SAMPLING MILK FOR EXAMINATION

SAMPLING MILK FOR EXAMINATION

For proper examination of milk the first pre-requisite is the accurate sampling of milk. Probably more errors are caused through careless preparation of samples than in the actual performance of the tests. The whole body of milk from which a sample is to be drawn should be uniform in its composition and any sample of milk drawn out of it for analysis must be a true representative of the whole body of milk. The factors disturbing the uniformity of composition of milk are mainly the separation and partial churning of fat. Thorough mixing of milk must be ensured either by stirring with a long handled dipper if the container is big or by pouring from one vessel to another or by shaking gently. If the sample bottle has been standing for some time resulting in the separation of fat in the cream layer or lumps of fat appear, the bottle may be heated in a water-bath up to 38°C (100.4°F) to melt the fat before mixing.

Under no circumstances should milk be shaken very violently since viscosity does not allow air bubbles to rise easily. Moreover, agitation of milk tends to churn with the result that even distribution of fat becomes difficult.

When milk from an individual animal is to be sampled, the whole amount of milk is taken and a sample drawn from the middle of the vessel containing it. For usual analysis of milk, collect 250 to 500 ml of the sample in a wide-mouthed bottle. When sample is only to be collected for carrying out the fat test, 50 to 60 ml would suffice.

Samples of milk should be examined as fresh as possible, since important changes in its physical condition, chemical composition and bacterial content may take place even within a short time. When samples are to be sent some distance to a laboratory for examination,

they should be first preserved by the addition of certain preservatives. Twenty drops of formaline to a litre of milk or half a gram of potassium dichromate to a litre of milk is enough for the purpose. The milk should better be sent in a previously sterilized bottle closed by a sterilized stopper. When samples are taken with a view to carry out bacteriological examination, antiseptics and preservatives should not be added.

In taking a milk sample for the purpose of bacteriological examination, the milk is mixed thoroughly and not less than 10 ml are collected in a previously sterilized bottle or tube fitted with a sterilized stopper. A sterilized pipette should be used for collection of milk sample. This sample bottle should be properly labeled as to the history of sample, time of sampling, etc., and packed in an ice-box and dispatched to the laboratory for examination. Such samples should be examined as soon as possible. No antiseptics/preservatives should be added to the samples meant for this purpose.

There are two types of samples, individual and composite. An individual sample is that which is obtained from each separate delivery of milk by a producer or that obtained and sampled separately from each animal. A composite sample is defined as a sample containing an aliquot portion of milk from the same source over a period of several days. Such a sample is placed in a properly labeled bottle and tested after a week or so. The results obtained would give an average figure for the total amount of milk received during the period covered by the composite sample. The system is accurate, simple and saves labour and time.

Since composite samples are kept for long periods, preservative needs to be added to prevent souring. The preservatives commonly used are mercuric-chloride (0.1 percent), formaline and potassium-dichromate. Composite samples to which mercuric-chloride has been added should be coloured to prevent any body mistaking the bottles for normal milk as it is a violet poison.

Composite samples should preferably be not kept for longer than ten days. The sample bottles be kept at a cool dark place not exposed to the sun. They better be kept locked so that they cannot be tempered with. Each bottle should be shaken daily to prevent its getting hard and dried on the top of the fluid. When it does become hard and dried, the best is to mix the contents with a small brush and warm the bottle but the heat applied should not be more than 32.2°C (90°F).

EXERCISES

1.                  Why is it necessary to mix the whole body of milk before obtaining a sample from it?

2.                  What is a representative sample of milk?

Differentiate between an individual sample and a composite sample of milk?

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