Food Chemistry

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 hydro-
lysis 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 conjunc-
tion 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 TriacylglycerolHydrolases(Lipases).......................

Lipases (cf 2.2.6) hydrolyze only emulsified acyl
lipids; they are active on a water/lipid interface.

Table 3.21.Lipid hydrolysis occurring during potato
tuber homogenization

μmoles/ga

Acyl lipids Free fatty

acids

Potato 2. 34 0. 70
Homogenateb 2. 04 1. 40
Homogenateb
kept for 10 min at 0◦C1. 72 1. 75
Homogenate
kept for 10 min at 25◦C0. 54 2. 90

aPotato tissue fresh weight.
bSliced potatoes were homogenized for 30 sec at 0◦C.

Lipases differ from esterase enzymes since the

latter cleave only water-soluble esters, such as tri-

acetylglycerol.

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 lipase-

type 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 Mr= 48 ,000. The enzyme cleaves the

following types of acyl glycerols with a de-

creasing 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

Fig. 3.17. A hypothetical model of pancreatic li-

pase fixation of an oil/water interphase (according to

Brockerhoff, 1974)