Enzymes as Biocatalysts for Lipid-based Bioproducts Processing - Food and Industrial Bioproducts and Bioprocessing - page 340

ROCOR 1

H 2 C

OCOR 1

H 2 C

OH

+

Catalyst

OCOR 2 HC +

ROH

HC OH

ROCOR 2

+

+

OCOR 3

H 2 C

ROCOR 3

H 2 C

OH

Triglyceride Alcohol Mixture of

Alkyl Esters

Glycerol

Triacylglycerides + Alcohol Diacylglycerides + FAAE

+ Alcohol Monoacylglycerides + FAAE

Diacylglycerides

Monoacylglycerides + Alcohol Glycerol + FAAE

Figure 14.5 Preparation of fatty acid alkyl esters by transesterification.

Thus, a combination of different enzymes is found to be more efficient in catalyzing FAAE
production in comparison to the single lipase system.
Table 14.4 shows a list of CALB lipase-catalyzed transesterification reactions using metha-
nol as the acyl acceptor. Although methanol is the cheapest source of acyl acceptor, it has a
harmful effect on biocatalysts. In countries where ethanol production from renewable feedstocks
is economically feasible, ethanol is the more preferable acyl acceptor (Kojima et al ., 2004 ).
Stoichiometrically, a 3:1 molar ratio of alcohol to oil is needed for complete transesteri-
fication reaction. Nevertheless, the solubility of short chain alcohols, namely methanol and
ethanol, in vegetable oil is only 1/2 and 2/3 of the stoichiometric amount, respectively. The
insoluble alcohols form either emulsion droplets under continuous stirring or a heterogeneous
alcohol phase in equilibrium conditions, with the oil phase leading to inactivation of
enzymes and, subsequently, lower conversion rate (Shimada et al ., 1999 ). To minimize the
effect of lipase inactivation, Shimada and co-workers (1999) performed a stepwise addition
of methanol. This procedure was found to be effective with close to complete conversion of
FAAE (98.4%). The use of organic solvents to improve substrate miscibility is deemed
undesirable considering the cost of recovery and also increasing environmental concerns.
The cost concern also applies to the use of ionic liquids despite them being green solvents
(Ha et al ., 2007). On the other hand, recycling or addition of FAAE as the co-solvent seems
a promising possibility for keeping the reaction media homogeneous and, thus, increasing
mass transfer and reaction rates (Fjerbæk et al ., 2009 ).
The quality of feedstock plays an important role in enzymatic processing. Alternative oil
sources, such as waste frying oils, inedible vegetable oils and waste oils from processing
industries, which are cheaper than edible vegetable oils, are potential feedstocks for FAAE
production. Biocatalysts are preferable for such type of feedstocks. Lipases can simultane-
ously esterify and transesterify FFA and oils. Thus, it can convert low quality oils, such as
high FFA oils and restaurant greases, and inedible oils, such as Jatropha and castor oil, into
FAAE at reasonably good yields. Basic catalysts have the tendency of forming soap with
such feedstocks. Although acid catalysts, for example hydrochloric or sulfuric acid, do not
have such drawbacks, the reaction rates are much slower (Robles-Medina et al ., 2009 ).
Enzymatic FAAE production does have its constraints. The major constraint to enzymatic
production of FAAE is substrate immiscibility. Both the substrates, oil and alcohol, have