Steam explosion treatment for ethanol production from branches pruned from pear trees by simultaneous saccharification and fermentation

Assessment of different Bacillus coagulans strains for l-lactic acid production from defined media and gardening hydrolysates: Effect of lignocellulosic inhibitors

Cellulose valorisation has been successfully addressed for years. However, the use of hemicellulosic hydrolysates is limited due to the presence of C5-sugars and inhibitors formed during pretreatment. Bacillus coagulans is one of the few bacteria able to utilize both C6- and C5-sugars to produce l-lactic acid, but its susceptibility to the lignocellulosic inhibitors needs further investigation. For such a purpose, the tolerance of different B. coagulans strains to increasing concentrations of inhibitors is studied. The isolated A162 strain reached the highest l-lactic acid productivity in all cases (up to 2.4 g L^-1  h^-1), even in presence of 5 g L^-1 of furans and phenols. Remarkably, most of furans and phenolic aldehydes were removed from defined media and hemicellulosic gardening hydrolysate after fermentation with A162. Considering the high productivities and the biodetoxifying effect attained, A162 could be pointed out as a great candidate for valorisation of mixed sugars from hemicellulosic hydrolysates with high inhibitors concentration, promoting the implementation of lignocellulosic biorefineries.

Efficient utilization of hydrolysates from steam-exploded gardening residues for lactic acid production by optimization of enzyme addition and pH control

The expansion of urban green areas has boosted the accumulation of gardening lignocellulosic residues that could be potentially used to produce platform chemicals like lactic acid. However, when using lignocelluloses, pretreatment step, such as steam explosion, is often needed to favour sugar release. Considering that the conversion of glucose from cellulose has been widely addressed, this work is focused on the valorisation of the steam-exploded gardening liquid fraction rich in hemicellulosic sugars. Since oligomeric sugars are usually solubilized during steam explosion, an enzymatic hydrolysis step was required in some cases to increase the monosaccharides content. Although the presence of inhibitors released during pretreatment (e.g. formic acid) hindered hydrolysis yields, the addition of hemicellulases and the enzyme dosage optimization resulted in 85%, 89% and 95% of glucose, xylose and arabinose release from soluble oligomers, respectively. Lactobacillus pentosus CECT4023T was used for lactic acid fermentation of C6 and C5 sugars from the hydrolysate with the highest sugars concentration, that did not require enzymatic hydrolysis. Xylose consumption was hampered due to the inhibitory effect of acids that produced pH drop. Different pH control systems were applied and automatic NaOH addition in bioreactor resulted in 21 g L^-1 of lactic acid (95% of the maximum theoretical yield) that implied 44% increase in lactic acid production when compared with flask fermentation. These results provide new insights for the valorisation of emerging lignocellulosic materials like gardening residues into high added-value products.

Review of Second Generation Bioethanol Production from Residual Biomass

In the context of climate change and the depletion of fossil fuels, there is a great need for alternatives to petroleum in the transport sector. This review provides an overview of the production of second generation bioethanol, which is distinguished from the first generation and subsequent generations of biofuels by its use of lignocellulosic biomass as raw material. The structural components of the lignocellulosic biomass such as cellulose, hemicellulose and lignin, are presented along with technological unit steps including pretreatment, enzymatic hydrolysis, fermentation, distillation and dehydration. The purpose of the pretreatment step is to increase the surface area of carbohydrate available for enzymatic saccharification, while minimizing the content of inhibitors. Performing the enzymatic hydrolysis releases fermentable sugars, which are converted by microbial catalysts into ethanol. The hydrolysates obtained after the pretreatment and enzymatic hydrolysis contain a wide spectrum of sugars, predominantly glucose and xylose. Genetically engineered microorganisms are therefore needed to carry out co-fermentation. The excess of harmful inhibitors in the hydrolysate, such as weak organic acids, furan derivatives and phenol components, can be removed by detoxification before fermentation. Effective saccharification further requires using exogenous hemicellulases and cellulolytic enzymes. Conventional species of distiller's yeast are unable to ferment pentoses into ethanol, and only a very few natural microorganisms, including yeast species like Candida shehatae, Pichia (Scheffersomyces) stipitis, and Pachysolen tannophilus, metabolize xylose to ethanol. Enzymatic hydrolysis and fermentation can be performed in a number of ways: by separate saccharification and fermentation, simultaneous saccharification and fermentation or consolidated bioprocessing. Pentose-fermenting microorganisms can be obtained through genetic engineering, by introducing xylose-encoding genes into metabolism of a selected microorganism to optimize its use of xylose accumulated in the hydrolysate.

Impact of co-pretreatment of calcium hydroxide and steam explosion on anaerobic digestion efficiency with corn stover

Anaerobic digestion (AD) is an effective way to utilize the abundant resource of corn stover (CS). In this light, Ca(OH)₂ pretreatment alone, steam explosion (SE) pretreatment alone, and co-pretreatment of Ca(OH)₂ and SE were applied to improve the digestion efficiency of CS. Results showed that AD of co-pretreated CS with 1.0% Ca(OH)₂ and SE at 1.5 MPa achieved the highest cumulative methane yield of [Formula: see text], which was 61.54% significantly higher (p < .01) than untreated CS. The biodegradability value of CS after co-pretreatment enhanced from 43.03% to 69.52%. Methane yield could be well fitted by the first-order model and the modified Gompertz model. In addition, composition and structural changes of CS after pretreatment were analyzed by a fiber analyzer, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The validated results indicated that co-pretreatment of Ca(OH)₂ and SE was efficient to improve the digestion performance of CS and might be a suitable method for agricultural waste pretreatment in the future AD industry.

Conversion of steam-exploded cedar into ethanol using simultaneous saccharification, fermentation and detoxification process

In this study, we investigated the simultaneous saccharification, fermentation and detoxification SSDF process of steam-exploded cedar using a detoxification microorganism, Ureibacillus thermosphaericus A1, to facilitate efficient ethanol production. Steam explosion was applied as a pretreatment before enzymatic saccharification followed by alcohol fermentation. The highest glucose conversion rate was observed in the sample pretreated with a steam pressure of 45atm for 5min. Alcohol production by a heat-tolerant yeast, Saccharomyces cerevisiae BA11, was inhibited strongly by inhibitory materials present in the steam-exploded cedar, such as formic acid, furfural, and 5-hydroxymethylfurfural. The maximum amount of ethanol, i.e., 0.155g ethanol/g dry steam-exploded cedar, which corresponded to 74% of the theoretical ethanol yield, was obtained using the SSDF when U. thermosphaericus A1 degraded the inhibitory materials. A fed batch SSDF culture, in which U. thermosphaericus A1 was used to maintain low concentrations of inhibitory materials, was effective for increasing the ethanol concentration.