Standardization of Milk

i. Purpose and Definition

We know that liquid milk sold in the market is of different types with regard to its composition. Since the milk available to the processor may not necessarily be of the same composition as desired in the milk to be marketed, it is a very common practice to adjust the composition as per the requirement. Also, compositional modification (or adjustment) is necessary if the milk is to be converted into a certain product. A product must conform to the legal standards prescribed for it, or the quality standards set by the manufacturer. Product manufacture without appropriate compositional manipulation of milk may lead to poor quality product, or a product that does not meet legal requirements, or it may be an economical loss to the processor.

Standardization thus refers to the process by which the milk composition is adjusted to the desired level. The most commonly considered compositional parameters are fat and SNF for market milk, although sometimes fat alone may be taken into account for standardization. For certain specific, product,-manufacturing applications even protein content may be adjusted. Accordingly, the process of standardization involves lowering or raising the level of a particular constituent(s) to the desired value specially fat.

 

ii. Standardization of Milk for Fat

Often milk may be standardized to a certain value of a single component i.e. fat or SNF alone. This can be achieved by adding to the milk, a calculated quantity of a fat-rich product such as cream if the fat level is to be raised, or a low-fat or fat-free component e.g. skim milk, if the milk has excess fat. The fat content of milk can be reduced also by separating a calculated amount of cream of known fat percentage.

The calculation of the quantity of cream or skim milk to be added to milk, or cream or skim milk to be separated from it can be made by a simple method known as the Pearson’s square method. It consists in drawing an arbitrary square (or, a rectangle), placing at the left concerns of the square, the values of fat content of the two products to be mixed and at the centre of the square, the desired fat percentage. Then, subtractions are made diagonally across the square, the smaller value being deducted from the larger one, and the differences are entered at the correspondingly opposite corners on the right- hand side. These two new values at the right corners are summed to obtain a third value. All the three values placed at the right represent the proportions or relative amounts of the given products to be mixed (top right figure for the amount of the product to the top left, bottom right figure for the product to the bottom left, and the sum for the final product). The following is an example of such a calculation:

500 kg of milk testing 6.5% fat to be standardized to 3.1% fat using skim milk containing 0.05% fat.



Thus mixing of 3.05 kg of 6.5% fat milk with 3.40 kg of the skim milk will yield 6.45 kg of milk containing the desired fat level i.e. 3.1%. Therefore, the quantity of skim milk required to be added to 500 kg whole milk will be

(3.4 x 500)/3.05=3.05

Accordingly, 557.38 kg of 0.05% skim milk mixed with 500 kg of 6.5% fat whole milk will yield 1057.38 kg of milk having 3.1% fat.The single-component (fat-based) standardization is commonly used for cream meant for butter-making. It generally involves adjusting the fat percentage of a high-fat cream to the desired level by mixing it with the calculated quantity of skim milk (or, whole milk). Blending of the two components i.e. cream and skim milk or whole milk can be carried out by transferring the calculated quantities of the two(one after the other), to a tank (or, silo) with a provision for adequate mixing.

Continuous, on-line blending is much more desirable in a large-scale operation. This can be achieved on the cream separator itself by allowing sufficient cream to remix with the skim milk so that the mixture is a milk with the desired fat content; the balance cream flows through the cream line into the cream tank. This requires that the separator is fitted with a standardizing device.

 

iii. Standardization of Milk for Fat and SNF

When milk is required to be standardized for both fat and SNF, the basis of calculation of the quantity of skim milk or cream to be added is the ratio of fat to SNF, and the total solids (TS) content. If the desired fat-to-SNF ratio is higher than the actual ratio in the available milk, then skim milk will be required to be added. On the other hand, when the desired ratio is lower, cream needs to be blended into the milk. It is, therefore, necessary that both the fat and SNF contents of the milk to be standardized, and those of cream or skim milk to be used are known. If the fat content of cream or skim milk (separated from a milk of known fat and SNF contents) is known, the SNF content can be estimated as under

i) SNF in cream, % = SNFm x ((100 - fc)/(100-fm))..... (Eq. 3)
                           
ii) SNF in skim milk = SNFm x ((100 - fs)/(100-fm)).... (Eq. 4)
                            
where,

SNFm = SNF percentage in milk

fc = fat percentage in cream

fm = fat percentage in milk

fs = fat percentage in skim milk.

The amount of skim milk or cream required to be added to a given quantity of milk(so as to attain the desired levels of fat and SNF in it) can be worked out by using the following formulae:

i) Qc = Qm x ((R x SNFm )/ (fc - (R x SNFc ))- Fm)..... (Eq. 5)
            
ii) Qs = Qm x ((fm /R) - SNFm)/ (SNFs - (fs /R))....... (Eq. 6)
              
where,

Qm =Quantity of milk to be standardized

Qc = Quantity of cream required

Qs = Quantity of skim milk required

R = Fat/ SNF ratio desired.

fc = Fat percentage in cream

fm = Fat percentage in milk

fs= Fat percentage in skim milk

SNFm = SNF percentage in milk

SNFc = SNF percentage in cream

SNFs = SNF percentage in skim milk.

Alternatively, an algebraic method can be used taking ‘x’ quantity of cream or skim milk of known fat and SNF contents required to be added to the given quantity of milk with certain fat and SNF levels, and then solving. (for x) an equation of the desired fat-SNF ratio:

where all values except that of x are known.