Very High Gravity Bioethanol Revisited: Main Challenges and Advances

Abstract

Over the last decades, the constant growth of the world-wide industry has been leading to more and more concerns with its direct impact on greenhouse gas (GHG) emissions. Resulting from that, rising efforts have been dedicated to a global transition from an oil-based industry to cleaner biotechnological processes. A specific example refers to the production of bioethanol to substitute the traditional transportation fuels. Bioethanol has been produced for decades now, mainly from energy crops, but more recently, also from lignocellulosic materials. Aiming to improve process economics, the fermentation of very high gravity (VHG) mediums has for long received considerable attention. Nowadays, with the growth of multi-waste valorization frameworks, VHG fermentation could be crucial for bioeconomy development. However, numerous obstacles remain. This work initially presents the main aspects of a VHG process, giving then special emphasis to some of the most important factors that traditionally affect the fermentation organism, such as nutrients depletion, osmotic stress, and ethanol toxicity. Afterwards, some factors that could possibly enable critical improvements in the future on VHG technologies are discussed. Special attention was given to the potential of the development of new fermentation organisms, nutritionally complete culture media, but also on alternative process conditions and configurations.

Pathway engineering strategies for improved product yield in yeast-based industrial ethanol production

“…Improving ethanol yield on fermentable sugars is by no means the only target of metabolic engineering studies related to yeast-based ethanol production. Other targets of intensive research include the reduction of processing costs by expression of polysaccharide hydrolases [ 136 ], extending substrate range to convert more fermentable substrates in crude industrial media [ 137 , 138 ], improving performance at high temperature to improve heat economy and cope with process temperature profiles [ 139 ], increasing yeast tolerance to process inhibitors and ethanol [ 139 , 140 ], improving osmotolerance of engineered strains with reduced glycerol formation [ 16 , 141 ] and simplification of nutritional requirements of industrial strains [ [142] , [143] , [144] , [145] ]. In addition, integration of corn-fiber from 1.5G processes [ 21 ] and reducing the need for antibiotics [ 139 , 146 ] are actively explored.…”

Increasing Ethanol Tolerance and Ethanol Production in an Industrial Fuel-ethanol <em>Saccharomyces cerevisiae</em> Strain

“…The fermentation of very high gravity (VHG) mediums has for long received considerable attention, as it can increase the fermentation rate and the amount of ethanol produced, reducing capital costs and the risk of bacterial contamination [50][51]. VGH technology promotes less process water and energy requirements, specially promoting considerable savings on the energy for distillation, one of the most energy consuming steps in fuel ethanol production, as well as reducing the volume of vinasse (the resulting residue after distillation of ethanol), which can have significant impacts in the environmental water supply [28,29,52].…”

Integrated Management of Industrial Wastewater in Food Sector

“…Corncob is another food waste that can produce biofuels through pyrolysis. The produced fuel can be employed as a biofuel in addition to producing other valuable chemicals [145]. Three principles govern the circular economy: protecting and enhancing regular capital; the reorganization of resources by remanufacturing, restoring, and reusing materials inside their technical and biological cycles; and, finally, the utilization of food manufacturing byproducts and nutrients [146].…”