Food Lipids: Chemistry, Nutrition, and Biotechnology

alters the native conformation of the protein by disrupting hydrogen bonding and
hydrophobic interactions, thereby leading to reduced activity and stability (72).
Lipases differ in their sensitivity to solvent type. An important solvent characteristic
that determines the effect of solvent in enzymatic catalysis is the polarity
of the solvent. Solvent polarity is measured by means of the partition coefficient of
a solvent between octanol and water (73), and this is taken as a quantitative measure
of polarity, otherwise known as log P value (74). The catalytic activity of enzymes
in solvents with log P < 2 is usually lower than that of enzymes in solvents with
log P > 2. This is because hydrophilic or polar solvents can penetrate into the
hydrophilic core of the protein and alter the functional structure (75). They also strip
off the essential water of the enzyme (67). Hydrophobic solvents are less able to
remove or distort the enzyme-associated water and are less likely to cause inactivation
of enzymes (61).
In choosing a solvent for a particular reaction, two important factors must be
taken into consideration: the solubility of the reactants in the chosen solvent and the
need for the chosen solvent to be inert to the reaction (61). Other factors that must
be taken into account in determining the most appropriate solvent for a given reaction
include solvent density, viscosity, surface tension, toxicity, flammability, waste disposal,
and cost (61). A report by Akoh and Huang (62) on the effect of solvent
polarity on the synthesis of SLs using IM 60 lipase from Rhizomucor miehei showed
that nonpolar solvents such as isooctane and hexane produced 40 mol% of disubstituted
SL, while a more polar solvent such as acetone produced 1.4% of the same
SL. Claon and Akoh (76) found that with SP 435 lipase from Candida antarctica, a
higher log P value does not necessarily sustain a higher enzyme activity. Some
experimentation is therefore necessary in selecting solvents for enzymatic reactions.
pH. Enzymatic reactions are strongly pH dependent in aqueous solutions. Studies
on the effect of pH on enzyme activity in organic solvents show that enzymes
‘‘remember’’ the pH of the last aqueous solution to which they were exposed (65,70).
That is, the optimum pH of the enzyme in an organic solvent coincides with the pH
optimum of the last aqueous solution to which it was exposed. This phenomenon is
called pH memory. A favorable pH range depends on the nature of the enzyme, the
substrate concentration, the stability of the enzyme, the temperature, and the length
of the reaction (77).
Thermostability. Temperature changes can affect parameters such as enzyme
stability, affinity of enzyme for substrate, and preponderance of competing reactions
(78). Thermostability of enzymes is a major factor the industry considers prior to
commercialization of any enzymatic process, mostly because of the potential for
saving energy and minimizing thermal degradation. Thermostability of lipases varies
considerably with enzyme origin: animal and plant lipases are usually less thermostable
than microbial extracellular lipases (49).
Several processes that lead to the irreversible inactivation of enzymes involve
water as a reactant (79). This characteristic of enzymes makes them more thermostable
in water-restricted environments such as organic solvents. Enzymes are usually
inactivated in aqueous media at high temperatures. Several studies have been reported
on the effect of temperature on lipase activity (64,76,80). Zaks and Klibanov
(80), who studied the effect of temperature on the activity of porcine pancreatic
lipase, showed that in aqueous solution at 100�C, the lipase is completely inactivated
within seconds, whereas in dry tributyrin containing heptanol, the lipase had a shelf
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