Milk Quality Tests
Somatic Cell Count – milk quality indicator
The Somatic Cell Count
The Somatic Cell Count (SCC) is a main indicator of milk quality. The majority of somatic cells are leukocytes (white blood cells) – which become present in increasing numbers in milk usually as an immune response to a mastitis-causing pathogen – and a small number of epithelial cells, which are milk-producing cells shed from inside of the udder when an infection occurs.
The SCC is quantified as the number of cells per ml of milk. In general terms:
An individual cow SCC of 100,000 or less indicates an ‘uninfected’ cow, where there are no significant production losses due to subclinical mastitis.
A threshold SCC of 200,000 would determine whether a cow is infected with mastitis. Cows with a result of greater than 200,000 are highly likely to be infected on at least one quarter.
Cows infected with significant pathogens have an SCC of 300,000 or greater.
The SCC in the milk increases after calving when colostrum is produced before the cow settles into lactation, and tends to rise towards the end of lactation, most likely due to the concentrating effect of lower amounts of milk being produced. SCCs vary, however, due to many factors, including seasonal and management effects.
Dairy farmers are financially rewarded for low herd SCCs and penalised for high ones, because cell counts reflect the quality of the milk produced and how mastitis can affect its constituent parts, having implications for its keeping abilities, its taste and how well it can be made into other dairy products such as yoghurt or cheese. Milk contracts often define several SCC ‘thresholds’ and any respective bonus for attaining them. Milk with an SCC of more than 400,000 is deemed unfit for human consumption by the European Union.
Essentially, a lower SCC indicates better animal health, as somatic cells originate only from inside the animal’s udder. SCC monitoring is important because as the number of somatic cells increases, milk yield is likely to fall, primarily due to the damage to milk-producing tissue in the udder caused by mastitis pathogens and the toxins they produce, particularly when epithelial cells are lost.
A particularly low SCC is sometimes regarded as a sign of poor immune response, but in general terms this need not be necessarily true; it may be the case that there is simply a low level of current infection. Immune response is best measured by how quickly the immune system reacts to the disease challenge, not how many white blood cells are present before infection occurs.
Cell counts tend to reflect a response to contagious mastitis pathogens: the Bactoscan count, on the other hand, indicates the level of bacterial contamination from external sources, such as insufficient cleaning of the milking equipment or poor udder and teat preparation prior to milking, and can indicate a high level of environmental pathogens.
The PortaSCC 5min Somatic Cell Count Test provides a simple and easy to use method to determine Somatic Cell counts of your milk
The Resazurin Test
The Resazurin Test is conducted similar to the Methylene Blue reduction test with the judgement of quality based either on the colour produced after a stated period of incubation or on the time required to reduce the dye to given end-point.
Resazurin is available in tablet form. One tablet to 50ml cold sterile deionised water gives a 0.005% solution. Make up fresh daily.
The disc contains seven glass colour standards, ranging from blue through mauve and purple to pink and finally colourless.
Taking the sample 2,3. When a composite sample is taken from all the cans in a consignment the sample from each can should be proportional to the volume of milk in the can. Otherwise the sample is best taken after the milk from all cans has been mixed.
The milk to be sampled should be vigorously mixed by stirring and plunging with a sterile dipper, and the sample should be taken from well below the surface. The sample should be poured into a sterile bottle and immediately stoppered.
1 Hour test 2
Shake the sample bottle 25 times up and down, with an excursion of about 300mm., and then pour the milk into a sterile test tube up to the 10ml. mark. Both the mouth of the sample bottle and the test tube should be flamed using a slightly yellow bunsen flame.
Take 1ml. of the resazurin solution in a sterile pipette, inserting the pipette 15mm. into the mouth of the tube and expelling the solution by blowing. Take care that the tip of the pipette does not touch the sides of the test tube.
Replace the sterile stopper in the tube and mix the contents by inverting the tube twice.
After adding resazurin solution to a batch of 10 tubes, in the case of 10 samples, place the tubes in the water-bath at 37.5 ±5oC and note the time. The level of the water in the bath must be maintained above the level of milk in the tubes for the whole period of incubation.
After exactly 60 minutes remove the tubes from the bath and examine immediately. Any tube showing no colour is recorded as 0. Very pale pink, pink and white mottling, or deep pink at the top with paler pink below record as ½.
Mix the contents of the other tubes by inverting twice, wipe off any water from the outside of the tube and place the tube in the right-hand slot at the back of the comparator stand so that it rests on the ramp and comes below the colourless centre of the disc.
A “blank” of similar milk without resazurin is poured into an identical tube and placed on the left-hand ramp to act as a background for the coloured glass standards. When Guernsey or Jersey milk is being tested, special care must be taken to use a blank from the same milk. The use of a blank in this manner compensates for the variable colour inherent in milk
Place the comparator and stand on a bench facing a standard source of white light, such as the Lovibond Daylight 2000 Unit or, failing this, North daylight. Make sure that both tubes are equally illuminated.
Rotate the disc until a colour match is obtained or until the sample colour falls between two successive standards. Read off the value of the matching colour from the indicator window in the comparator, interpolating half values between standards as required, i.e. record a 3½ sample with a colour falling between standard 3 and 4.
Any sample giving a disc reading of 4 or above on this test may be regarded as satisfactory.
Products required available from LABTEK Services:
- RZN-100Resazurin Tablets (100units)
- LOVIBOND Comparator
- Resazurin Disc – Bacterial Content of Milk
- Reductase Tubes Marked 10ml RT510100 Reductase 5/10ml Tubes (100units)
- Reductase Tube Rubber Stopper RTS010190 Stoppers Rubber Red Solid (pk of 20)
Methylene Blue Reduction Test
The Methylene Blue reduction test is based on de-colorization of a dye due to milk quality. The removal of oxygen from milk and the formation of reducing substances during bacterial metabolism causes the colour to disappear. The greater number of bacteria present the faster the dye color will disappear, thus the time of reduction is an indication of the number of organisms in the milk.
Methylene Blue Procedure:
- Sterilize all glassware and rubber stoppers either in autoclave or in boiling water.
- Boil 200ml of distilled water in a light resistant (amber) stoppered flask and then add one methylene blue tablet, tablet should be completely dissolved and solution cooled before use. Fresh Solution should be prepared weekly.
- Add 1ml of Methylene Blue solution into a test tube and add 10ml of Milk and stopper the tube.
- Place tubes immediately in waterbath or store in refrigerator for more convenient time of incubation, bring samples to 35deg C within 10mins, when temperature reaches 36Deg C slowly invert tube a few times. Record this as the beginning of the incubation period, cover to keep out of light.
- Check samples for de-colorisation after 30mins, make subsequent readings at hourly intervals thereafter.
Products required from LABTEK Services Ltd:
- MB100 Methylene Blue Tablets (100units)
- RT510100 Reductase Tubes 5/10ml (100units) RTS010190 Stopper, Rubber, Red Solid (pk of 10)
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