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