Quality Testing of Milk using Colorimetric Chemical Analysis and Lovibond® Colour Standards
IntroductionColorimetric chemical analysis is commonly used to determine the amount of a particular chemical present in a sample, using the depth of colour produced by the action of chemical reagents as a measure of concentration. As summarised below, several substances that are useful in the quality testing of milk can be determined colorimetrically using appropriate reagents and Lovibond® test discs containing series of coloured glass filters, which have been calibrated in accordance with the specific test procedures. These tests are ideal for field applications; they are based on simple, robust and inexpensive apparatus, employ straightforward methods that give rapid results, yet achieve a standard of accuracy that is more than sufficient for such purposes.
1. The Efficiency of Pasteurisation – The Phosphatase Test
The Phosphatase test is a chemical measure for the efficiency of milk processing, revealing whether the products have been adequately heat-treated and the possible causes of under-pasteurisation or whether they have been contaminated with raw products. The Phosphatase test is valuable as a rapid test, designed to detect minor processing faults after an incubation period as short as 30 minutes, compared with incubation of up to 24 hours for alternative methods. It is used widely as a routine test in dairy plant control and has been adopted as the official test method in the UK and France.
2. Quality Test based on the Resazurin Reduction Method
The Resazurin test is used as a rapid indication of the bacterial content of milk. Resazurin gives milk a characteristic blue colour and the test is based on the ability of bacteria in the milk to reduce the blue dye. The quality of the milk is judged by noting the degree of colour change – from blue through mauve and purple and pink and finally colourless – after a stated period of incubation, or the time required reducing the dye to a predetermined colour.
The Resazurin test is ideal for testing milk at the point of delivery and for regular periodic checks on quality. Although it is most applicable to raw milk, it has also been used to test pasteurised milk, raw and pasteurised cream, foremilk samples for detecting mastitis plus the cleanliness of shipping cans and other containers.
3. Estimating Shelf-Life by Measuring Developed Lactic AcidThis test for milk and milk products is based on a relationship between developed lactic acid and keeping quality. Milk proteins are precipitated by means of reagents and ferric chloride is added to the filtrate to produce a yellow complex. The intensity of this yellow colour is proportional to the concentration of lactic acid present.
Equipment Required: Each of these colorimetric tests can be carried out using a selection of inexpensive and simple to use equipment:
- the Lovibond® Comparator 2000 in conjunction with the
- Milk Viewing Stand, DB415,
- the appropriate reagents and
- test discs containing coloured, graded glass filters (see table) and
- test tubes, AF215TT.
- Also available are portable and laboratory lighting units for accurate colour matching irrespective of ambient lighting.
Other Chemical Analysis Tests:
4. Alizarol (68%) Test
This test gives an indication of the heat stability of milk and is used for testing the stability of the milk proteins before processing.
The test is used as a platform (acceptance/rejecting) test for raw milk for acceptability either on the farm, tanker or milk reception at the factory.
|In summary, colour standards indicate:|
|1.||Light purple (fresh milk)|
|3.||Brownish-yellow (slightly acid)|
5. Lactognost Test
Qualitative analysis of phosphatase in milk, whey, cream and butter
Kit with dosed reagents for determination of alkaline phosphatase activity (limit test).
The test enables the determination of H.T.S.T. heating of milk and similar products or the determination of raw milk in pasteurized milk.
2 Years on the Lovibond® comparator and disc set