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.
Highlights
The application of process engineering strategies to achieve high-productivity fermentation systems is considered a key issue in the bioethanol industry
20.1% (v/v) of ethanol and the glucose residual decreased to 60 g/L. Another example points to the findings previously reported by Laluce et al [60], who conducted an optimization study regarding critical variables on very high gravity (VHG) fermentation, such as sugar concentration, temperature, and inoculum size
Special attention has been given to the physiological stresses undertaken by cells, either from high initial sugar levels or from high final ethanol concentrations
Summary
The application of process engineering strategies to achieve high-productivity fermentation systems is considered a key issue in the bioethanol industry. According to Hu et al [22], ethanol can induce the production of heat shock-like proteins, causing a reduction of mRNA and protein accumulation, and a denaturation of intracellular proteins and glycolytic enzymes, which will directly affect critical metabolic capacities involved in cell growth and fermentation. Another key target structure on yeasts is their cell membrane where ethanol can cause an increase of fluidity, resulting in a loss of membrane integrity [23]. According to Piper et al [24], many of these changes induced from stressful levels of ethanol are identical to those caused by thermal stress
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