Page 192 - Agriculture_Form_Three

Agriculture for Secondary Schools

(i) Colour and smell: Good milk must be of creamy whitish colour and should
have normal smell.
(ii) Absence of dirt and other foreign materials: Sometimes dirt, cow dung,
insects, etc. may fall into the milk during milking and collection. These foreign
materials should not be present in the milk.
(iii) Density of the milk: Do not add water into the milk as it may cause spoilage of
the milk and the products made from it. A lactometer is used to detect if water
has been added into milk (adulteration).

Tests for assessing milk quality

Tests for assessing milk quality include lactometer, Clot On Boiling (COB) as well
as alcohol methylene blue and resazurin tests. These tests are elaborated in the
following sub-sections:
Lactometer: Lactometer measures the density of a sample of milk presented at
the collection centre. The lactometer reading is compared with expected standard
values of normal milk. The density of normal milk depends on its composition and
temperature at the time of measurement. The normal value ranges between 1.026
g/ml and 1.035 g/ml when the reading is taken at 20 C. When water is added into
°
milk, the lactometer will give values lower than those given in the range. If soluble
materials, for example, sugar, salt, and wheat flower are added into the milk, the
lactometer will read values higher than the range given. The milk which deviates
from normal lactometer reading should be rejected.

Clot-On-boiling (COB): This is done by taking a small amount of milk sample in a
container or a spoon and hold it over a flame. If the milk coagulates on heating, that
milk should be considered to be already undergoing spoilage and therefore should
not be accepted for processing or mixing with clean milk.

Alcohol test: A small amount of milk is mixed with alcohol in a test tube. The tube
should then be agitated thoroughly. Milk will curdle if it is spoiled or in the process
of getting spoiled.

Methylene Blue and Resazurin test: These two tests give indication of microbial
activities in the milk. When a solution of Resazurin or Methylene Blue is mixed in
a milk sample, the milk turns into blue colour. If there is a high microbial activity in
the milk, the blue colour will turn into pink within a short period of time.

Milk processing
Milk can be sold when it is still raw, but it is not safe as it can easily be destroyed
by microbial activities. The best way is to process and market as whole milk or as
various processed forms (refer to Figure 8.10).

Student’
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