A very high gravity (VHG) repeated-batch fermentation system using an industrial strain of Saccharomyces cerevisiae PE-2 (isolated from sugarcane-to-ethanol distillery in Brazil) and mimicking industrially relevant conditions (high inoculation rates and low O(2) availability) was successfully operated during fifteen consecutive fermentation cycles, attaining ethanol at 17.1 ± 0.2% (v/v) with a batch productivity of 3.5 ± 0.04 g l(-1) h(-1). Moreover, this innovative operational strategy (biomass refreshing step) prevented critical decreases on yeast viability levels and promoted high accumulation of intracellular glycerol and trehalose, which can provide an adaptive advantage to yeast cells under harsh industrial environments. This study contributes to the improvement of VHG fermentation processes by exploring an innovative operational strategy that allows attaining very high ethanol titres without a critical decrease of the viability level thus minimizing the production costs due to energy savings during the distillation process.

Increasing environmental and sustainability concerns, caused by current population growth, has promoted a raising utilization of renewable bio-resources for the production of materials and energy. Recently, nanocellulose (NC) has been receiving great attention due to its many attractive features such as non-toxic nature, biocompatibility, and biodegradability, associated with its mechanical properties and those related to its nanoscale, emerging as a promising material in many sectors, namely packaging, regenerative medicine, and electronics, among others. Nanofibers and nanocrystals, derived from cellulose sources, have been mainly produced by mechanical and chemical treatments; however, the use of cellulases to obtain NC attracted much attention due to their environmentally friendly character. This review presents an overview of general concepts in NC production. Especial emphasis is given to enzymatic hydrolysis processes using cellulases and the utilization of pulp and paper industry residues. Integrated process for the production of NC and other high-value products through enzymatic hydrolysis is also approached. Major challenges found in this context are discussed along with its properties, potential application, and future perspectives of the use of enzymatic hydrolysis as a pretreatment in the scale-up of NC production.

On a near future, bio-based economy will assume a key role in our lives. Lignocellulosic materials (e.g., agroforestry residues, industrial/solid wastes) represent a cheaper and environmentally friendly option to fossil fuels. Indeed, following suitable processing, they can be metabolized by different microorganisms to produce a wide range of compounds currently obtained by chemical synthesis. However, due to the recalcitrant nature of these materials, they cannot be directly used by microorganisms, the conversion of polysaccharides into simpler sugars being thus required. This conversion, which is usually undertaken enzymatically, represents a significant part on the final cost of the process. This fact has driven intense efforts on the reduction of the enzyme cost following different strategies. Here, we describe the fundamentals of the enzyme recycling technology, more specifically, cellulase recycling. We focus on the main strategies available for the recovery of both the liquid- and solid-bound enzyme fractions and discuss the relevant operational parameters (e.g., composition, temperature, additives, and pH). Although the efforts from the industry and enzyme suppliers are primarily oriented toward the development of enzyme cocktails able to quickly and effectively process biomass, it seems clear by now that enzyme recycling is technically possible.

BackgroundThe optimization of industrial bioethanol production will depend on the rational design and manipulation of industrial strains to improve their robustness against the many stress factors affecting their performance during very high gravity (VHG) or lignocellulosic fermentations. In this study, a set of Saccharomyces cerevisiae genes found, through genome-wide screenings, to confer resistance to the simultaneous presence of different relevant stresses were identified as required for maximal fermentation performance under industrial conditions.ResultsChemogenomics data were used to identify eight genes whose expression confers simultaneous resistance to high concentrations of glucose, acetic acid and ethanol, chemical stresses relevant for VHG fermentations; and eleven genes conferring simultaneous resistance to stresses relevant during lignocellulosic fermentations. These eleven genes were identified based on two different sets: one with five genes granting simultaneous resistance to ethanol, acetic acid and furfural, and the other with six genes providing simultaneous resistance to ethanol, acetic acid and vanillin. The expression of Bud31 and Hpr1 was found to lead to the increase of both ethanol yield and fermentation rate, while Pho85, Vrp1 and Ygl024w expression is required for maximal ethanol production in VHG fermentations. Five genes, Erg2, Prs3, Rav1, Rpb4 and Vma8, were found to contribute to the maintenance of cell viability in wheat straw hydrolysate and/or the maximal fermentation rate of this substrate.ConclusionsThe identified genes stand as preferential targets for genetic engineering manipulation in order to generate more robust industrial strains, able to cope with the most significant fermentation stresses and, thus, to increase ethanol production rate and final ethanol titers.