Front Matter

Most previous work on the lipase applications in lipid modification has been con-

ducted in batch reactors, including those tubes or flasks with shaking or magnetic

stirring. Most of these studies were conducted in solvent systems and/or in milli-

gram/gram levels. One kilogram and up to 20-kg-scale studies were also performed

in solvent-free systems (Xu et al., 1998a). It was found that acyl migration could not

be avoided in batch reactors, especially in large-scale systems. The optimization with

response surface methodology (RSM) was conducted for the production of structured

lipids in batch reactors in the solvent-free system. The undesired acyl migration

could be reduced to lower levels in optimal conditions (Xu et al., 1998d). How-

ever, a certain degree of compromise was required to obtain both high acyl incor-

poration and low acyl migration (Xu et al., 1998e). The cause of strong acyl migra-

tion in batch reactors is mostly due to the longer reaction time needed for a certain

degree of acyl incorporation as the ratio between enzyme and substrate is generally

low. The breakdown of enzyme particles by stirring could be another important

reason for the high level of acyl migration, because most enzyme carriers catalyze

acyl migration (Freeman and Morton, 1966; Fureby, 1995). This is one of the parti-

cular drawbacks when using batch reactors for the production of specific structured

lipids among other common features such as batch operation, high volume, enzyme

loss, extra filtration, etc.

11.6.2 Packed-bed reactors

Packed-bed reactors (PBR) are the most frequently used reactors in commercial-

scale operations for immobilized enzymes. They are best used continuously and

on a large scale so as to minimize labor costs and overheads, and to facilitate con-

trol, resulting in more reproducible product quality compared to batch processes.

PBR require relatively low power input and have the lowest reactor volume because

the highest ratio of enzyme to substrate is maintained in the enzyme bed. This system

will result in the highest reaction rate and the least reaction time needed to reach a

certain extent of conversion. For an ideal PBR, the performance of the reactor can be

described as:

XSKmlnð 1 XÞ¼

kEtR

Ve

¼

kE

VF

ð 20 Þ

whereXis the proportion of substrate converted,Sis the initial substrate concentra-

tion,Kmis the Michaelis constant,kEis the maximum activity of the total enzyme in

the reactor,tRis the residence time,Vis the volume of the packed-bed,eis the

voidage of the bed, andmFis the volumetric flow rate. The characteristic behavior

of the lipase-catalyzed acidolysis between oils and fatty acids in PBR was studied

previously (Mu et al., 1998; Xu et al., 1998b).

One important experimental parameter is the termS/Km,thedimensionless sub-

strate concentration, which is usually defined asb. This term is especially useful

when considering to make a process design and experimental set-up. The term is

derived from the Michaelis – Menten equation as follows:

204 11 Modification of Oils and Fats by Lipase-Catalyzed Interesterification