Market Milk

Module 10. Quality assurance in milk processing

Lesson 44
MICROBIOLOGICAL QUALITY OF MILK

44.1 Introduction

It is necessary to test microbiological quality of milk at a number of points along the chain from producer to consumer for public health and economic considerations.

44.2 Dye Reduction Tests

These methods are based on changes produced by bacteria due to their chemical constituents, enzymatic activity or on products of metabolic activity. They are widely used in dairy industry for their simplicity, quickness and low cost. They are helpful in judging the hygienic quality of milk, detecting abnormal milk and grading of raw milk supplies. The basis of the dye reduction tests is the ability of the bacteria to produce enzymes which can transfer hydrogen from a substrate to a redox dye which undergoes change in color. The rate of reduction of color depends upon the enzyme activity which is taken as the index of number of organisms present in milk. The reduction time is inversely related to the bacterial count of the sample. These tests are used for non-refrigerated, bulked raw milk.

44.2.1 Methylene blue reduction test

The redox potential of milk rises by +0.3 V during milking and subsequent handling. At this potential, methylene blue, if added to milk, will be in the oxidized state and will be blue in colour. When milk with dye is kept at a specific temperature, bacterial cells multiply and consume oxygen in the process. This results in decrease in the redox potential of milk and the dye becoming colourless.

To 10 ml milk in a test tube is added 1.0 ml of methylene blue test solution. The test tube is plugged with cotton and placed in water bath at 37°C. The tube is monitored for possible colour changes every 30 min. The time in hours, at which discoloration takes place is noted and compared with suggested standard for grading of milk as below (Table 44.1):

Table 44.1 BIS guidelines for the MBR test

* Reduction times are noted as whole hours e.g. if the colour
disappears between 1.5 and 2.5 h, the result will be noted as 2 h

44.2.2 Resazurin reduction test

As bacteria multiply, they consume dissolved oxygen resulting in the decrease of redox potential of milk. Resazurin acts as an oxidation-reduction indicator. The changes in colour take place in two stages. First the blue colour of resazurin changes to pink, owing to the compound resofurin. This colour change is irreversible. During the second stage, the resofurin is reduced to hydroresofurin, which is white in colour. The series of colour changes can be compared with standard colour discs in a Lovibond comparator and expressed in terms of disc number (6-10). Depending on the conditions and requirements, the test can be performed for 10 min, one hour or three hours. The former is generally used to weed out poor quality milk at the receiving platform.

A stock solution of resazurin is prepared by dissolving 0.05 g resazurin powder in 100 ml distilled water. The solution is heated for 30 min, cooled and stored in a cool dark place. The test is performed by mixing resazurin bench solution (one ml stock solution diluted to 10 ml with distilled water) with a sample of milk and incubating in a water bath at 37°C for the required period. The colour developed is compared using a resazurin disc. The milk is graded as per standard given in Table 44.2.

Table 44.2 Standard for resazurin test

44.3 Microbial Enumeration Methods

Direct methods are used to count cells microscopically or examine the ability of the viable cells to grow and form colonies. They are more sensitive and accurate than indirect methods, but take more time. The most widely used method is the plate count method, where serial dilutions of the sample are plated out on an agar medium in which the organisms grow. The plates are incubated at the appropriate temperature for a period of time (24-72 h). The colonies formed on the plates are counted and the results expressed as colony forming units (cfu) per gram or ml of the sample used for testing.

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.