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Food Lipids: Chemistry, Nutrition, and Biotechnology

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volume per year. The volumetric activity is determined as the mass of product obtained<br />

per liter of reactor per hour, whereas the operational stability is the half-life<br />

of the immobilized enzyme (80). The most common reactor systems used include<br />

fixed bed, batch, continuous stirred tank, <strong>and</strong> membrane reactors.<br />

A. Fixed Bed Reactor<br />

A fixed bed reactor is a form of continuous flow reactor, where the immobilized<br />

enzyme is packed in a column or as a flat bed, <strong>and</strong> the substrate <strong>and</strong> product streams<br />

are pumped in <strong>and</strong> out of the reactor at the same rate. The main advantages of fixed<br />

bed reactors are their easy application to large scale production, high efficiency, low<br />

cost, <strong>and</strong> ease of operation. A fixed bed reactor also provides more surface area per<br />

unit volume than a membrane reactor system (7). A model fixed bed reactor for<br />

interesterification would consist of two columns in series: one for the reaction <strong>and</strong><br />

a precolumn for fat-conditioning steps such as incorporation of water. Reservoirs<br />

attached to the columns would contain the feed streams <strong>and</strong> product streams. A pump<br />

would be required to keep the flow rate through the system constant, <strong>and</strong> the system<br />

would have to be water-jacketed to keep the reaction temperature constant (Fig. 15).<br />

Since water is required in minimal amounts for hydration of the enzyme during<br />

the reaction, the oil is first passed through a precolumn containing water-saturated<br />

silica or molecular sieves, which would allow the oil to become saturated with sufficient<br />

water to allow progression of the interesterification reaction without increasing<br />

the rate of hydrolysis. Interesterification in a fixed bed reactor can lead to increases<br />

in product formation through increased residence time in the reactor. Complete conversion<br />

to products will never be achieved, <strong>and</strong> with an increase in product levels,<br />

a loss in productivity will occur (89). Using a fixed bed reactor with a silica pre-<br />

Figure 15 Fixed bed reactor for immobilized lipase–catalyzed interesterification.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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