermentable sugars. Even though, Saccharomyces has the ability to grow under anaerobic conditions,

Biodiesel fuel

Biodiesel fuel is produced by the transesterification, also called alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except that an alcohol is employed instead of water (Fig. 4). Suitable alcohols for transesterification include methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanol are most commonly used, especially methanol because of its low cost and its physical and chemical advantages. This process has been

Conclusion and future prospects

The S. cerevisiae displaying biomass degrading enzymes has been proved as whole-cell biocatalysts for direct conversion of starchy materials and lignocellulosic materials to ethanol. The genes encoding these enzymes were fused with the anchor genes and were introduced in to S. cerevisiae. The yeast cells harbouring these fused genes were successfully utilized the starch and lignocellulosic material as the sole carbon source. Even though several reports successfully developed strategies for

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      Similarly to the use of plant lipases, the use of fungi, bacteria and yeasts as whole-cell biocatalysts is a cost-effective alternative to extracellular lipases [126,127]. In addition, methodologies for the production of biodiesel using whole-cells as catalysts in non-edible oil from Jatropha curcas with immobilized Rhizopus oryzae cells, showed better conversion rates ​​than the commercial enzyme Novozymes 435, which had its catalytic activity inhibited by the presence of water added to the reaction medium [128,129]. In general, lipase selection is based on its specificity and stability in different solvent systems [130].

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