Rate-constraining changes in surface properties, porosity and hydrolysis kinetics of lignocellulose in the course of enzymatic saccharification

Abstract

BackgroundExplaining the reduction of hydrolysis rate during lignocellulose hydrolysis is a challenge for the understanding and modelling of the process. This article reports the changes of cellulose and lignin surface areas, porosity and the residual cellulase activity during the hydrolysis of autohydrolysed wheat straw and delignified wheat straw. The potential rate-constraining mechanisms are assessed with a simplified kinetic model and compared to the observed effects, residual cellulase activity and product inhibition.ResultsThe reaction rate depended exclusively on the degree of hydrolysis, while enzyme denaturation or time-dependent changes in substrate hydrolysability were absent. Cellulose surface area decreased linearly with hydrolysis, in correlation with total cellulose content. Lignin surface area was initially decreased by the dissolution of phenolics and then remained unchanged. The dissolved phenolics did not contribute to product inhibition. The porosity of delignified straw was decreased during hydrolysis, but no difference in porosity was detected during the hydrolysis of autohydrolysed straw.ConclusionsAlthough a hydrolysis-dependent increase of non-productive binding capacity of lignin was not apparent, the dependence of hydrolysis maxima on the enzyme dosage was best explained by partial irreversible product inhibition. Cellulose surface area correlated with the total cellulose content, which is thus an appropriate approximation of the substrate concentration for kinetic modelling. Kinetic models of cellulose hydrolysis should be simplified enough to include reversible and irreversible product inhibition and reduction of hydrolysability, as well as their possible non-linear relations to hydrolysis degree, without overparameterization of particular factors.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0431-3) contains supplementary material, which is available to authorised users.

Testing scientific models using Qualitative Reasoning: Application to cellulose hydrolysis

“…The former derives from the common belief that the viability of a model structure increases with the number and diversity of the observations it can explain 2 . The latter has been more often addressed in the domain via comparative analysis of concurrent kinetics models of (ligno)cellulose hydrolysis 19 , 38 – 40 . Comparative analysis helps pointing out processes that can best account for the observed kinetics, e.g.…”

“…linear vs quadratic relation) affect significantly the model outputs, while not being strictly part of the model structure. Thus, comparative analysis studies can report concurrent kinetic models of (ligno)cellulose hydrolysis with equally high goodness of fit 38 , 40 , which is just another illustration of the non-uniqueness of model results and of the difficulty to assess the “veracity” of a mathematical model in natural science 2 . Secondly, experimental data are often context-dependent, which has two consequences: it makes the parameterized model also context-dependent and it hinders the use of datasets from different sources.…”

Structural changes of lignin in biorefinery pretreatments and consequences to enzyme-lignin interactions

“…Nevertheless, removal or redistribution of lignin (Chundawat et al 2011;Donohoe et al 2008) and hemicelluloses also simultaneously increases the total volume of enzyme-accessible micropores within the substrate, a factor that correlates with hydrolysability (Torr et al 2016). While pretreatments generally have a major effect on lignin surface area and chemistry, only minor changes have been observed in surface-displayed phenolic OH groups during enzymatic hydrolysis, regardless of the degree of saccharification (Pihlajaniemi et al 2016a). State of the art imaging techniques and surface sensitive chemical analysis procedures should be taken into wider use to analyse the surface distribution of lignin.…”

Structural changes of lignin in biorefinery pretreatments and consequences to enzyme-lignin interactions - OPEN ACCESS

“…Nevertheless, removal or redistribution of lignin (Chundawat et al 2011;Donohoe et al 2008) and hemicelluloses also simultaneously increases the total volume of enzyme-accessible micropores within the substrate, a factor that correlates with hydrolysability (Torr et al 2016). While pretreatments generally have a major effect on lignin surface area and chemistry, only minor changes have been observed in surface-displayed phenolic OH groups during enzymatic hydrolysis, regardless of the degree of saccharification (Pihlajaniemi et al 2016a). State of the art imaging techniques and surface sensitive chemical analysis procedures should be taken into wider use to analyse the surface distribution of lignin.…”