Milk is amphoteric in reaction which turns blue litmus to Red and Red litmus to blue. The acidity of milk is of two kinds.

1.      Natural Acidity or apparent Acidity: The Acidity that come from the component of the milk such as albumin, casein, phosphate citrate and carbon dioxide.
2.      Acquired or developed acidity:  the acidity that results from fermentation of   lactose to lactic acid by streptococcus lactis by lactic acid fermentation.
3.      Real Acidity (Titratable Acidity) (TA %): Generally the acidity of milk means the total acidity (Natural + developed) or tirratable acidity. It is determine by titrating a know volume of milk with standard alkali to the point of an indicator line phenolphthalein.
pH of fresh raw milk normally 6.4-6.8 with an average 6.6. Higher figures equal or greater than 7.7 indicates mastitis. Lower figures than the normal indicate lower keeping quality.
Material Required:
1. Milk sample.
2. N/10 NaOH solution.
3. Phenolphthalein indicator
Apparatus:
1.      10 ml capacity pipette.
2.      100 ml conical flask or porcelain dish.
3.      50 ml capacity burette with stand.
4.      Porcelain tile.
5.    Glass rod.
Procedure:
  1. Fill the burette with N/10 NaOH solution.
  2. Take 9 ml milk with the pipette in porcelain dish/conical flask.
  3. Add 3-4 drops of phenolphthalein indicator and stir with glass rod.
  4. Take the initially reading of the alkali in the burette at the lowest point of meniscus.
  5. Rapidly titrate the contents with N/10 NaOH solution continue to add alkali drop by the drop and stirring the content with glass rod till first definite change to pink colour which remains constant for 10 to 15 seconds.
  6. Complete the titration within 20 seconds.
  7. Note down the final burette reading.
  8. Repeat the titration after storage one hour at room temperature and note down the reading.

    Calculation:
                                       

For pH

pH is the concentration of hydrogen ions It determined by two methods:
Colorimetric method (PH paper)
Electrometric method (PH meter)


Colorimetric method (PH paper)
In this method ph strips are used and colour change is the indicator of ph
.
 Electrometric method (PH meter)
pH meter using an electric current .It is more precise method
Procedure:    
1. The PH meter should be turned on 30 minutes before
2. Rinse the glass electrodes with distill water and wipe with fin tissue.
3. Immerse electrodes in freshly prepared buffered solution (PH 7) & do the adjustment.
4. Take the electrodes out & rinse with distill water &wipe with fin tissue.
 5. Immerse the electrodes in the milk sample &read.      

Precaution
1.      Between measurements electrodes should be stored in distilled water.
2.      Use fresh N/10 NaOH solution.



Sr. No
Time
Temperature
Initial reading of NaOH
Final reading of NaOH
% acidity
pH