Abstract
The aim of this work was to assess the utility of seven different kraft pulps produced from softwood (pine), hardwood (poplar, birch and beech), wheat straw and hemp (bast and harl) as potential sources of sugar feedstocks for fermentation processes. The pulps contained low amounts of hemicelluloses (1.9–8.2% d.w.) and lignin (1.7–15% d.w.). The crystallinity index values ranged from 55% (wheat straw pulp) to 79% (hemp bast pulp), while the average DP varied from around 230 (hemp bast pulp) to 1482 (poplar and birch pulps). The results of enzymatic hydrolysis showed that not only the residual lignin content but also the cellulose crystallinity index decided on the sugar yields while the average polymerization degree had a weak impact. More reducing sugars were obtained from the hardwood pulps and wheat straw pulp (100% d.w.) than from the pine pulp (around 89% d.w.) and two hemp pulps (40.5% d.w. and 44.7% d.w. from the bast and harl pulps, respectively). Glucose was the dominating (69–79% w/w) soluble sugar in enzymatic hydrolysates of the pulps. The sugar profiles of these hydrolysates make them suitable sugar feedstocks for fermentation processes.
Highlights
Various types of lignocellulosic biomass have been increasingly used as renewable raw materials for the production of fuels and chemicals because of the depletion of fossil fuels, accumulation of greenhouse gases and other problems related to theWood Sci Technol (2017) 51:873–885 growing pollution of natural environment
Kraft pulping is a severe and highly efficient technology of lignocellulosic biomass pretreatment that was proved by the high cellulose content in the seven kraft pulps, ranging from around 76% d.w. for the hemp harl pulp to around 96% for the hemp bast pulp, and the low concentrations of extractives (Table 3)
According to the latter authors, cellulose degradability increased with the decrease in the amount of xylan remaining after lignocellulosic biomass pretreatment
Summary
Various types of lignocellulosic biomass have been increasingly used as renewable raw materials for the production of fuels and chemicals because of the depletion of fossil fuels, accumulation of greenhouse gases and other problems related to theWood Sci Technol (2017) 51:873–885 growing pollution of natural environment. The principal factor deciding on the use of plant biomass for conversion to sugar feedstocks for fermentation processes is the overall content of cellulose and hemicelluloses (holocellulose) and the percentage of lignin. The latter, highly cross-linked polymer protects fibers of cellulose and hemicelluloses from the attack of hydrolytic enzymes. Delignification of vegetal biomass is carried out using various chemical, physicochemical and biological pretreatments (Chaturvedi and Verma 2013; Singh et al 2016). Another objective of pretreatment, apart from partial lignin removal, is reduction in cellulose crystallinity, to make it more susceptible to enzymatic degradation. Because of the high lignin level, varying between 16 and 28% d.w., each of these materials requires delignification before enzymatic digestion
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