In the recent article of Dr. Alankar Vaidya and colleagues from Scion which is a Crown Research Institute dedicated to improving the international competitiveness of the New Zealand forest industry. The authors describe an investigation of wet vibratory ball milling as an effective pretreatment, before the enzymatic saccharification of the most recalcitrant lignocellulosic substrates such as Pinus radiata softwood using relatively low enzyme inputs. In this study, wet ball milling mechanism is proposed (see the Figure), based on the observed micromorphological changes and changes in the cellulose crystallinity with different ball milling time. A mathematical model validates the proposed ball milling mechanism. This research work is now published in the journal, Bioresource Technology. According to the team, radiata pine wood chips obtained from a sawmill were treated with steam under pressure and then refined to produce substrate that was diluted with water prior to the ball milling.
One third of the glucan was converted to glucose at enzyme loadings as low as 2 FPU/g of dry substrate, after 60 minutes of ball milling. They attributed this rapid enzymatic conversion of cellulose exposed in fibrils torn from cell walls. They use scanning electron microscopy to characterize micromorphological changes in the substrates that were ball milled for different times. Based on the observed micromorphological changes, the cellulose present in the substrate is assigned to different categories. Carbon-13 NMR spectroscopy was also used in their study to investigate whether exposure of cellulose in the crystallites changed during the ball milling process.
Alankar A. Vaidya the first author in the paper said, ball milling performed for less than 120 min showed extensive fiber breakage and defibrillation of the broken fibers exposing more and more cellulose to enzymatic hydrolysis. However, over-milling caused compression of the porous fragments to compact globular particles with a granular texture, decreasing accessibility of enzymes to cellulose. From the carbon-13 NMR spectroscopy investigations, partial loss of interior cellulose in crystallites occur when the fiber breakage was completed. The authors found that wet ball milling performed for optimum time can provide highly digestible softwood substrate at low enzyme loadings. At a low enzyme loading of 2 FPU/g of substrate and milling time of 120 min a total monomeric sugar yield of 306 g/kg of substrate was obtained which is higher than conventional pretreatment method such as steam exploded wood.
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