Enzymes are organic catalytst, which are found in plant and animal cells. The enzymes bring about metabolic reactions but they don’t undergo any chemical change. They are colloidal and proteinous in nature and are classified as per the reaction performed e.g., lipase, the fat splitting enzyme. The activity is affected by pH, heat, light etc. Enzyme in milk gain entry via udder or externally.Milk enzymes are technologically important. They are related with flavour (e.g.,lipase). Study and knowledge of these enzymes is essential to understand their role.
Functions of enzymes
The following functions are related to enzymes.
Functions of enzymes
The following functions are related to enzymes.
- Oxidising enzymes (e.g., peroxidase)
- Lipolytic enzyme hydrolyzing fat (e.g., lipase)
- Decomposing H 2 O 2 (e.g., catalase)
- Decomposes phosphorous esters.(e.g., phosphatase)
- Lactose hydrolyzing enzyme (e.g., lactase)
- Reductase as reducing enzyme (e.g., MBR test)
- Proteolytic enzymes hydrolyzing protein (e.g., protease)
- Hydrolysing aldehyde (e.g., xanthine oxidase)Peroxidase, lipase, catalase, reductase, phosphatase, xanthine oxidase, lactase are all present in freshly drawn milk. Other enzymes enter via bacterial contamination.
i)Peroxidase: Peroxidase is present in milk as lactoperoxidase enzyme. The enzyme is destroyed between 70-80 0 C. Lactoperoxidase enzyme act on H 2 O 2 in the presence of thiocyanate ions, forming hypothiocyanate ions (OSCN - ) are lethal to microbes. This enzyme has been used in milk to improve shelf life during transportation of milk from distant places to milk plant. The enzyme is also used as an index of detecting proper heating of milk as it is destroyed at 70 0 C, especially for detecting high temperature heat treatment of milk.
ii) Phosphatase: Phosphatase catalyse the hydrolysis of phosphate esters. Alkaline phosphatase is the most important milk enzyme. It is destroyed by pasteurization of milk. At the temperature of pasteurization of milk tubercle bacili bacteria present in milk are also destroyed. The inactivation of this enzyme is thus taken as the process of destruction of TB organisms. Under health consideration pasteurization of milk is mandatory in various countries. Phosphatase test has been developed to ascertain if the milk has been properly pasteurized. So as to ensure the destruction of Micobacterium tuberculosis which is destroyed at a temperature wherein alkaline phosphatase is inactivated.
iii) Lipases: Lipases hydrolyse milk fat into corresponding fatty acids and glycerol.In milk they are linked with hydrolytic rancidity of milk fat releasing butyric acid. Excessive presence of butyric acid in milk causes rancid flavour defect.This defect may also be present in butter. They are destroyed at 63 0 C when heated for 20 minutes.
iv) Proteases: Proteins are hydrolysed by proteases to simple compounds such as proteose, peptone, amino acid and other compounds. They are inactivated in the presence of salt or preservative. Proteases are destroyed by heating milk between 70-80 0 C. Proteolytic enzymes have been employed externally for preparing different varieties of cheese. These enzymes primarily hydrolyze Catalase casein.
v) Reductase: Reductase are enzymes of bacterial origin. These enzymes are capable of reducing certain dyes to their colourless leuco-compounds. It has been shown that generally speaking the reduction time at 38 0 C is approximately proportional to the number of bacteria.. They are used as measure of microbial population and determine the extent of contamination of milk by bacteria. This is possible through methylene blue reduction test (MBRT). The blue dye is reduced to a colourless compound in the presence of reductase. The earlier the dye lost its blue colour greater is the contamination.
vi) Catalase: Catalase catalyses the decomposes hydrogen peroxide as per the following reaction
ii) Phosphatase: Phosphatase catalyse the hydrolysis of phosphate esters. Alkaline phosphatase is the most important milk enzyme. It is destroyed by pasteurization of milk. At the temperature of pasteurization of milk tubercle bacili bacteria present in milk are also destroyed. The inactivation of this enzyme is thus taken as the process of destruction of TB organisms. Under health consideration pasteurization of milk is mandatory in various countries. Phosphatase test has been developed to ascertain if the milk has been properly pasteurized. So as to ensure the destruction of Micobacterium tuberculosis which is destroyed at a temperature wherein alkaline phosphatase is inactivated.
iii) Lipases: Lipases hydrolyse milk fat into corresponding fatty acids and glycerol.In milk they are linked with hydrolytic rancidity of milk fat releasing butyric acid. Excessive presence of butyric acid in milk causes rancid flavour defect.This defect may also be present in butter. They are destroyed at 63 0 C when heated for 20 minutes.
iv) Proteases: Proteins are hydrolysed by proteases to simple compounds such as proteose, peptone, amino acid and other compounds. They are inactivated in the presence of salt or preservative. Proteases are destroyed by heating milk between 70-80 0 C. Proteolytic enzymes have been employed externally for preparing different varieties of cheese. These enzymes primarily hydrolyze Catalase casein.
v) Reductase: Reductase are enzymes of bacterial origin. These enzymes are capable of reducing certain dyes to their colourless leuco-compounds. It has been shown that generally speaking the reduction time at 38 0 C is approximately proportional to the number of bacteria.. They are used as measure of microbial population and determine the extent of contamination of milk by bacteria. This is possible through methylene blue reduction test (MBRT). The blue dye is reduced to a colourless compound in the presence of reductase. The earlier the dye lost its blue colour greater is the contamination.
vi) Catalase: Catalase catalyses the decomposes hydrogen peroxide as per the following reaction
2H 2 O 2 2H 2 O+O 2
Catalase content varies in milk from different animals and within the same species. It is also affected by feed given to the animal. Catalase content is high in colostrum, mastitis milk and milk contaminated with mastitis or colostrum milk or bacterial contamination. It tends to parallel leucocyte count. It increases with multiplication of bacteria in milk. It is destroyed when milk is heated to about 65 0 C or over.
vii) Xanthine oxidase: A variety of substances are oxidized by this enzyme including xanthine, hypoxanthine, aldehyde, oxypurines, etc. Thus in the presence of O 2 and an aldehyde following reaction takes place:
RCHO +H 2 O+O 2 ⎯Xanthine oxidase ⎯⎯ ⎯ ⎯ ⎯ ⎯ →RCOOH+H 2 O 2
Xanthine oxidase is a prominent enzyme of milk and was discovered as early as 1902.
Xanthine oxidase content varies from cow to cow and increases with stage of lactation. It is associated with fat globules. It can be isolated from cream or buttermilk. The following table gives the data for inactivation of the enzymes in milk.
Catalase content varies in milk from different animals and within the same species. It is also affected by feed given to the animal. Catalase content is high in colostrum, mastitis milk and milk contaminated with mastitis or colostrum milk or bacterial contamination. It tends to parallel leucocyte count. It increases with multiplication of bacteria in milk. It is destroyed when milk is heated to about 65 0 C or over.
vii) Xanthine oxidase: A variety of substances are oxidized by this enzyme including xanthine, hypoxanthine, aldehyde, oxypurines, etc. Thus in the presence of O 2 and an aldehyde following reaction takes place:
RCHO +H 2 O+O 2 ⎯Xanthine oxidase ⎯⎯ ⎯ ⎯ ⎯ ⎯ →RCOOH+H 2 O 2
Xanthine oxidase is a prominent enzyme of milk and was discovered as early as 1902.
Xanthine oxidase content varies from cow to cow and increases with stage of lactation. It is associated with fat globules. It can be isolated from cream or buttermilk. The following table gives the data for inactivation of the enzymes in milk.
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| Inactivation temperature of enzymes |
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