Abstract
Increasing demand and uncertain availability of fossil
fuels urge us to find alternative resources available in
large quantities especially for the petrol-based
transportation sector. Lignocellulosic biomass, available
worldwide in plant cell walls, is a promising alternative
feedstock. It can be depolymerised to sugar monomers,
which provide potential raw material for sugar
platform-based production of fuels and chemicals.
However, the enzymatic saccharification of lignocellulose
to platform sugars is hindered primarily by the
complexity of lignocellulosic substrates as well as by
the performance of the hydrolytic enzymes involved. This
study focuses on various rate limiting factors such as
the decrease in the reactivity and accessibility of the
substrates which slow down the hydrolysis, on auxiliary
enzymes needed for the efficient solubilisation of
cellulose, as well as on the adsorption of enzymes.
Consequently, solutions to these limitations were sought
to improve the efficiency of biomass conversion
processes.
Following the morphological and structural changes in the
substrate during hydrolysis revealed that the average
crystal size and crystallinity of cellulose remained
constant while particle size generally decreased (Paper
I). In particular, cellulose microfibrils were proposed
to be hydrolysed one-by-one in fibre aggregates by
peeling off cellulose chains layer-by-layer from the
outer crystals of microfibril aggregates. Microscopic
observation showed that almost intact particles remained
in the residue even after 60% conversion.
Lignocellulose is a complex network of lignin and
polysaccharides. Lignin was found to impede the
hydrolysis of cellulose, and its extensive removal
doubled the conversion yields of softwood (Paper II). On
the other hand, accumulation of lignin during hydrolysis
did not affect hydrolysability by commercial cellulase
preparations. Residual hemicelluloses, especially
glucomannan, were resistant to enzymatic hydrolysis but
could be removed together with lignin during
delignification. This suggests that especially
glucomannans are bound to lignin as lignin-carbohydrate
complexes. In addition, cellulose, xylan and glucomannan
were shown to be structurally interlinked in softwood
(Paper IV). The hydrolysis yield of these polysaccharides
remained below 50% without the simultaneous hydrolysis of
all polysaccharides. Synergism between the solubilisation
of cellulose and hemicelluloses was found, and the
release of glucose, xylose and mannose was in linear
correlation.
The adsorption and desorption of enzymes were followed
during hydrolysis (Paper III). After a quick initial
adsorption, slow desorption and re-adsorption of enzymes
was observed in alkaline delignified spruce. On the other
hand, unproductive adsorption to lignin as well as enzyme
inactivation was predicted to play a primary role in the
irreversible adsorption of cellulases in steam pretreated
spruce or Avicel during hydrolysis when no desorption of
cellulases could be detected.
This study showed for the first time that increasing
substrate concentration could compensate for the absence
of carbohydrate binding modules (CBMs) in hydrolytic
enzymes (Paper V). The performance of cellulases lacking
CBMs was comparable to that of cellulases comprising CBM
at 20% substrate concentration. At the same time, over
60% of the enzymes without CBMs could be recovered at the
end of the hydrolysis. Thus, the major part of hydrolytic
enzymes without CBMs could potentially be recovered in
industrial high consistency processes.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 9 Nov 2012 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-7936-5 |
Electronic ISBNs | 978-951-38-7937-2 |
Publication status | Published - 2012 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- lignocellulose
- accessibility
- enzymatic hydrolysis
- Trichoderma reesei
- glycoside hydrolases
- cellulases
- xylanases
- mannanases
- enzyme adsorption
- CBM