2009_Book_FoodChemistry

188 3 Lipids
since the free C 4 –C 12 fatty acids (cf. Tables 3.3
to 3.5 for odor threshold values) are responsible
for the rancid aroma defect. On the other hand,
lipolysis occurring during the ripening of cheese
is a desired and favorable process because
the short-chain fatty acids are involved in the
build-up of specific cheese aromas. Likewise,
slight hydrolysis of milk fat is advantageous in
the production of chocolate.
Linoleic and linoleiic acid released by hydrolysis
and present in emulsified form affect the
flavor of food even at low concentrations. They
cause a bitter-burning sensation (cf. Table 3.9).
In addition, they decompose by autoxidation
(cf. 3.7.2.1) or enzymatic oxidation (cf. 3.7.2.2)
into compounds with an intensive odor. In fruits
and vegetables enzymatic oxidation in conjunction
with lipolysis occur, as a rule, at a high
reaction rate, especially when tissue is sliced or
homogenized (an example for rapid lipolysis is
shown in Table 3.21). Also, enzymatic hydrolysis
of a small amount of the acyl lipids present can
not be avoided during disintegration of oil seeds.
Since the release of higher fatty acids promotes
foaming, they are removed during oil refining
(cf. 14.4.1).
Enzymes with lipolytic activity belong to the
carboxyl-ester hydrolase group of enzymes
(cf. 2.2.6).
3.7.1.1 Triacylglycerol Hydrolases (Lipases)
Lipases (cf 2.2.6) hydrolyze only emulsified acyl
lipids; they are active on a water/lipid interface.
Lipases differ from esterase enzymes since the
latter cleave only water-soluble esters, such as triacetylglycerol.
Lipase activity is detected, for example, in milk,
oilseeds (soybean, peanut), cereals (oats, wheat),
fruits and vegetables and in the digestive tract of
mammals. Many microorganisms release lipasetype
enzymes into their culture media.
As to their specificity, lipases are distinguished
according to the criteria presented in Table 3.22.
The lipase secreted by the swine pancreas has
been the most studied. Its molecular weight
is M r = 48,000. The enzyme cleaves the
following types of acyl glycerols with a decreasing
rate of hydrolysis: triacyl- >diacyl-
≫monoacylglycerols. Table 3.22 shows that
pancreatic lipase reacts with acyl residues at
positions 1 and 3. The third acyl residue of
a triacylglycerol is cleaved (cf. Reaction 3.52)
only after acyl migration, which requires a longer
incubation time.
(3.52)
The smaller the size of the oil droplet, the larger
the oil/water interface and, therefore, the higher
the lipase activity. This relationship should not
Table 3.21. Lipid hydrolysis occurring during potato
tuber homogenization
µmoles/g a
Acyl lipids Free fatty
acids
Potato 2.34 0.70
Homogenate b 2.04 1.40
Homogenate b
kept for 10 min at 0 ◦ C 1.72 1.75
Homogenate
kept for 10 min at 25 ◦ C 0.54 2.90
a Potato tissue fresh weight.
b Sliced potatoes were homogenized for 30 sec at 0 ◦ C.
Fig. 3.17. A hypothetical model of pancreatic lipase
fixation of an oil/water interphase (according to
Brockerhoff , 1974)