Food Chemistry

3.7 Changes in Acyl Lipids of Food 189

Table 3.22.Examples of the specificity of lipases

Specificity Lipase from

Substrate specific
Monoacylglycerides Rats (adipose tissue)
Mono- and diacyl- Penicillium camembertii
glycerides
Triacylglycerides Penicilliumsp.
Regiospecific
1,3-Regioselective Pancreas, milk,
Aspergillus niger
sn-2-Regioselective Candida antarctia
Non-regiospecific
Oats, castor,Aspergillus
flavus
Acyl residue-specific
Short chain fatty acids Penicillium roqueforti
cis-9-Unsaturated fatty Geotrichum candidum
acids
Long chain fatty acids Botrytis cinerea
Stereospecifica
sn-1 Pseudomonas neruginosa
sn-3 Rabbit (digestive tract)

a 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 exam-

pleofalipaseofPseudomonas 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 un-

der 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 sub-

strate. 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 activ-

ity. More precise information about the changes

to be expected is obtained through storage expe-

riments during which the fatty acids are quantita-

tively determined by gas chromatographic anal-

ysis. Table 3.24 shows the change in the con-

centrations of free fatty acids in sweet cream

butter together with the resulting rancid aroma

notes.

Table 3.23.Heat inactivation of a lipase ofPseu-

domonas fluorescencedissolved in skim milk

Temperature D-valuea

◦C(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).