Enzyme-modifi ed Dairy Ingredients 323the other hand, lipolytic rancidity has been
observed in foods stored at - 29 ° C ( - 20 ° F).
Some lipases, referred to as non - specifi c,
hydrolyze fatty acids at any position in the
triglyceride. Non - specifi c lipases break down
triglycerides to free fatty acids and glycerol.
Other lipases, referred to as specifi c, hydro-
lyze fatty acids esterifi ed at one or three posi-
tion of glycerol. The products of specifi c
lipase reactions are free fatty acids and di -
and mono - glycerides. Lipases, in addition to
positional specifi city, also exhibit specifi city
towards the type of fatty acid. Pancreatic
lipase is specifi c for shorter chain fatty acids
esterifi ed to glycerol. Milk fat contains short
chain fatty acids.
Pre - gastric esterases are useful in generat-
ing specifi c fl avors reminiscent of Italian
cheeses. Esterases act on soluble substrates
as opposed to lipases, which require insolu-
ble substrates. Pre - gastric esterases are also
known as pre - lingual lipases — a misnomer.
The main sources of esterases are kid, calf,
and lamb. These enzymes, used in traditional
Italian piquant cheeses, are not acceptable to
vegetarians and are very expensive. As
judged by enzyme activity on tributyrin sub-
strate, pre - gastric esterases are 1,000 times
more expensive than pancreatic lipase. The
cost disadvantage of pre - gastric esterases
coupled with animal virus issues have led
to the use of microbial esterases. Besides
being less expensive, microbial esterases
are protease free and useful for vegetarian
products.
The free fatty acids that are generated are
converted to fl avor compounds by reactions
such as thiolation, oxidation, and esterifi ca-
tion, yielding compounds such as thiolesters,
γ - or δ - lactones, ethyl esters, and alken - 2 -
ols. Thiol ester formation involves sulphur -
containing amino acids such as methionine
and cysteine, whereas interaction of free fatty
acids with tryptophan results in indole, and
tyrosine forms phenol. The amino acid
leucine reacts with α - ketoglutarate to form
α - isoketocaproate. Phenylalanine catabolismabsence of general and proline - specifi c pep-
tidase activities. The Rhizomucor niveus
preparation contained little or no general or
endopeptidase activity. Esterase activity was
found in all of the preparations evaluated,
with only two Aspergillus preparations con-
taining lipase activity.
Lipid Modifi cation
Enzymatic modifi cation of lipids is facili-
tated by lipases and esterases. Lipases hydro-
lyze triglycerides to free fatty acids and
mono - and di - glycerides. Lipases require an
insoluble substrate to be present at an inter-
face; with triglycerides the interface is created
by emulsifying the substrate in an aqueous
medium. A potential hindrance to lipolysis is
the generation of surface - active mono - and
di - glycerides that block the interface. If such
surfactants are not controlled, the rate of
lipolysis continues to decrease. One method
of controlling the release of surfactants is to
include divalent cations, e.g. Ca^2 +^ , which can
form insoluble soaps.
As noted above, plants, animals, and
microorganisms produce lipases. Plant lipases
derived from castor bean or wheat germ are
not commercially viable sources of enzyme.
Animal lipases are mainly obtained from
porcine or bovine pancreas. Other enzymes
derived from animals are caprine (kid and
goat) pre - gastric lipases and esterases.
Microbial lipases are extracellular
enzymes and in some instances inducible.
The synthesis of the enzyme is under feed-
back control of mono - and di - glycerides and
glycerol concentrations in the growth
medium. Some microbial lipases are glyco-
proteins with carbohydrate moieties facilitat-
ing the secretory process. Most microbial
lipases have pH optima in the alkaline range
(pH 8 to 9) and altering composition of
the growth medium by salts and emulsifi ers
can shift the optima to an acidic range.
Most industrial microbial lipases are active
between 30 ° C to 40 ° C (86 ° F to 104 ° F). On