Front Matter

were also observed in the lipase-catalyzed acidolysis (Xu et al., 1998b). In a system

of the lipase-catalyzed acidolysis between an oil and a medium-chain fatty acid, for

example fish oil and caprylic acid, the molecular weight of the oil is 5- to 6-fold that

of the free fatty acid, and the diffusion coefficient of the oil will be much smaller than

that of the acid. This is certainly one of the reasons leading to the longer reaction time

needed to reach equilibrium when higher substrate molar ratios between fatty acids

and substrate oil is used (Xu et al., 1998b).

11.5 Stability and reusability of lipases

The stability of an enzyme depends on the characteristic nature of the enzyme and the

conditions under which it is used. The factors that stabilize and inactivate enzymes

are not well understood. Immobilization can influence the operational stability; both

increases and decreases in stability having been observed previously. Generally the

activity of enzymes may be reduced due to immobilization; however, the stability is

often enhanced by immobilization because the tertiary structure and/or conformation

of the enzymes can be held in active forms by enzyme-support linkage. For the

irreversible thermal deactivationand in the absence of diffusional restrictions, en-

zyme activity (A) decays exponentially with timeA¼A 0 exp (–kdt), wherekdis the

deactivation coefficient,tis time, andA 0 is the initial activity. For Lipozyme IM, a

first-order model was proposed by Hansen and Eigtved (1987) and Posorske et al.

(1988).

Foulingis one of the drawbacks of packed-bed reactors. Fouling causes a loss in

the activity of immobilized enzyme by deposition onto the surface of enzyme par-

ticles of colloidal materials or polar compounds that exist in the substrate; this pre-

vents access of the substrate to the enzyme. The extent of fouling depends on the

presence, amount, and nature of the fouling materials and on the location of the

enzyme in/on the support materials; fouling is accentuated when viscous substrates

are used. In the system of lipase-catalyzed lipid modifications, the stability of lipases

is affected by lipid quality. It was found that minor compounds in oils and fats, such

as lipid hydroperoxides, phospholipids, emulsifiers, chlorophyll, carotenoids, lipid

polymers, heavy metal ions and even some antioxidants, had effects on the stability

of lipases (Xu et al., 1998c). It has also been shown that highly unsaturated oils and

fats as substrates reduce the lifetime of the enzymes used (Hansen and Eigtved, 1987;

Posorske et al., 1988). Therefore, high-quality starting oils and fats and careful pro-

tection of lipid oxidation during processing are necessary requirements to ensure a

longer stability of lipases. However, the refining of oils and fats is also costly, and so

a compromise must be found between the quality of lipids and the stability of lipases.

202 11 Modification of Oils and Fats by Lipase-Catalyzed Interesterification