Thermopressurized diluted phosphoric acid pretreatment of ligno(hemi)cellulose to make free sugars and nutraceutical oligosaccharides

Abstract

Ligno(hemi)cellulosics (L(h)Cs) as sugarcane bagasse and loblolly pine sawdust are currently being used to produce biofuels such as bioethanol and biobutanol through fermentation of free sugars that are often obtained enzymatically. However, this bioconversion requires a pretreatment to solubilize the hemicellulose fractions, thus facilitating the action of the cellulolytic enzymes. Instead of the main free monosaccharides used in these current models, the modulation of thermopressurized orthophosphoric acid as a pretreatment, in the ranges of 3-12 atm and pH 1.5-2.5, can produce nondigestible oligosaccharides (NDOS) such as xylo-oligosaccharides (XOS) because heteroxylan is present in both types of hardwood and softwood hemicelluloses. A comparative thin-layer chromatographic analysis of the hydrolytic products showed the best conditions for NDOS production to be 7 atm/water, pH 2.25 and 2.50, and 8.5 atm/water for both sources. Particular hydrolysates from 7 atm (171 °C) at pHs 2.25 and 2.50 both for cane bagasse and pine sawdust, with respective oligosaccharide contents of 57 and 59 %, once mixed in a proportion of 1:1 for each plant source, were used in vitro as carbon sources for Bifidobacterium or Lactobacillus. Once both bacteria attained the stationary phase of growth, an unforeseen feature emerged: the preference of B. animalis for bagasse hydrolysates and, conversely, the preference of L. casei for pine hydrolysates. Considering the fact that nutraceutical oligosaccharides from both hemicelluloses correspond to higher value-added byproducts, the technology using a much diluted thermopressurized orthophosphoric acid pretreatment becomes an attractive choice for L(h)Cs.

Non-alkaline solubilization of arabinoxylans from destarched wheat bran using hydrothermal microwave processing and comparison with the hydrolysis by an endoxylanase

“…A c c e p t e d M a n u s c r i p t A c c e p t e d M a n u s c r i p t 5 bagasse and pine sawdust [23], with good yields and leading to linear xylans and xylo-1 oligosaccharides.…”

“…glycoproteins might be a hypothesis), whereas galactose was 23 around 1% and mannose and rhamnose were less than 0.5% (not shownfor a more detailed 24 composition, see [21]). These minor saccharides are part of the hemicellulose fraction, which A c c e p t e d M a n u s c r i p t 13 may also contain low amounts of methyl, uronates, acetate and ferulate groups; these were not 1 assessed here because off-topic at this stage of the study.…”

“…For each of the six monosaccharides detected, the calculations were made as if all were 21 monomers, then the content in free monomers (as described above) was subtracted, giving the 22 amounts of non-monomers. These values were then "converted" to the equivalent 23 polymerized masses by multiplying by the ratio: (molar mass of the monomer -18)/(molar A c c e p t e d M a n u s c r i p t 9 mass of the monomer). The sum of those six values and the total free monomers gave the total 1 released saccharides (on a basis of dw of the solubilized matter).…”

“…So, a fine monitoring of the process would be necessary to 22 reach high extraction yields and controlled products. Microwave heating at lab scale has been 23 arising as a good means to predict the effects of heat treatment on a larger scale [31] and 24 furthermore microwaves have been reported as a valuable tool to reduce processing time and A c c e p t e d M a n u s c r i p t 6 energy in biomass treatments. A microwave-assisted extraction of xylooligosaccharides from 1 sugarcane bagasse in acidic media at less than 100 °C was reported [32], however applying 2 pressure enables generally much shorter treatment times and higher yields.…”

“…21 Protein content was determined after Kjeldahl procedure (a factor of 6.25 was used for 22 conversion of nitrogen to crude protein), according to AOAC methods [37]. 23 Hemicellulose, cellulose and lignin contents of DWB were determined (three independent 24 replicates) by the method described by Van Belgium) were added and the pH was adjusted to 5.6 with HCl 1 M. According to product 8 datasheet, the activity was 105 IU/g (1 IU is the amount of enzyme which liberates 1 µmole of 9 reducing sugars, expressed in eq. xylose, from xylan per min at pH 4.5 and 30 °C).…”