MARKET MILK

Module 10. Quality assurance in milk processing

Lesson 41
QUALITY ASSESSMENT OF RAW MILK

41.1 Introduction

Milk being a product of biological origin, is extremely vulnerable to attack by microbes. It is also a good vehicle for additives and adulterants without any apparent changes in its appearance. Thus, milk is prone to several post-secretion changes, some natural and some man-made. Most dairy plants usually follow chemical quality control with the intention of payments to contractors/producers, processing of milk and meeting mandatory requirements for end products.

41.2 Quality Assessment of Raw Milk

The quality of dairy products depends upon quality of raw milk used in their manufacture, processing and handling. The poor socio-economic conditions of the rural producers pose serious problems in producing raw milk of good quality, which is further deteriorated during subsequent handling and transportation. The quality of the incoming raw milk is assessed to check its suitability for processing through various quick tests called ‘platform tests’. Samples are drawn from the milk supplied by each producer and certain tests performed to assess its acceptability.

41.2.1 Visual and sensory tests

Milk received at the dairy plant is thoroughly screened for the presence of any objectionable material floating on top of the container. It is also tested for any objectionable flavour by taking 20-30 ml of milk in the mouth and rolling it in the palate and mouth cavity for assessing taste and flavour, which should be typical of milk.

41.2.2 Sediment test

This test assesses the cleanliness of milk received at the dairy plant and is performed by using a sediment tester. A sediment disc is inserted in the space provided and the tester is dipped to the bottom of the can. Milk is collected slowly from different parts of the bottom of the can by pulling the plunger upwards. The sediment tester is then removed and the plunger pressed down to empty the tester. The sediment disc is removed and compared with standard sediment disc. Alternatively, it may also be weighed and compared to the weight of an unused sediment disc. The milk is graded in accordance to standard as per Table 41.1 below.

Table 41.1 Standards for sediment (BIS)

41.2.3 Clot-on-boiling (COB) test

COB test indicates the suitability of milk for pasteurization and other heat treatment processes. Five ml of milk in a test tube is held over a flame and allowed to boil. The formation of flakes or clots indicates that the milk has high developed acidity and is unsuitable for pasteurization or high heat treatments.

41.2.4 Alcohol test

Milk intended for high heat processing such as condensing and UHT processing has to be highly heat stable. Milk sample (5 ml) is mixed well with equal amount of ethyl alcohol, and observed for formation of flakes or clots. Formation of small flakes indicates that the milk is abnormal either due to high acidity or disturbed salt balance. Such milk is not suitable for high heat treatment.

41.2.5 Alcohol–alizarin test

This test illustrates the suitability of milk for high heat treatment and also gives an idea about its acidity. To 5 ml milk in a test tube is added an equal amount of alcohol-alizarin solution (0.2%). The contents are mixed and observed for formation of flakes and colour and the observations matched with the following table (Table 41.2).

Table 41.2 Observation chart for Alcohol-alizarin test

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.