5. Acidity of milk — determination, factors influencing acidity of milk

Question 1

Methods of determination

Answer 1

• acidity of milk is measured by: - titratable acidity

- pH

Question 2

pH

Answer 2

• important factor to be measured, defined as the negative logarithm of the amount of hydrogen ions (in moles) per litter of liquid
• measures the acidity or alkalinity of a solution
• determines processes such as: curding of milk, action of enzymes, growth of bacteria, taste, etc
• influences:
• activity of enzymes
• growth of microorganisms - technological properties
• pH values of milk:
• physiological value of cow milk: 6,5-6,7 - colostrum: < 6
• mastitic milk: > 7,5
• bacterial deterioration lowers the pH

Question 3

pH measurements:

Answer 3

• by addition of pH indicator into the solution:
• the indicator colour varies depending on the pH of the solution
• common indicators: litmus paper, phenolphthalein, methyl orange, phenol red, bromothyl blue,
  bromocresol green, and bromocresol purple
• by using a pH meter together with pH selective electrodes
• by using pH paper:
• indicator paper that turns colour corresponding to a pH on a colour key
• the pH paper us usually small trips of paper that has been soaked in an indicator solution and
  used for approximations

Question 4

Titratable acidity

Answer 4

• milk has buffering capacity i.e. resistance to changes in pH on addition of acid or base
• most important buffering components are phosphate, proteins, citrate —> can bind a small
  amount of acids/alkaline —> pH value is unchanged but titratable acidity will change
• buffering capacity of milk is often estimated by determining its titratable acidity
• recommended value for bulk milk sample: 6,2-7,8° using SH
• measured by two methods mainly (practical):
• Soxhlet-Henkel method: used in Czech Republic - Thörner method: mostly used in Sweden

Question 5

Soxhlet-Henkel method:

Answer 5

• volume of milk titrated with NaOH solution
• from pH of milk to the pH of end point of indicator phenolphthalein
• SH = amount of 0,25 mol.L^-1 NaOH used for titration x 2

Question 6

Thörner method:

Answer 6

• determined after its dilution with water
• similar principle as SH
• °T’ = amount 0,1 mol/L^-1 NaOH used for titration x 10

Question 7

Orientation methods:

Answer 7

• used for detection of acidity in milk
• e.g. alcohol test and the clot-on-boiling test give a rough indication of the acidity of milk
• clot on boiling test: sour milk is curdled during boiling and that’s why this test is used for
  detection of whether the milk may be boiled
• alcohol test: if the acidity of milk is higher than 9 SH, and diluted alcohol solution is added into
  milk —> milk is curdled

Question 8

Test with indicator papers

Answer 8

• very fast
• used in dairy plants when the milk is received
• indicator papers are dipped into the milk samples and the changes of indicator paper are
  compared with the standard strips
• results of the test depends on the quality of the indicator: light, moisture, and long period of
  storage deteriorate their quality

Question 9

Expression of titratable acidity:

Answer 9

• °SH — Sohlet-Henkel degrees:
• normal milk: 6,2-7,8 °SH
• °Th — Thörner degrees:
• normal milk: 17 Th
• acidity in degree SH may be converted into degree Thörner and conversely:
• SH=°T/2,5
• °T=SHx2,5
• °D — Domic degrees: normal milk has 13-15 °D
• % l.a. — percent lactic acid:
• obtained as °D, divide the reading by 10
• frequently used in UK, USA, Canada, Australia, and New Zealand

Question 10

Factors influencing acidity of milk Factors influencing milk pH

Answer 10

• stage of lactation: colostrum has a pH of < 6
• health of cow: mastitic milk has a pH of > 7-7,5
  Factors influencing titratable acidity
• breed
• lactation period:
  
  • colostrum: 15 SH
    2nd week of lactation: 7,1 SH
    end of lactation: 6,3 SH
• composition of milk
• presence of contaminants
• age of cow:
• young: 7,5-8,5
• old: 5,5 SH
• health of cow:
• mastitis: < 6 SH
• herd with subclinical mastitis: bulk milk —> 5,8-6,2 SH
• nutrition
• milk with high content of protein
• metabolic disorders:
• acidosis: 6,8 SH
• alkalosis: 5,72 SH - ketosis: 5,92 SH

Question 11

Secondary changes of titratable acidity:

Answer 11

• fermentation of lactose: bacterial activity

- added water 1% lowers titratable acidity by about 0,1-0,05 SH - detergents