322 DAIRY CHEMISTRY AND BIOCHEMISTRY Cathepsin D(EC 3. 423. 5). It has been known for more than 20 years that milk also contains an acid proteinase, (optimum pH 4.0) which is now known to be cathepsin D, a lysozomal enzyme. It is relatively heat labile ted by 70C x 10 min). 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 z,,and B-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 significan than plasmin, but more work on the subject is necessary 8.2. 3 Lipases and esterases(EC 3. 1.1.-) es catalyse the development of hydrolytic rancidity in milk, and, uently, lipases and lipolysis in milk have been studied extensively Ik contains three types of esterase 1. A-type carboxylic ester hydrolases(arylesterase; 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 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 in322 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