The document discusses several enzymes found in milk, including hydrolases, lipases, plasmin, alkaline phosphatase, acid phosphatase, ribonuclease, and lysozyme. Hydrolases are responsible for hydrolyzing milk constituents and lipases play a role in off flavors. Plasmin exists as plasminogen and converts to active plasmin through plasminogen activators. Alkaline phosphatase is used to determine adequate pasteurization and acid phosphatase is more heat stable. Ribonuclease protects milk from viruses by inhibiting RNA-dependent DNA polymerase. Lysozyme is an important protein in human milk and acts as a bactericide by attacking bacterial cell walls.

1. MILK ENZYMES
Hydrolases

2. Hydrolases
• The enzymes which are responsible for the hydrolysis of various milk
constituents are included in this group.
• This enzymes play a significant role in dairy industry especially the
lipases which are responsible for the development of off flavours in
milk and milk products.
• Similarly some of these hydrolases are useful in the detection of
efficiency of milk processing. Enzymes which have the bacteriostatic
action are also included in this group of enzymes

3. 1. Lipase
2. Plasmin
3. Milk alkaline phosphatase
4. Acid phosphatase / acid phosphomonoesterase
5. Ribonuclease (RNase)
6. Lysozyme

4. Plasmin
• Plasmin is part of a complex protease–protease inhibitor system in
milk, commonly referred to as the PL system
• Plasmin exists in milk primarily in its zymogen form, plasminogen
(PG)
• PG can be converted into active PL by plasminogen activators (PA)
• The PL system also includes plasminogen activator inhibitors (PAI)
and plasmin inhibitors (PI)
• The conversion of PG to PL can be slowed by the action of PAI.
• The proteolysis of casein induced by PL can be slowed by the action
of PI.
• Plasmin inhibitor and PAI are present mainly in milk serum

6. • β-casein is the preferred substrate for plasmin and its hydrolysis
results in the production of γ-caseins and proteose-peptones
• C-terminal peptide- γ casein
• N-terminal peptide- Proteose peptone

7. Milk alkaline phosphatase
• This is a phosphomonoesterase
• The scientific name for this enzyme is orthophosphomonoester
phosphohydrolase
• Two major isozymes have been identified, α and β phosphatase,
mainly located in the milk plasma and fat globule membrane,
respectively
• It contains about five atoms of zinc per dimeric molecule
• Its optimal pH for hydrolysis is alkaline

8. • Milk alkaline phosphatase is used as the method of preference for
determining whether the milk has been pasteurized adequately
• Inactivation of alkalinephosphatase by pasteurization is an index of
destruction of Mycobacterium tuberculosis
• It is possible to determine the inactivation of phosphatase enzyme by
easy chemical methods
• The usual substrates are phenyl phosphate, p-nitrophenyl-phosphate
or phenolphthalein phosphate which are hydrolysed to inorganic
phosphate and phenol, p-nitrophenol or phenolphthalein, respectively

9. • Phenol is colourless but forms a coloured complex on reaction with
one of several reagents, e.g. 2,6-dichloroquinonechloroimide, with
which it forms a blue complex
• p-Nitrophenol is yellow at the alkaline pH of the assay
• Phenolphthalein is red at the alkaline pH of the assay
ALP

10. Acid phosphomonoesterase
• A second phosphatase present in milk has a pH optimum at about 4.0
• It is primarily in the milk plasma. Its concentration is quite low
(compared to alkaline phosphatase), though higher in colostrum.
• Both the alkaline and the acid phosphatase can release inorganic phos-
phatase from caseins and from soluble esters
• The acid phosphatase is the more active of the two at the pH of milk.
These two enzymes differ greatly in susceptibility to inactivation by
heat
• Very heat stable (LTLT pasteurization causes only 10-20% inactivation)

11. Ribonuclease
• RNase is present in bovine, caprine, human, porcine and buffalo milks
• Bovine milk contains 11–25 mg L−1 of the enzyme, most of which is in the milk
serum
• RNase activity in bovine milk is thus more than three times higher than in human
milk, which contains 3 mg L−1
• Generally, RNase activity is higher in colostrum (three to five times) and mastitic
milk (two times) than in normal milk
• It catalyze the hydrolysis of ribonucleic acid (RNA) on the 3′ side of pyrimidine
nucleotides
• RNase may protect milk from viruses since it is reported to inhibit the activity of
RNA-dependent DNA polymerase, an enzyme involved in the replication of
viruses

12. Lysozyme
• It is quantitatively an important fraction of the proteins of human milk (400 mg
liter -1)
• It is a powerful bactericide as it attacks polysaccharides of the bacterial cell wall,
causing lysis of the bacteria
• Lysozyme (muramidase or N-acetylmuramylhydrolase) hydrolyse the β(1-4)-
linkage between muramic acid and N-acetylglucosamine of mucopolysaccharides
of the bacterial cell wall
• Although the lysozyme content in bovine milk contains only about 0.1 mg. liter-
1 it could be isolated from it
• The lysozyme of human milk appears to be identical to that found in other human
secretions; it is a polypeptide of 129 residues, with MW 14,602 Da