Sampling of Milk and Milk Products for Different Tests

Objectives:
To be familiar with different procedures of collecting representative sample of milk and milk products for evaluation of analysis.

Relevant Information:
Correct sampling of milk and its products in dairy industry is important.  The error in sampling will lead to have erroneous results.  The sample may be required for chemical, bacteriological and physical examination.  The basic principles of sampling of milk are same in all cases. Various factors interplay to have faulty sampling.  For example, lack of thorough mixing of milk before the samples are drawn, Lack of hygienic conditions, use of unsterilized equipment, utensils and glassware’s.

Precautions:
1. Make sure that all the glassware’s, equipments and instruments are cleaned, sterilized and dry.
2. Sampling of milk should be done at 90⁰F to 104⁰F.

Apparatus:
Plunger:
ISI specification, 1 meter long with a disc having diameter of 150mm and six holes in the disc.

Long Handle Dipper:
ISI specification. Dipper is fitted with solid handle at least 150 mm long and capacity shall not be less than 80 mi.

Sample Bottle:
Capacity of sample bottle shall be 100,150,250 ml. for collecting the sample for chemical analysis. Plastic bottles may also be used.

Procedure:
Sampling of milk and milk products carried out by the following procedures.

1. Sampling of Fresh Milk:
   Freshly drawn milk contains air and gases. For correct sampling, one should wait until milk is at least one hour old.2. Sampling from Individual Containers:
   Mix thoroughly the milk from one container to another container for five times to ensure uniformity of milk.
   Do not allow the milk to stand for longer than five minutes after mixing and take required quantity of milk with the help of a dipper.

   3. Sampling from Several Containers:
   1. Mix the milk thoroughly with the plunger.
   2. Take proportionate quantity of milk in a separate vessel.
   3. Repeat this procedure for all cans.
   4 .Mix the milk from separate vessels in one from which proportionate quantity of milk samples from different cans are taken.
   5. Take final sample from vessel with the help of a dipper.

   4. Sampling of Cold Canned Milk:
   1. In cold can milk there is formation of cream line; break the cream line before sampling.
   2. Dump the cans into weight vat/tipping tank.
   3. Record the temperature of the milk.
   4. Adjust the temperature of the milk to 90-104⁰F.
   5. Stir, the milk thoroughly with plunger or agitator.
   6. Take small quantity of sample at three or more different places from the vat.

   5. Sampling of Partially Churned Milk and Cream:
   1. Heat the milk to a temperature of 90-104⁰F.
   2. Stir, the milk thoroughly and take representative sample immediately.
   This procedure can be followed when the milk or cream has “cream plug” layer of partially churned fat.  Such condition occurs when the milk is agitated during transportation.  Milk churns easily at a temperature of 26.5 to 29.5⁰C hence agitation at this temperature should be avoided.

   6. Sample of Cream:
   Warm the cream to 122⁰F Mix it thoroughly and take a representative sample.

   7. Sampling of Butter:
   Remove three or four cores with a sample Trier from various parts of the products mass.  Core should be extending from exterior to the centre.  Keep the combined cores in wide and necked bottle for analysis.
   
   8. Sampling of Dahi:
   Entire dahi should be rendered homogenous carefully with a thin wire brush.  Take representative sample.
   
   9. Sampling of Khoa:
   1. Sift the khoa through a 20 meshsieve.
   2. Grind any residue left and sift into 20 mesh sieve.
   3. Mix the whole khoa again and protect it from absorption of moisture.
   4. Take representative sample.

Composite Sample:
A composite milk sample is one which when properly prepare represents two or more lots of milk.  The sample must be taken in proportion to the amount of milk in each lot.  Such milk is placed in a properly labeled bottle and tested after a week or two.  The results obtained would give an average figure for total amount of milk received during the period.  The purpose of this is to reduce the number of analysis and loss of chemicals, labour etc.
The volume of composites sample should not be less than 175 ml.  The milk is preserved by adding preservatives to prevent souring since composite samples are kept for longer period.
I. Formalin 36% is added @ 0.1 ml. for 25 ml. of milk.
II. Dichromate of potash: required for metal container 6 – 8 grains for ½ liter milk.

Care of Sample:
i. Label the sample properly.
ii. During transportation, sample should not be exposed to sunlight, or not to be exposed to near volatile odours as milk picks them up immediately.
iii. Use an air tight container, it should have 100 – 250 ml. capacity and should be rubber stopper.
iv. Keep the sample in a cool place at 45 – 60⁰F.
v. In composite milk sample each time when milk is added, the sample must be mixed thoroughly.