Ethanol production from rape straw by a two-stage pretreatment under mild conditions.

The growing interest on rape oil as raw material for biodiesel production has resulted in an increasing availability of rape straw, an agricultural residue that is an attractive renewable source for the production of second-generation bioethanol. Pretreatment is one of the key steps in such a conversion process. In this work, a sequential two-stage pretreatment with dilute sulfuric acid (130 °C, 60 min, 2% w/v H2SO4) followed by H2O2 (1-5% w/v) in alkaline medium (NaOH) at low temperature (60, 90 °C) and at different pretreatment times (30-90 min) was investigated. The first-acid stage allows the solubilisation of hemicellulose fraction into fermentable sugars. The second-alkaline peroxide stage allows the delignification of the solid material whilst the cellulose remaining in rape straw turned highly digestible by cellulases. Simultaneous saccharification and fermentation with 15% (w/v) delignified substrate at 90 °C, 5% H2O2 for 60 min, led to a maximum ethanol production of 53 g/L and a yield of 85% of the theoretical.

Correction: Gómez-Cruz et al. Valorisation of Exhausted Olive Pomace by an Eco-Friendly Solvent Extraction Process of Natural Antioxidants. Antioxidants 2020, 9, 1010.

The author wishes to make the following correction to this paper [...].

Valorisation of Exhausted Olive Pomace by an Eco-Friendly Solvent Extraction Process of Natural Antioxidants.

Exhausted olive pomace (EOP) is the waste generated from the drying and subsequent extraction of residual oil from the olive pomace. In this work, the effect of different aqueous solvents on the recovery of antioxidant compounds from this lignocellulosic biomass was assessed. Water extraction was selected as the best option for recovering bioactive compounds from EOP, and the influence of the main operational parameters involved in the extraction was evaluated by response surface methodology. Aqueous extraction of EOP under optimised conditions (10% solids, 85 ºC, and 90 min) yielded an extract with concentrations (per g EOP) of phenolic compounds and flavonoids of 44.5 mg gallic acid equivalent and 114.9 mg rutin equivalent, respectively. Hydroxytyrosol was identified as the major phenolic compound in EOP aqueous extracts. Moreover, these extracts showed high antioxidant activity, as well as moderate bactericidal action against some food-borne pathogens. In general, these results indicate the great potential of EOP as a source of bioactive compounds, with potential uses in several industrial applications.

Evaluation of steam explosion pre-treatment for enzymatic hydrolysis of sunflower stalks.

Sunflower stalks, a largely available and cheap agricultural residue lacking of economic alternatives, were subjected to steam explosion pre-treatment, the objective being to optimize pre-treatment temperature in the range 180-230°C. Enzymatic hydrolysis performed on the pre-treated solids by a cellulolytic complex (Celluclast 1.5L) and analysis of filtrates were used to select the best pre-treatment temperature. Temperature selection was based on the susceptibility to enzymatic hydrolysis of the cellulose residue and both the cellulose recovery in the solid and the hemicellulose-derived sugars recoveries in the filtrate. After 96h of enzymatic action, a maximum hydrolysis yield of 72% was attained in the water-insoluble fiber obtained after pre-treatment at 220°C, corresponding to a glucose concentration of 43.7g/L in hydrolysis media. Taking into account both cellulose recovery and hydrolysis yield, the maximum value of glucose yield referred to unpretreated raw material was also found when using steam pre-treated sunflower stalks at 220°C, obtaining 16.7g of glucose from 100g of raw material. With regard to the filtrate analysis, most of the hemicellulosic-derived sugars released during the steam pre-treatment were in oligomeric form, the highest recovery being obtained at 210°C pre-treatment temperature. Moreover, the utilisation of hemicellulosic-derived sugars as a fermentation substrate would improve the overall bioconversion of sunflower stalks into fuel ethanol.

Conversion of olive tree biomass into fermentable sugars by dilute acid pretreatment and enzymatic saccharification.

The production of fermentable sugars from olive tree biomass was studied by dilute acid pretreatment and further saccharification of the pretreated solid residues. Pretreatment was performed at 0.2%, 0.6%, 1.0% and 1.4% (w/w) sulphuric acid concentrations while temperature was in the range 170-210 degrees C. Attention is paid to sugar recovery both in the liquid fraction issued from pretreatment (prehydrolysate) and that in the water-insoluble solid (WIS). As a maximum, 83% of hemicellulosic sugars in the raw material were recovered in the prehydrolysate obtained at 170 degrees C, 1% sulphuric acid concentration, but the enzyme accessibility of the corresponding pretreated solid was not very high. In turn, the maximum enzymatic hydrolysis yield (76.5%) was attained from a pretreated solid (at 210 degrees C, 1.4% acid concentration) in which cellulose solubilization was detected; moreover, sugar recovery in the prehydrolysate was the poorest one among all the experiments performed. To take account of fermentable sugars generated by pretreatment and the glucose released by enzymatic hydrolysis, an overall sugar yield was calculated. The maximum value (36.3 g sugar/100 g raw material) was obtained when pretreating olive tree biomass at 180 degrees C and 1% sulphuric acid concentration, representing 75% of all sugars in the raw material. Dilute acid pretreatment improves results compared to water pretreatment.

Xylitol production by Debaryomyces hansenii and Candida guilliermondii from rapeseed straw hemicellulosic hydrolysate.

This study evaluated the possibility of using rapeseed straw hemicellulosic hydrolysate as a fermentation medium for xylitol production. Two yeast strains, namely Debaryomyces hansenii and Candida guilliermondii, were used for this bioconversion process and their performance to convert xylose into xylitol was compared. Additionally, different strategies were evaluated for the hydrolysate detoxification before its use as a fermentation medium. Assays in semi-defined media were also performed to verify the influence of hexose sugars on xylose metabolism by the yeasts. C. guilliermondii exhibited higher tolerance to toxic compounds than D. hansenii. Not only the toxic compounds present in the hydrolysate affected the yeast's performance, but glucose also had a negative impact on their performance. It was not necessary to completely eliminate the toxic compounds to obtain an efficient conversion of xylose into xylitol, mainly by C. guilliermondii (YP/S=0.55g/g and 0.45g/g for C. guilliermondii and D. hansenii, respectively).

Valorisation of olive agro-industrial by-products as a source of bioactive compounds.

A large amount of olive-derived biomass is generated yearly in Spain, which could be used as a potential source of bioactive compounds. The present work evaluates the recovery of natural antioxidants from olive tree pruning (OTP) and olive mill leaves (OML). For this purpose, the effect of different solvents on the total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity was evaluated. The solvent was found to have a significant effect (p < 0.05) on the TPC, TFC, and the DPPH, ABTS, and FRAP activity, affording similar results for the extracts from the two by-products. The extracts obtained using 50% ethanol showed high TPC (23.85 and 27.54 mg GAE/gdw for OTP and OML, respectively) and TFC (52.82 and 52.39 mg RE/gdw for OTP and OML, respectively). Also, the OTP and OML extracts exhibited notable antioxidant activity as measured by the ABTS method (45.96 and 42.71 mg TE/gdw, respectively). Using pyrolysis-gas chromatography/mass spectrometry, 30 bioactive compounds were detected in both extracts. Additionally, UPLC-DAD-ESI-MS allowed the identification of 15 compounds in the samples. Furthermore, the antioxidant extracts were found to inhibit the growth of several food pathogenic bacteria. This research demonstrates that these by-products from olive grove farming are a good source of antioxidant compounds with antibacterial properties, which have potential applications in the food and pharmaceutical industries.

Bifidobacterial growth stimulation by oligosaccharides generated from olive tree pruning biomass.

This work aims to evaluate the prebiotic potential of oligosaccharides (OS) obtained from autohydrolysis of olive tree pruning biomass (OTPB). Two selected fractions (F1 and F2) were characterized and used in in vitro fermentations by two Bifidobacterium spp. (B. adolescentis and B. longum) and one fecal inoculum. The fraction F1 presented a lower average degree of polymerization (DP) mainly with OS ranging from 3 to 6 DP, whereas the fraction F2 corresponded to a pool of unsubstituted and acetylated oligomers with DP between 4 and 19. In the fermentation by Bifidobacterium, F1 supported a higher biomass formation, OS consumption and organic acids production than F2. With the fecal inoculum, the accumulation of organic acids, as the sum of acetate, propionate and butyrate, was similar for F1 and F2 (107 and 101mM, respectively). The bifidobacteria counts also increased during the incubation time for both OS fractions.

Combined acid/alkaline-peroxide pretreatment of olive tree biomass for bioethanol production.

Olive tree biomass (OTB) can be used for producing second generation bioethanol. In this work, extracted OTB was subjected to fractionation using a sequential acid/alkaline oxidative pretreatment. In the first acid stage, the effects of sulfuric acid concentration and reaction times at 130°C were investigated. Up to 71% solubilization of hemicellulosic sugars was achieved under optimized conditions (2.4% H2SO4, 84min). In the second stage, the influence of hydrogen peroxide concentration and process time were evaluated at 80°C. Approximately 80% delignification was achieved under the best operational conditions (7% H2O2, 90min) within the experimental range studied. This pretreatment produced a substrate with 72% cellulose that was highly accessible to enzymatic attack, yielding 82g glucose/100g glucose in delignified OTB. Ethanol production from both hemicellulosic sugars solubilized in the acid pretreatment and glucose from enzymatic hydrolysis of delignified OTB yielded 15g ethanol/100g OTB.

Ethanol production from pretreated olive tree wood and sunflower stalks by an SSF process.

Olive tree wood and sunflower stalks are agricultural residues largely available at low cost in Mediterranean countries. As renewable lignocellulosic materials, their bioconversion may allow both obtaining a value-added product, for fuel ethanol, and facilitating their elimination. In this work, the ethanol production from olive tree wood and sunflower stalks by a simultaneous saccharification and fermentation (SSF) process is studied. As a pretreatment, steam explosion at different temperatures was applied. The water insoluble fractions of steam-pretreated sunflower stalks and steamed, delignified olive tree wood were used as substrates at 10% w/v concentration for an SSF process by a cellulolytic commercial complex and Saccharomyces cerevisiae. After 72-h fermentation, ethanol concentrations up to 30 g/L were obtained in delignified steam-pretreated olive tree wood at 230 degrees C and 5 min. Sunflower stalks pretretated at 220 degrees C and 5 min gave maximum ethanol concentrations of 21 g/L in SSF experiments.

Optimization of uncatalyzed steam explosion pretreatment of rapeseed straw for biofuel production.

Rapeseed straw constitutes an agricultural residue with great potential as feedstock for ethanol production. In this work, uncatalyzed steam explosion was carried out as a pretreatment to increase the enzymatic digestibility of rapeseed straw. Experimental statistical design and response surface methodology were used to evaluate the influence of the temperature (185-215°C) and the process time (2.5-7.5min). According to the rotatable central composite design applied, 215°C and 7.5min were confirmed to be the optimal conditions, considering the maximization of enzymatic hydrolysis yield as optimization criterion. These conditions led to a maximum yield of 72.3%, equivalent to 81% of potential glucose in pretreated solid. Different configurations for bioethanol production from steam exploded rapeseed straw were investigated using the pretreated solid obtained under optimal conditions as a substrate. As a relevant result, concentrations of ethanol as high as 43.6g/L (5.5% by volume) were obtained as a consequence of using 20% (w/v) solid loading, equivalent to 12.4g ethanol/100g biomass.

Pretreatment of olive tree biomass with FeCl3 prior enzymatic hydrolysis.

Olive tree biomass (OTB) is an agricultural residue which can be used as raw material for bioethanol production. OTB was pretreated with 0.05-0.275 M FeCl(3) solutions at 120-180 °C for 0-30 min. Enzymatic hydrolysis yields were used for assessing pretreatment performance. Optimum FeCl(3) pretreatment conditions were found to be 152.6 °C, 0.26 M FeCl(3) for 30 min. Under such conditions, 100% of hemicellulose was removed, and enzymatic hydrolysis of pretreated solids resulted in a yield of 36.6g glucose/100g of glucose in the raw material. Hemicellulosic sugar recovery in the prehydrolysate was 63.2%. Results compare well with those obtained by other pretreatment strategies on the same raw material, confirming FeCl(3) solutions as a new, feasible approach for bioethanol production.

Dilute sulfuric acid pretreatment of sunflower stalks for sugar production.

In this work the pretreatment of sunflower stalks by dilute sulfuric acid is studied. Pretreatment temperature and the concentration of acid solution were selected as operation variables and modified according to a central rotatable composite experimental design. Based on previous studies pretreatment time was kept constant (5 min) while the variation range for temperature and acid concentration was centered at 175°C and 1.25% (w/v) respectively. Following pretreatment the insoluble solids were separated by filtration and further submitted to enzymatic hydrolysis, while liquid fractions were analyzed for sugars and inhibitors. Response surface methodology was applied to analyze results based on the combined severity of pretreatment experiments. Optimized results show that up to 33 g of glucose and xylose per 100g raw material (65% of the glucose and xylose present in the raw material) may be available for fermentation after pretreatment at 167°C and 1.3% sulfuric acid concentration.

Dilute acid pretreatment of rapeseed straw for fermentable sugar generation.

The influence of the main pretreatment variables on fermentable sugar generation from rapeseed straw is studied using an experimental design approach. Low and high levels for pretreatment temperature (140-200 °C), process time (0-20 min) and concentration of sulfuric acid (0.5-2% w/v) were selected according to previous results. Glucose and xylose composition, as well as sugar degradation, were monitored and adjusted to a quadratic model. Non-sugar components of the hydrolysates were also determined. Enzymatic hydrolysis yields were used for assessing pretreatment performance. Optimization based on the mathematical model show that total conversion of cellulose from pretreated solids can be achieved at pretreatment conditions of 200 °C for 27 min and 0.40% free acid concentration. If optimization criteria were based on maximization of hemicellulosic sugars recovery in the hydrolysate along with cellulose preservation in the pretreated solids, milder pretreatment conditions of 144 °C, 6 min and 2% free acid concentration should be used.

Hydrothermal pre-treatment of rapeseed straw.

As a first step for ethanol production from alternative raw materials, rapeseed straw was studied for fermentable sugar production. Liquid hot water was used as a pre-treatment method and the influence of the main pre-treatment variables was assessed. Experimental design and response surface methodology were applied using pre-treatment temperature and process time as factors. The pretreated solids were further submitted to enzymatic hydrolysis and the corresponding yields were used as pre-treatment performance evaluation. Liquid fractions obtained from pre-treatment were also characterized in terms of sugars and no-sugar composition. A mathematical model describing pre-treatment effects is proposed. Results show that enzymatic hydrolysis yields near to 100% based on pretreated materials can be achieved at 210-220 degrees C for 30-50 min, equivalent to near 70% of glucose present in the raw material. According to the mathematical model, a softer pre-treatment at 193 degrees C for 27 min results in 65% of glucose and 39% of xylose available for fermentation.

Liquid hot water pretreatment of olive tree pruning residues.

Olive tree pruning generates an abundant, renewable lignocellulose residue, which is usually burnt on fields to prevent propagation of vegetal diseases, causing economic costs and environmental concerns. As a first step in an alternative use to produce fuel ethanol, this work is aimed to study the pretreatment of olive tree pruning residues by liquid hot water. Pretreatment was carried out at seven temperature levels in the range 170-230 degrees C for 10 or 60 min. Sugar recoveries in both solid and liquid fractions resulting from pretreatment as well as enzymatic hydrolysis yield of the solid were used to evaluate pretreatment performance. Results show that the enzyme accessibility of cellulose in the pretreated solid fraction increased with pretreatment time and temperature, although sugar degradation in the liquid fraction was concomitantly higher.