Food Lipids: Chemistry, Nutrition, and Biotechnology

vinyl chloride. In a membrane such as microporous polypropylene, the pores have
dimensions of 0.075 by 0.15 �m and the fibers have an internal diameter of 400
�m, providing 18 m 2 of surface area per gram of membrane (82). With a hydrophilic
membrane such as cellulose, the oil phase circulates through the inner fiber side
whereas the aqueous components circulate on the shell side (63). Immobilization of
lipase can be accomplished by submerging the fibers in ethanol, rinsing them in
buffer, then submerging them in lipase solution (82). Another method involves dispersing
the enzyme in the oil phase and using ultrafiltration to deposit the lipase on
the inner fiber side. One of the substrates can diffuse through the membrane toward
the interface where the enzyme is immobilized. van der Padt et al. (63) used hollow
fibers made from cellulose to perform gylcerolysis of decanoic acid. Using a hydrophilic
membrane bioreactor, the lipase activity was similar to the activity in emulsion
systems. The hydrophilic membrane was found to be more effective for glycerolysis
since the lipase was immobilized on the oil phase side, with the membrane preventing
it from diffusing into the glycerol phase and being lost. Hoq et al. (96) used a
hydrophobic polypropylene membrane to esterify oleic acid and glycerol. The lipase
was adsorbed on the glycerol side, resulting in the loss of some enzyme in this
phase. Therefore, use of a hydrophobic membrane would require the addition of
more lipase to prevent losses in activity (7,64). Membrane reactors have been used
in glycerolysis and acidolysis reactions and have an advantage over more conventional
stirred tank reactors in that the reaction and separation of substrates and product
can be accomplished in one system. Having the substrates and products separated
during the reaction is especially useful during the esterification reaction where water
is produced. Hoq et al. (96,97) found that during esterification of oleic acid and
glycerol, the excess water produced could be removed by passing the oleic acid
stream through molecular sieves, thereby preventing losses in productivity from
hydrolysis.
E. Fluidized Bed Reactor
Fluidized bed reactors are reactors in which the immobilized enzyme and support
are kept suspended by the upward flow of substrate or gas at high flow rates (80)
(Fig. 18). The advantages of fluidized bed reactors are that channeling problems are
eliminated, there is less change in pressure at high flow rates and less coalescence
of emulsion droplets. Also, particulates do not have to removed from the oil and
there are no concentration gradients (7). The main disadvantage of fluidized bed
reactors is that small concentrations of enzyme can be used since a large void volume
is required to keep the enzyme and support suspended. Mojovic et al. (98) used a
gas lift reactor to produce a cocoa butter equivalent by interesterifying palm oil
midfraction. These authors immobilized lipase encapsulated in lecithin reverse micelles
in hexane; the reaction in the gas lift reactor was more efficient than in a
stirred batch reactor. Equilibrium was reached 25% earlier and productivity was 2.8
times higher in the gas lift reactor.
VI. FACTORS AFFECTING LIPASE ACTIVITY DURING
INTERESTERIFICATION
In considering all of the factors involved in enzymatic interesterification, all components
of the system must be examined; namely pH, water content, temperature,
substrate composition, product composition, and lipase content.
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