Front Matter

11.7 Control of water content/activity

The control of water content or water activity is a key factor of lipase-catalyzed

interesterification in the microaqueous biocatalysis system. As discussed in Section

12.3, water not only maintains the activity of lipases but also participates in the

interesterification. The optimal control of water content affects the reaction rate,

product yield, product selectivity, and operational stability. A general profile of ef-

fects of water content on the first two variables is shown in Figure 9. This shows that

at lower water content, the yield of the product may be high, but the rate of reaction

may be lower. In contrast, at higher water content the rate of reaction becomes higher,

but the yield drops (Yamane, 1987). However, there will be an optimal water content

for the reaction from the engineering point of view. Water content in the reaction

system is directly related to the content of diacylglycerols and the degree of acyl

migration (Xu et al., 1998d; 1999). On the other hand, water control during the

operation of the process is very important for the control of lipase stability, as de-

monstrated previously in long-term operations in packed-bed reactors (Xu et al.,

1998b).

The control of water content during lipase-catalyzed interesterification lies in the

removal of water from, and addition to, the system. Removal of water can be per-

formed by vacuum, addition of molecular sieve, or by circulating the liquid through a

silica gel bed (Kyotani et al., 1988b), or bubbling the liquid with inert gas for la-

boratory operations. The addition of water to lipases can be conducted by spraying

water directly onto the lipase and conditioning for a certain time, or by on-line in-

jection and mixing for batch and continuous bioreactors. In laboratory experiments,

the water content control for continuous operation in packed-bed reactors was con-

ducted by mixing the water in substrate (Xu et al., 1998b) or substrate passing

through a water-saturated silica gel bed (Hansen and Eigtved, 1987). For monitoring

and automatic controls of water content, a water sensor connected to the reaction

system is required.

Water activityis a dynamic parameter for an enzymatic reaction system. Lipases

were shown to have similar water activity profiles in different reactions (Wehtje and

Adlercreutz, 1997). In another study, it was found that reaction rate with suspended

Lipozyme IM showed similar dependence on water activity in different organic sol-

vents (Valivety et al., 1992). The control of water activity for a lipase-catalyzed

reaction system is, however, practically difficult due to mass transfer limitations

of water vapor diffusion, especially for continuous operations in packed-bed reac-

tors. Halling (1994) studied the theoretical background and practical control of water

activity in enzymatic reactions. Water activity in laboratory experiments is usually

regulated by saturated salt solutions or salt pairs (Halling, 1994). The control of

water activity is normally performed before reaction takes place, as control during

the reaction is much more difficult for a number of reasons. Vaidya (1996) described

several types of reactor configurations for the continuous enzymatic esterification

within situwater activity control. Wehtje et al. (1993) also described a method

in which a silicone tubing was used to control the water activity of the reaction

mixture continuously during biocatalysis in organic media. However, these systems

are rather limited to small-scale operations or to certain applications. A practical

system for water activity control requires further investigation for large-scale bio-

11.7 Control of water content/activity 209