Introduction
Oligodextran is a dextran derivative with a relatively low molecular
weight (Mw) (< 70 kDa), which has been widely used as a
prebiotic in food and as an antiphlogistic and antioxidant in
pharmaceutical applications 1,2. Production of
oligodextran through enzymatic hydrolysis of industrial grade dextran is
a safe and efficient process that minimizes secondary pollution (e.g.
the generation of large amounts of organic wastewater in traditional
acid hydrolysis) 3,4. The combination of
ultrafiltration (UF) with an enzymatic reactor, however, could purify
the resulting oligodextran and narrow its Mw distribution which in turn
would enhance oligodextran bioactivity 5. Compared
with the traditional enzymatic reactor and membrane separation hybrid
process, which are performed separately, an enzymatic membrane reactor
(EMR) that simultaneously performs enzymatic reaction and membrane
separation enables enzyme reuse, reduces product inhibition, and avoids
excessive hydrolysis of target products 6. Several
researchers have investigated the feasibility of EMR for oligodextran
production 7-10. For example, Torras et al.7 successfully fabricated a UF membrane loaded with
dextranase for manufacturing oligodextrans. The Mw of oligodextran can
be controlled by the loading amount of enzyme 7 and
the molecular weight cut-off (MWCO) of the membrane 8.
In our own previous study 9,10 the membrane fouling
that resulted from the enzyme was found to enable narrowing of membrane
pore size distribution while oligodextran products of uniform Mw
distribution were obtained.
In spite of extensive experimental investigation of EMR performance, few
efforts have been devoted to understanding the interaction between
enzymatic reaction and membrane filtration, yet these are critical for
the performance of the overall EMR process. Establishment of a reliable
model, which describes the transport phenomena of oligodextran and
enzymatic hydrolysis reactions, enables us to systematically investigate
the influences of operating conditions and membrane properties on the
performance of EMR and to optimize reactor design and membrane
fabrication. However, most of previous studies dedicated to modelling
EMR have focused on kinetic modeling and have considered only
mass-transfer mechanisms 11-13 in which the membrane
only serves as a barrier or carrier of enzymes. In these studies the
effect of membrane sieving on the transport mechanisms of the product
has therefore not been taken into account. This is a critical knowledge
gap that needs to be systematically addressed to guide the optimization
of EMR for oligodextran production because the membrane also functions
as a selective barrier in tailoring the molecular weight of the product.
To the best of our knowledge no mathematical model regarding the EMR for
oligodextran production has so far been reported.
The aim of the current study was therefore to develop a mathematical
model to characterize the transport phenomena in EMR and predict EMR
oligodextran production performance on the basis of the kinetics of
dextran hydrolysis, the force balance model, and the stagnant film model
as an approach for achieving process optimization. Our model evaluates
membrane properties (pore size and porosity) and operating conditions
(operating pressure and agitation speed) in relation to filtration
performance. Furthermore, feeding modes (water feeding vs. substrate
feeding) were also explored. Our work elaborates the inherent
relationship between the operating parameters of EMR and its production
efficiency, which can provide important insights into fundamental
understanding and designing of EMR systems.