Dairy Chemistry And Biochemistry

322 DAIRY CHEMISTRY AND BIOCHEMISTRY

Cathepsin D (EC3.4.23.5). It has been known for more than^20 years that
milk also contains an acid proteinase, (optimum pH x 4.0) which is now
known to be cathepsin D, a lysozomal enzyme. It is relatively heat labile
(inactivated by 70°C x 10min). Its activity in milk has not been studied
extensively and its significance is unknown. At least some of the indigenous
acid proteinase is incorporated into cheese curd; its specificity on zsl- and
p-caseins is quite similar to that of chymosin but it has very poor
milk-clotting activity (McSweeney, Fox and Olson, 1995). It may contribute
to proteolysis in cheese but its activity is probably normally overshadowed
by chymosin, which is present at a much higher level.

Other proteinases. The presence of low levels of other proteolytic enzymes

in milk has been reported (see Fox and McSweeney, 1996). Most of these

originate from somatic cells, and their level increases during mastitic

infection. The presence of cathepsin D, a lysozomal enzyme, in milk suggests

that all the lysozomal proteinases are present in milk although they may not

be active. These minor proteinases are considered to be much less significant

than plasmin, but more work on the subject is necessary.

8.2.3

Lipases catalyse the development of hydrolytic rancidity in milk, and,

consequently, lipases and lipolysis in milk have been studied extensively.

Lipases and esterases (EC 3.1.1.-)

Milk contains three types of esterase:

1. A-type carboxylic ester hydrolases (arylesterases; EC 3.1.1.2), which
   hydrolyse aromatic esters, e.g. phenylacetate; they show little activity on
   tributyrin, and are not inhibited by organophosphates.


2. B-type esterases (glycerol tricarboxyl esterases, aliphatic esterases, lipases;
   EC 3.1.1.3): they are most active on aliphatic esters although they show
   some activity on aromatic esters; they are inhibited by organophosphates.


3. C-type esterases (cholinesterase; EC 3.1.1.7; EC 3.1.1.8): they are most
   active on choline esters but hydrolyse some aromatic and aliphatic esters
   slowly; they are inhibited by organophosphates.

In normal milk, the ratio of A : B : C esterase activity is about 3 : 10: 1 but
the level of A-esterase activity increases considerably on mastitic infection. A
and C esterases are considered to be of little technological significance in
milk.
Classically, lipases hydrolyse ester bonds in emulsified esters, i.e. at a
water/oil interface, although some may have limited activity on soluble
esters; they are usually activated by blood serum albumin and Ca2+ which
bind free fatty acids, which are inhibitory. Little lipolysis normally occurs in