Mercedes Ballesteros

Nowadays, the use of biofuels has become an unavoidable solution to the depletion of fossil fuels and global warming. The controversy over the use of food crops for the production of the first-generation biofuels and deforestation caused by the second-generation ones has forced the transition to the third generation of biofuels, which avoids the use of arable land and edible products, and does not threaten biodiversity. This generation is based on the marine and freshwater biomass, which has the advantages of being abundant or even invasive, easy to cultivate and having a good energetic potential. Bioethanol production from Ulva lactuca, a local marine macroalgae collected from the west coast of Algiers, was examined in this study. Ulva lactuca showed a good energetic potential due to its carbohydrate-rich content: 9.57% of cellulose, 6.9% of hemicellulose and low lignin content of 5.11%. Ethanol was produced following the separate hydrolysis and fermentation process (SHF), preceded by a thermal acid pretreatment at 120 °C during 15 min. Enzymatic hydrolysis was performed using a commercial cellulase (Celluclast 1.5 L), which saccharified the cellulose contained in the green seaweed, releasing about 85.01% of the total glucose, corresponding to 7.21 g/L after 96 h of enzymatic hydrolysis at pH 5 and 45 °C. About 3.52 g/L of ethanol was produced after 48 h of fermentation using Saccharomyces cerevisiae at 30 °C and pH 5, leading to a high ethanol yield of 0.41 g of ethanol/g of glucose.

The present work reviews the role of biogas as advanced biofuel in the renewable energy system, summarizing the main raw materials used for biogas production and the most common technologies for biogas upgrading and delving into emerging biological methanation processes. In addition, it provides a description of current European legislative framework and the potential biomethane business models as well as the main biogas production issues to be addressed to fully deploy these upgrading technologies. Biomethane could be competitive due to negative or zero waste feedstock prices, and competitive to fossil fuels in the transport sector and power generation if upgrading technologies become cheaper and environmentally sustainable.

Lignocellulosic ethanol production requires high substrate concentrations for its cost-competitiveness. This implies the presence of high concentrations of insoluble solids (IS) at the initial stages of the process, which may limit the fermentation performance of the corresponding microorganism. The presence of 40–60% IS (w/w) resulted in lower glucose consumption rates and reduced ethanol volumetric productivities of Saccharomyces cerevisiae F12. Yeast cells exposed to IS exhibited a wrinkled cell surface and a reduced mean cell size due to cavity formation. In addition, the intracellular levels of reactive oxygen species (ROS) increased up to 40%. These ROS levels increased up to 70% when both lignocellulose-derived inhibitors and IS were simultaneously present. The general stress response mechanisms (e.g. DDR2, TPS1 or ZWF1 genes, trehalose and glycogen biosynthesis, and DNA repair mechanisms) were found repressed, and ROS formation could not be counteracted by the induction of the genes involved in repairing the oxidative damage such as glutathione, thioredoxin and methionine scavenging systems (e.g. CTA1, GRX4, MXR1, and TSA1; and the repression of cell cycle progression, CLN3). Overall, these results clearly show the role of IS as an important microbial stress factor that affect yeast cells at physical, physiological, and molecular levels.

The determination of chemical composition of lignocellulose biomass by wet chemistry analysis is labor-intensive, expensive, and time consuming. Near infrared (NIR) spectroscopy coupled with multivariate calibration offers a rapid and no-destructive alternative method. The objective of this work is to develop a NIR calibration model for olive tree lignocellulosic biomass as a rapid tool and alternative method for chemical characterization of olive tree pruning over current wet methods. In this study, 79 milled olive tree pruning samples were analyzed for extractives, lignin, cellulose, hemicellulose, and ash content. These samples were scanned by reflectance diffuse near infrared techniques and a predictive model based on partial least squares (PLS) multivariate calibration method was developed. Five parameters were calibrated: Lignin, cellulose, hemicellulose, ash, and extractives. NIR models obtained were able to predict main components composition with R²_cv values over 0.5, except for lignin which showed lowest prediction accuracy.

Cassava peels provide a cheap non-food biomass waste that can be hydrolyzed to simple sugars as a useful feedstock. Unlike most crop wastes, they have high starch content as well as lignocellulose. In this study, an enzymatic treatment of cassava peels by various concentrations of amylase and glucoamylase is considered. Steam explosion pre-treatments reduced rate and yield of hydrolysis. Milled peels suspended at 10% w/v yielded a maximum reducing sugar of 0.41 g (as glucose) per gram of peels. HPLC analysis showed that levels of soluble oligosaccharides remained low throughout. A pretreatment with amylase at 95 °C slightly increased rates although final yield was the same. Additional treatment with cellulolytic enzymes increases the total hydrolysis yield to 0.61 g (as glucose) per gram of peels representing 91% of the carbohydrate in cassava peels.

Eucalyptus biomass was studied as a feedstock for sugars release using an alkaline extrusion plus a neutralization-based pretreatment. This approach would be a first step in a bioconversion process aimed at obtaining fuel bioethanol from eucalyptus biomass. The best operation conditions of extrusion (screw speed, temperature, liquid to solid ratio and NaOH amount) that lead to an effective destructuration of lignocellulose and enhanced sugar release were investigated. Two process configurations, with and without filtration inside the extruder, were tested. In the case without filtration, washed and not washed extrudates were compared. It was demonstrated that filtration step was convenient to remove inorganic salts resulting from neutralization and to promote the mechanical effect of extrusion, but limitations in the machine used in the work prevented testing of temperatures above 100 °C using this configuration. In the no filtration strategy, a temperature of 150 °C allowed attaining the highest glucan and xylan conversion rates by enzymatic hydrolysis of extruded biomass, almost 40% and 75%, respectively, of the maximum yield that could be attained if all carbohydrates contained in raw eucalyptus were converted to sugars. Some of the mechanisms and individual effects underlying alkaline extrusion of eucalyptus were figured out in this work, providing guidelines for a successful pretreatment design that needs to be further studied.

Lactic acid is a versatile chemical that can be produced via fermentation of lignocellulosic materials. The heterolactic strain Lactobacillus pentosus CECT 4023 T, that can consume glucose and xylose, was studied to produce lactic acid from steam exploded wheat straw prehydrolysate. The effect of temperature and pH on bacterial growth was analyzed. Besides, the effect of oxygen on lactic acid production was tested and fermentation yields were compared in different scenarios. This strain showed very high tolerance to the inhibitors contained in the wheat straw prehydrolysate. The highest lactic acid yields based on present sugar, around 0.80 g g ⁻¹ , were obtained from glucose in presence of 25%, 50%, and 75% v v ⁻¹ of prehydrolysate in strict anaerobiosis. Lactic fermentation of wheat straw hydrolysate obtained after enzymatic hydrolysis of the prehydrolysate yielded 0.39 g of lactic acid per gram of released sugars, which demonstrated the high potential of L. pentosus to produce lactic acid from hemicellulosic hydrolysates. Results presented herein not only corroborated the ability of L. pentosus to grow using mixtures of sugars, but also demonstrated the suitability of this strain to be applied as an efficient lactic acid producer in a lignocellulosic biorefinery approach. © 2018 American Institute of Chemical Engineers Biotechnol. Prog ., 35: e2739, 2019

Olive tree crops, extensively cultivated in Southern European countries, yield large amounts of olive tree pruning (OTP) biomass. This could be used within the framework of a bio-based economy that maximizes the utilization of biomass resources in a sustainable way. In the present work, the techno-economic feasibility of an OTP-based integrated biorefinery is evaluated by the process simulation software Aspen Plus, while the process is aimed at the production of ethanol, xylitol, antioxidants and electricity. Overall, the proposed plant could perform economically, and it is self-sufficient from an energy resource point of view. The plant as designed yields around 109 l of ethanol, 27 kg of xylitol and 43 kg of antioxidants per ton of OTP biomass, with an estimated production cost of 0.24 € l⁻¹, 1.48 € kg⁻¹ and 5.12 € kg⁻¹, respectively. In a 10-year period, the economic profitability of the biorefinery plant is within a positive investment balance, with a net present value (NPV) of 32.1 M€ and a payback period of 5–6 years. These figures point out the opportunities for placing in the market several OTP-based products. Based on these data, the construction of small-scale OTP-based lignocellulosic biorefineries seems to be a realistic scenario.