PHYSICAL CHEMISTRY OF MILK

Module 8. Acids and bases

Lesson 20
ACIDITY OF MILK

20.1 Introduction

The acidity of a solution results from the ionization of groups that split off or bind protons. Milk being a biological fluid secreted from mammary cells (gland) has to maintain isotonicity with the blood plasma. Further the proteins in milk exist in its native colloidal state only when the pH of milk is maintained constantly within the given range. Any change in the pH would destabilize the proteins and result in precipitation and gelation. The determination of acidity of milk is a rapid measure to understand the stability of milk during heat processing.

20.2 Acidity of Milk

The hydrogen ion concentration of milk is about 10-66 per litre which means that pH value for milk is 6.6. Freshly drawn milk is amphoteric to litmus paper that is to say that it will turn the red litmus to blue and blue litmus to red. The pH of normal healthy cow milk will range between 6.4 to 6.6 while the pH of buffalo milk is in the range of 6.7 – 6.8. If the animal is suffering from udder disease like mastitis the pH would be higher than the normal milk.

Due to the variation in the composition which is the result of the mammary gland activity a difference in the pH and buffering capacity of individual lots of fresh milk was observed. In general the pH is lower in colostrum (pH 6.0) and higher in cases of mastitis (Up to pH 7.5) than normal milk of mid lactation. Colostrums and mastitis milk differs radically from normal milk in the proportion of proteins, and certain salts (A radical difference in the colostrum and mastitis milk from normal milk could be observed in which the proportion of proteins and certain salts were observed to be different). Milks of low phosphorus, casein, and Ca²⁺ tend to be low in titratable acidity while excessive acidity is related to hyperketonemia, inadequate calcium and excessive concentrates in the ration.

Table. 20.1 Titratable acidity and pH of milk

(Source: Fundamental of Dairy Chemistry Wong et. al.,1988)

Perusal of the Table 20.1 reveal that there appears to be a reduction in the mean titratable acidity over a period of study. The drop in the maximum values is consistent with an improvement in the microbiological quality of milk supply.

20.3 Types of Acidity

There are two types of acidity observed in milk

a. Natural acidity or apparent acidity

b. Developed acidity or true acidity

20.3.1 Natural acidity

Milk freshly drawn from the udder of a cow shows acidity due to its normal constituents. Such acidity is known as Natural acidity or apparent acidity. The constituents that contribute to this acidity are Casein, acid phosphates and citrates and to some extent the albumins and globulins, carbon dioxide. Colostrum is having high natural acidity due to its protein content. In early lactation also this value is higher which slowly returns to normal level during the second month of lactation. This level is maintained upto the last month of lactation. At that stage of lactation a slight decline in the acidity is observed. This is the normal phenomenon during the lactation of an animal. The variation in the natural acidity of fresh milk is also observed between the species, breed, individual animal, physiological condition of the udder etc. An increase in the solids non fat content of milk would also increase the natural acidity.

20.3.2 Developed acidity

During the microbial multiplication in milk the lactose would be converted into lactic acid which would result in increase in the acidity of milk and decrease in the pH value. This acidity is known as developed acidity. The sum of natural acidity and developed acidity is known as titratable acidity. The normal acidity of individual cow Milk ranges from 0.10 to 0.26% lactic acid. Milk having titratable acidity more than 0.18% Lactic acid is not suitable to prepare heat treated products as the milk will coagulate at or above that acidity. Heated milk will show an increase in acidity due to the changes in the casein complex and formation of acids by the degradation of lactose. Dilution of milk will decrease the acidity and increases the pH. The acidity of milk cannot be increased through feeding.

20.4 Methods of Determination of Acidity of Milk

As a routine method the total titratable acidity is determined by using the titration method where the milk sample is titrated against standard alkali to the phenolphthalein end point. This corresponds to the pH 8.3 or 8.4. and at this end point the color of the phenolphthalein would change from colourless in the acidic medium to pink in the alkaline medium. The milk is being titrated using the standard sodium hydroxide and the acidity is determined by calculating the acidity using the volume of the standard sodium hydroxide used for the titrating to the phenolphthalein end point. The total titratable acidity of milk is expressed as percent lactic acid. The pH of the milk does not truly represent the titratable acidity because of the non availability of the ionic groups to titration in the normal milk. It is also a matter of interest that the titratable acidity of a milk sample will vary depending upon the buffering capacity of the milk. The natural constituents protect the milk from developing acidity. Mastitis, even in mild or sub-clinical form, causes the acidity of the milk to be lower. In rare cases mastitis causes a high acidity in the milk

20.5 Importance of Acidity of Milk

Titratable acidity of milk could be used in several stages during milk processing, however we have to use it with considerable discretion.

• It is mostly used as a method to grade raw milk at the plant while the milk is received at the milk reception dock (RMRD). While using this we have to take into consideration the variation in the natural acidity of the milk and the buffering capacity of milk. The conditions of storage of milk would also influence the initial acidity which means that if the milk is held under room conditions for long time and there is higher microbial count in the milk the initial increase in the acidity cannot be detected. It is necessary that higher SPC count should also be associated with high acidity
• Titratable acidity could also be of use in monitoring the udder health of an animal. A change in the titratable acidity indicates the change in the udder health
• It could also be used as a measure to study the heat stability of milk during processing. Acidity of more than 0.18% lactic acid would result in the curdling during heat processing