MARKET MILK

Module 10. Quality assurance in milk processing

Lesson 43
CHEMICAL QUALITY OF MILK - FAT, SNF, TS AND ACIDITY

43.1 Introduction

Chemical tests performed in order to meet the mandatory standards for legal requirements constitute testing for major components of milk and milk products namely, fat, total solids, lactose, protein, moisture, ash etc. Speed, sensitivity and accuracy are key factors for choice of the various existing methods. Rapid instrumental methods have also evolved in the recent past. Therefore, many dairies use instruments such as Milko-tester, Infra-red milk analyzer, Milko-scan, Lactostar etc. for quick analysis of major constituents of milk.

43.2 Fat Determination

As the FSS Rules, 2011 (earlier PFA Act) prescribes the minimum level of milk fat for cow and buffalo milk in various regions of the country, it is important to determine fat content of raw milk being received at the dairy. There are two methods of determining fat content of milk namely the Gerber method, based on direct reading and Rose Gottlieb method, which is a gravimetric test. The former is more widely used as it is quicker than the gravimetric method. Figure 43.1 illustrates the Gerber fat testing assembly.

The Gerber test is based on the principle that when a definite quantity of sulphuric acid and amyl alcohol are added to a definite volume of milk, the proteins dissolve and fat is set free from within the globules. This fat remains in the liquid state due to heat produced by the acid. Upon centrifugation, fat being lighter separates on top of the solution as liquid fat column in butyrometer stem.

Ten milliliters of sulphuric acid (Gerber acid) is transferred into milk butyrometer (range 0–10%) using an automatic measure. The milk sample is mixed well and 10.75 ml is slowly transferred from the side of the butyrometer wall taking care not to wet its neck. Thereafter, one ml of amyl alcohol is added and the butyrometer closed with a lock stopper and shaken well. The contents are centrifuged for 5 min at 1100–1200 rpm and the fat that appears as a colourless column read directly on the butyrometer stem.

Fig. 43.1 Gerber fat testing assembly

The theory behind using 10.75 ml milk in the pipette is as follows:

Graduation in the Gerber butyrometer, i.e., 0-10 is calibrated in such a way that each 1% division represents 0.125 ml.
So, if milk has 10% fat, it represents 0.125*10 = 1.125 ml of fat.
This is equivalent to 1.25 g of fat (because density of fat = 0.9 mg/ml)
As per this, we should be pipetting 11.25 g of milk, but there are certain impurities due to isoamyl alcohol, which affects the fat reading. These impurities are estimated at 2.5-3% (average (2.5+3)/2 = 2.667%).
So the fat is 1.125-(1.125*2.667/100) = 1.095 g.
According to this we should be pipetting 10.95 g of milk.
This is equivalent to 10.65 ml of milk (10.95/1.02547, the denominator being the density of milk).
Since 0.1 ml residual milk remains in the glass pipette sticking to the walls, we take 10.75 ml of milk.

43.3 Solids-Not-Fat (SNF) Estimation

SNF is the collective term given to the various components of milk other than fat. The SNF content of milk is calculated by determining the specific gravity of milk. Lactometers are used for rapid determination of specific gravity of liquids. The method is based on the law of floatation which states that when a solid is immersed in a liquid it is subjected to upward thrust equal to the weight of liquid displaced by it and acting vertically upwards. Lactometers are variable immersion type hydrometers and calibrated in advance with liquids of known specific gravity. The SNF testing assembly is shown in Fig. 43.2.

Fig. 43.2 SNF testing assembly

The sample is mixed well avoiding incorporation of air or foam formation. The temperature of milk sample is adjusted to measuring temperature prescribed for the BIS lactometer (27ºC). Sufficient milk is poured into the glass or steel cylinder to allow free floating of lactometer. It is then placed in the milk and allowed to float till it stops and assumes a constant level. The lactometer reading and temperature of milk are recorded at the same time. This is the lactometer reading (LR). The corrected value or corrected lactometer reading (CLR) is obtained from the standard table for corresponding temperature. The SNF and/or TS content are calculated using the given formulas.

Percent SNF = CLR/4 + (0.25*fat %) + 0.44
Percent Total solids = CLR/4 + (1.25*fat %) + 0.44

43.4 Acidity Test

While the normal acidity of milk is due to its constituents like casein, citrates, phosphates and carbon dioxide, developed acidity in milk as a result of microbial action makes it unsuitable for high heat processing. Milk acidity is measured by titrating milk against a standard alkali solution using an indicator. Per cent acidity is determined by dividing number of ml of NaOH used with 10 and is expressed in terms of lactic acid. Acidity testing assembly is depicted in Fig. 43.3.

Fig. 43.3 Acidity checking assembly

Selected Readings

FAO. 1998. Milk Testing and Quality Control. Milk Processing Guide Series. Volume 2. FAO/TCP/KEN/6611 Project. Training Programme for Small Scale Dairy Sector and Dairy Training Institute – Naivasha. Kenya.
IS: SP : 18 (Part XI), 1981. Handbook of Food Analysis. Part XI., Dairy Products. Bureau of Indian Standards, New Delhi.
Marshall, R.T. 1993. Ed. Standard Methods for the Examination of Dairy Products. 16th ed. American Public Health Association, Washington, DC.
MIF. 2005. Analysis of Milk and Its Products: A Laboratory Manual. Milk Industry Foundation (U.S.). Daya Books, India.