Milk contains both indigenous and exogenous enzymes that catalyze important chemical reactions. Some of the most significant indigenous enzymes are lipoprotein lipase, plasmin, and alkaline phosphatase. Plasmin is a heat-stable protease that can cause bitterness in pasteurized milk and is responsible for proteolysis. Milk also contains lipases that cause rancidity by hydrolyzing milkfat. Alkaline phosphatase is destroyed during pasteurization and can be used to test for proper pasteurization. Exogenous enzymes are introduced from psychrotrophic bacteria and include heat-stable lipases and proteinases.

1. Chemistry of milk
enzymes

2. ENZYMES
 Enzymes are a group of proteins that
have the ability to catalyze chemical
reactions and the speed of such reactions
 Milk contains both indigenous and
exogenous enzymes
 Exogenous enzymes mainly consist of
heat-stable enzymes produced by
psychrotrophic bacteria: lipases, and
proteinases

3.  There are many indigenous enzymes that
have been isolated from milk.
 The most significant group are the
hydrolases:
 lipoprotein lipase
 Plasmin
 Alkaline phosphatase

5. Plasmin
 Plasmin is a proteolytic enzyme
 Most of the plasmin in milk is present as
the inactive plasminogen and associated
with the casein micelles
 It is very heat stable and responsible for
the development of bitterness in
pasteurized milk and UHT processed milk

6.  Milk contains one or more promoters
(especially urokinase) that catalyze the
hydrolysis of plasminogen to yield
plasmin.
 Plasmin can hydrolyze proteins to yield
large degradation products and is
responsible for production of γ -casein
and proteose peptones from β-casein.
 This reaction even proceeds at 5°

7.  In UHT milk products its proteolytic action
causes a bitter flavor and eventually can
solubilize the casein micelles(140°C for 15
s)
 Proteinases-
Milk also contains acid proteinases
 These originate from somatic cells in the
milk.
 The most active one is cathepsin D

8.  It is less heat resistant than plasmin.
 It is also associated with the casein
micelles, and it can cause some
proteolysis in cheese.
 Lipolytic Enzymes-
 A lipase enzyme splits fats into glycerol
and free fatty acids.
 It is bound largely to casein micelles.

9.  In milk, lipolysis causes a soapy, rancid off-
flavor
 The flavour is due to the short chain fatty
acids produced, particularly butyric acid.
 It is active at the fat/water interface but is
ineffective unless the fat globule membrane
is damaged or weakened.
 Rancidity arises from the hydrolysis of milkfat
by an enzyme called the lipoprotein lipase
(LPL) so that avoid mixing new and
homogenized milk because it leads to rapid
rancidity

10. Phosphatases
 Alkaline phosphatase which catalyzes the
hydrolysis of phosphoric monoesters
 The enzyme is destroyed by minimum
pasteurization temperatures, therefore, a
phosphatase test can be done to ensure
proper low pasteurization of milk
 Milk also contains an acid phosphatase it
occurs in the serum and is quite heat
resistant .
 These phosphatases catalyze the hydrolysis
of certain phosphoric esters in milk, but
slowly.

11. Antibacterial Enzymes
 Lactoperoxidase-
 The enzyme can catalyze oxidation of
thiocyanate (SCN) by H2O2 to OSCN and
other oxyacids, which inhibit most
bacteria.
 Milk is rich in lactoperoxidase (about 0.4
μM), but the thiocyanate concentration in
milk varies widely because it depends on
the cyanoglucoside content of the feed.

12.  thiocyanate together with a little H2O2 is
added to raw milk to prevent spoilage. In
this way, even in a warm climate spoilage
of milk can be delayed for many hours.
 Oxidoreductases-
 Xanthine oxidase can catalyze oxidation of
various substances
 The enzyme can reduce nitrate (which
occurs in milk only in trace quantities) to
nitrite.

13.  This property is put to use in the
manufacture of some cheeses, in which
nitrate is added to milk to prevent the
proliferation of the detrimental butyric
acid bacteria
 Superoxide dismutase-
 catalyzes the dismutation of superoxide
anion to hydrogen peroxide and triplet
oxygen

14.  The enzyme in milk is identical to that in
blood.
 Its biological function is to protect cells
from oxidative damage.

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