In general, the maximum adsorption is observed at pHs close
to the isoelectric point of the lipase [37]. In addition, porous
particulate supports are superior to nonporous supports for immobilisation
of lipases due to their greater surface area and chemical
properties. However, porous supports can have an internal morphology that allows not only the lipase binding, but also an easy
accessibility to substrate molecules in order to minimise diffusional
limitation. It appears that pore sizes best suited for lipase adsorption
are at least 100 nm diameter. Smaller pore sizes can result in
diminished availability of lipase molecules within the pores and in
restricted diffusing substrate molecules. Such limitations lead to a
lowered efficiency [26]. The affinity of a lipase for an adsorbent generally
increases with the hydrophobicity of the surface, and lipases
desorb more easily from hydrophilic than from hydrophobic surfaces.
The alcohols used in the transesterification process according
to its structure behave in different ways, e.g. methanol and ethanol
are not miscible in triglycerides at room temperature and mechanical
agitation is required to facilitate mass transfer. However, in the
course of the reaction there is the formation of an emulsion, which
in the case of the meta-analysis, are easily and rapidly dissolved
to form an inner layer rich in glycerol and another one on the top,
rich in methyl esters. In the ethanolysis case, these emulsions are
more stable, making the process of separation and purification of
methyl esters into something more complex. The transesterification
reaction involves the displacement of alcohol from an ester by
another, in a process similar to hydrolysis, except that alcohol is
used instead of water. This process is used to reduce the high viscosity
of triglycerides. Fig. 7 represents the general equation of the
transesterification reaction: