2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 189
Table 3.22. Examples of the specificity of lipases
Specificity
Substrate specific
Monoacylglycerides
Mono- and diacylglycerides
Triacylglycerides
Regiospecific
1,3-Regioselective
sn-2-Regioselective
Non-regiospecific
Acyl residue-specific
Short chain fatty acids
cis-9-Unsaturated fatty
acids
Long chain fatty acids
Stereospecific a
sn-1
sn-3
a
Lipase from
Rats (adipose tissue)
Penicillium camembertii
Penicillium sp.
Pancreas, milk,
Aspergillus niger
Candida antarctia
Oats, castor, Aspergillus
flavus
Penicillium roqueforti
Geotrichum candidum
Botrytis cinerea
Pseudomonas neruginosa
Rabbit (digestive tract)
Lipases differentiate between the sn-1 and sn-3
position in TGs.
be ignored when substrate emulsions are prepared
for the assay of enzyme activities.
A model for pancreatic lipase has been suggested
to account for the enzyme’s activity on the
oil/water interface (Fig. 3.17). The lipase’s
“hydrophobic head” is bound to the oil droplet
by hydrophobic interactions, while the enzyme’s
active site aligns with and binds to the substrate
molecule. The active site resembles that of serine
proteinase. The splitting of the ester bond occurs
with the involvement of Ser, His and Asp residues
on the enzyme by a mechanism analogous to that
of chymotrypsin (cf. 2.4.2.5). The dissimilarity
between pancreatic lipase and serine proteinase
is in the active site: lipase has a leucine residue
within this site in order to establish hydrophobic
contact with the lipid substrate and to align it
with the activity center.
Lipase-catalyzed reactions are accelerated by
Ca 2+ ions since the liberated fatty acids are
precipitated as insoluble Ca-salts.
The properties of milk lipase closely resemble
those of pancreatic lipase.
Lipases of microbiological origin are often very
heat stable. As can be seen from the example
of a lipase of Pseudomonas fluorescence
(Table 3.23), such lipases are not inactivated by
pasteurization, ultra high temperature treatment,
as well as drying procedures, e. g., the production
of dry milk. These lipases can be the cause
of decrease in quality of such products during
storage.
A lipase of microbial origin has been detected
which hydrolyzes fatty acids only when they have
a cis-double bond in position 9 (Table 3.22). It is
used to elucidate triacylglyceride structure. The
use of lipases in food processing was outlined under
2.7.2.2.14.
Lipase activities in foods can be measured very
sensitively with fluorochromic substrates, e. g.,
4-methyl umbelliferyl fatty acid esters. Of course
it is not possible to predict the storage stability
of a food item with regard to lipolysis based only
on such measurements. The substrate specificity
of the lipases, which can vary widely as shown
in Table 3.22, is of essential importance for the
aroma quality. Therefore, individual fatty acids
can increase in different amounts even at the same
lipase activity measured against a standard substrate.
Since the odor and taste threshold values of
the fatty acids differ greatly (cf. Tables 3.3–3.5),
the effects of the lipases on the aroma are very
variable. It is not directly possible to predict the
point of time when rancid aroma notes will be
present from the determination of the lipase activity.
More precise information about the changes
to be expected is obtained through storage experiments
during which the fatty acids are quantitatively
determined by gas chromatographic analysis.
Table 3.24 shows the change in the concentrations
of free fatty acids in sweet cream
butter together with the resulting rancid aroma
notes.
Table 3.23. Heat inactivation of a lipase of Pseudomonas
fluorescence dissolved in skim milk
Temperature
◦ C
D-value a
(min)
100 23.5
120 7.3
140 2.0
160 0.7
a
Time for 90% decrease in enzyme activity
(cf. 2.5.4.1).