Lektion 29/3 - Enzymes in milk

Question 1

Alkaline phosphatase
pH optimum
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Answer 1

• pH optimum ≈ 9

• Used to control that heat treatment (HTST) was carried out properly
• bildas phenol som är blå om ej rätt

• Significance in milk
– Dephosphorylation of phosphopeptides
in cheese, particularly in raw milk cheese

Question 2

Acid phosphatase
pH optimum
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Answer 2

• pH optimum ≈4
• Involved in cheese ripening by catalysing hydrolysis of phosphate groups from caseins
• Removal of phosphate makes casein more easily degraded by proteolytic enzymes
• Hypothesis: rate-limiting step in cheese ripening

Question 3

Sulfhydryl oxidase
pH optimum
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Answer 3

• pH optimum ≈7
• Creates disulfide bridges in proteins
• Has been used in processing, immobilized in a column, to oxidize thiols (-SH) to reduce the cooked flavour of UHT milk
• Enzyme levels in milk too low to have any
significant role in normal milk
• catalyserar s-s bryggor

Question 4

Superoxide dismutase (SOD)
pH optimum
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Answer 4

• SOD identified in many animal and bacterial cells, biological role to protect tissue against free oxygen radicals
• Scavenges superoxide radicals
• H2O2 produced may be reduced to H2O + O2 by e.g. catalase or peroxidase
• Possible role in milk is to inhibit lipid oxidation
• Milk contains several pro- and antioxidants and the
balance between these determines the oxidative
stability of milk, not individual factors

Question 5

Lactoperoxidase
pH optimum
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Answer 5

• högsta aktivitet i mjölk
• pH optimum 6.5
• A ubiquitous heme-containing peroxidase in milk, capable of forming free radicals and contribute to oxidative mechanisms in milk
• Activity limited by low levels of H2O2
• Bactericidal/ bacteriostatic function in mammary gland
• Used in milk preservation in tropical countries

Question 6

Plasmin
pH optimum
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Answer 6

• pH optimum ≈8
• Plasmin in milk identical to that in blood
– biological role hydrolysis of blood clots
– in milk plasmin related activites are associated to casein micelle

• Levels/ activity increase with severity of mastitis,
lactation week and number

• Physiological role in
– Regulation of mammary secretion
– Induction of mammary gland involution

•finns inhibitorer som inhiberar aktiverings substansen av plasmin och inhibitor som inhiberar dess aktivitet

Question 7

Importance of plasmin in dairy products:

Answer 7

• Importance of plasmin in dairy products:
– PP lost in whey during cheese making,
resulting in lower cheese yield
– Plasmin contributes to gelation of UHT milk
+ Plasmin important during cheese ripening
• Flavor
• Reduction in ripening time

Question 8

Lipoprotein lipase (LPL)
pH optimum
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Answer 8

• pH optimum≈8.5

• In blood LPL liberates fatty acids from lipoproteins and
chylomicrons
– A cofactor, apo-protein (part of certain lipoproteins), necessary for LPL to be active at oil-water interface

• Activity of LPL in milk normally low due to number of factors:
– pH of milk suboptimal
– Most enzyme bound to casein micelle, not MFG
– Triglycerides protected inside MFG, however, in the presence of certain blood lipoproteins LPL can penetrate the membrane and adsorb onto the fat
– Enzyme inhibitors present in milk (proteose peptones)

• Increased activity associated to leakage of lipoproteins from blood to milk, e.g. at end of lactation, during mastitis
• Increasing the number of milkings also leads to increased lipolysis

Question 9

why are milk enzymes of

interest?

Answer 9

• Physiological role
– Part of innate immune system
– Prevent oxidative processes (lipid oxidation)
– For many enzymes still not understood

• Use as indices for processing
– E.g. alcaline phosphatase

• Effects on quality of dairy products
– Plasmin
– Lipoprotein lipase