4. Milk proteins — composition, content, significance. Methods of determination

Question 1

Milk protein…

Answer 1

• big differences among species, but will mainly focus on bovine milk (protein content of 3,3%)
• milk proteins:
• casein: 80%
• serum proteins (whey proteins): 20% — called whey proteins because during production of
  cheeses casein is the most important one as it forms the coagulum, and the rest of the proteins stay in the whey, hence the name

Question 2

Casein

Answer 2

• phosphoprotein: contains phosphorous
• important for production of dairy products: create the coagulum
• many important aa, nutritive importance
• heat stable
• several different types in milk
• types:
• αs1-casein 38,5%
• αs2-casein 10,5%
• ß-casein 36;5% — second place - κ-casein 12,5%
• 𝛄-casein 2%
• synthesised in secretory cells: especially in rER
• gamma casein is very specific type, not synthesised in the secretory cells but is a degradation
  product of ß-casein — by the enzyme called plasmin, or bacterial proteases

Question 3

Serum proteins

Answer 3

• ß-lactoglobulin (secretory cells of mammary gland)
• a-lactalbumin (secretory cells of mammary gland) — part of complex for synthesis of lactose
• BSA(blood)
• immunoglobulins (blood)

Question 4

ß-lactoglobulin

Answer 4

• can be up to 10% of all proteins in milk, highest and most important - hydrophobic
• deficiency of hydroxyprolin
• high amount of sulfur aa: cysteins, cystines, caline, lysine
• SH groups of cysteins partly protect milk against oxidation, can bound heavy metals - nutritive importance
• higher amount in premature milk 80%

Question 5

a-lactalbumin

Answer 5

• similar to lysosome but without bactericidal effect
• allaa
• the most stable
• lactosasnthetase: transfer UDP galactose to glucose - higher amount in mastitic milk

Question 6

Bovine serum albumin (BSA)

Answer 6

• from blood: same pathway as Igs, connected together but still not clear how they could go into the milk
• identical to blood serum albumin
• only 1,2% in milk
• higher amount in mastitic milk

Question 7

Immunoglobulins

Answer 7

• IgG = main Ig, IgA, IgM
• all from blood
• significantly high amount in colostrum, important for passive transfer of immunity from mother to
  calf
• higher amount also in mastitic milk
• IgG:
• highest concentration, 3,3% of all proteins
• in colostrum: up to 80% of all serum proteins - 3 subtypes: IgG1, IgG2, IgG3

highest amount usually IgG1
in mastitic milk: higher amount of IgG2
- IgA and IgM:
- in very low levels
- more than 100x higher in colostrum in all types of IgG

Question 8

Miscellaneous proteins

Answer 8

• osteopontin = most important
• ß-2-microglobulin
• lipoproteins
• enzymes
• proteins binding vitamines: A, D
• protease-pepton
• trasnferrin: anti-bacterial effect, due to the ability to bind Fe which is a very important growing
  factor for certain microorganisms
• lactoferrin: same as transferrin

Question 9

4) Deterioration of milk protein

Answer 9

• enzyme action or exposure to light
• enzyme action:
• due to presence of proteolytic enzymes (milk proteases)
• sources:
• native milk: naturally present
• bacterial contamination: produced by these bacteria
• bacteria that are added intentionally for fermentation
• somatic cells: amount of it is also one of the signs for the quality of milk (cf. another lecture)
• most important proteolytic enzyme = plasmin
• causes proteolysis during cheese production
• cause the flavour and texture in cheeses
• undesirable proteolytic action can cause off flavour and poor quality of milk
• exposure to light:
• can degrade 2 aa: methionine and cystine
• causing off flavour and loss of nutritional quality - i.e. do not store milk in light

Question 10

Influence of heat treatment on milk proteins

Answer 10

• caseins are more stable to heat treatment
• normal temperature for normal pasteurisation: usually does not affect any nutritional or
  functional properties of casein
• higher temperature treatment can cause interactions between casein and whey proteins —> affect functional properties:
• ß-lactoglobulin can form a layer on the casein micelles —> micelles are then protected during
  the production of dairy products which prevents curd formation
• whey proteins are more sensitive to heat:
• pasteurisation does not affect nutritional and functional properties
• can be an advantage in case of yoghurt production —> can bind more water —> economically
  profitable as they get more volume
• denaturation usually only changes their physical structures but does not affect aa composition
  i.e. nutritional ppts
• higher temperature treatment can affect functional properties — especially those that are
  more sensitive

Question 11

5) Milk allergy

Answer 11

• ≠ lactose intolerance
• insufficient function or amount of enzyme lactase produced in the body
• milk allergy: immunological reaction of some allergens in the milk, which can be all proteins
• 2 big groups of possible allergens: caseins and whey proteins
• most important = casein
• people who are allergic to casein cannot eat cheeses
• whet proteins: 2 main allergenic proteins which is a-lactalbumin and ß-lactoglobulin

Question 12

6) Methods of determination

1. Kjeldahl method

Answer 12

• determines the content of N in the sample
• N in released from proteins and other nitrogenous compounds and converted to ammonia through acid digestion
• crude protein is estimated by multiplying the N value by 6,38, the average N content in milk protein

Question 13

Organic dye method

Answer 13

• amino black method
• based on the addition of a dye to the milk, which bind to proteins
• the colour intensity of the dye is reduced and the colour is measured by colorimeter
• the degree of colour reduction is related to the protein content of milk