1.2.2 Hydrolysis using fatty acid-selective lipases
Reduction of the saturated fatty acid content in oil
Another application of a specific lipase in triglyceride hydrolysis is the production of
a low-SAFA oil. For many decades, the lowering of blood cholesterol by changing
dietary fat consumption has been a hot issue. Nowadays, it is recommended that the
intake of total fat is limited, and especially that from saturated fats (American Die-
tetic Association, 1998); hence, there is considerable interest in low saturated fat
products.
Based on the typical selectivity of lipases fromFusarium oxysporum, it should be
possible to lower the level of saturated fatty acids in oil via selective hydrolysis. Both
intracellular and extracellular lipases from this microorganism have been described
which are reported to have a high affinity towards saturated fatty acids.
The extracellular lipase was shown to hydrolyze tristearin at a higher rate as com-
pared to triolein, both as single substrates. However, when mixed triglycerides were
tested, no significant selectivity for the saturated fatty acids was actually observed
(Hoshino et al., 1992).
Using an intracellular lipase fromFusarium oxysporum, hydrolysis of cotton seed
oil or groundnut oil yielded high levels of saturated fatty acids in the free fatty acid
fraction (Joshi and Dhar, 1987). Recalculation of the experimental data suggests that
this lipase even has an overall specificity factor in the range of 40^1.
Unfortunately, no more data are available on any further attempts to exploit this
rare fatty acid selectivity.
Another option for producing a low-SAFA oil is a two-step process based on the
high selectivity of Lipase B (Charton and Macrae, 1992) or Lipase I (Holmqvist et
al., 1997) fromGeotrichum candidum. This B-lipase has a very distinct preference
for handling unsaturated free fatty acids with acis-D-9 double bond, e.g., oleic or
linoleic acid.
For example, taking sunflower oil [88 % (w/w) unsaturated fatty acids] as sub-
strate,>99 % unsaturated fatty acids were obtained upon hydrolysis in a 1 : 1 oil/
water emulsion (Diks and Lee, 1999). From the experimental data it was derived that
the so-called ‘specificity factor’ was approximately 30. It was shown that the lipase
maintained its high selectivity up to 70 % degree of hydrolysis.
Of practical importance is the rather low stability of the lipase observed during
hydrolysis in emulsion. In solution, the lipase appeared quite stable, even at 40 8 C.
Once added to the oil/water emulsion, the lipase lost most of its activity within hours
at only 30 8 C. This discrepancy was attributed to a reduced stability of the lipase at
the oil/water interface (Diks and Lee, 1999).
Immobilization was shown not to effect the selectivity of theGeotrichumB-lipase
(Charton and Macrae, 1993). Moreover, the catalyst was active in organic solvents.
1.2 Free fatty acids 5
(^1) Specificity factor defined as S¼(S/U)
FFA/(S/U) (Diks and Lee, 1999), in which S is the fraction of
saturated fatty acids and U the fraction of unsaturated fatty acids. The specificity factor reflects the
relative net rate of hydrolysis of the saturated over the unsaturated fatty acid, corrected for their con-
centration difference in the starting oil.