CBSE Book Class 8 Science Activity A Friend Indeed

 MICRO ORGANISMS : FRIEND AND FOE

“Ma – you kept a bowl of milk in the kitchen last night – but now there is no milk”. How did it turn into  curd!". – exclaimed Vishnu. Ma smiled" 'Beta, I added a small amount of curd to milk last night – so it  turned into curd”. “But how did it happen?” – enquired Vishnu. Vishnu's  mother,  a  teacher  of  biology,  explained  how  milk  gets  converted  into  curd  by  the  action  of  friendly microbe called “Lacto bacillus bacteria”. They multiply fast when a small amount of curd is  added to lukewarm milk. She also explained that proper temperature (app. 30°C) has to be maintained  for multiplication of the bacteria. She encouraged Vishnu to experiment it himself:

You will need :Curd Sample, a compound microscope, slide, cover slip, needle, dropper, methylene blue stain, filter  paper.

What to do

• Take a drop of the 'Whey' i.e. the watery, part separated from the curd sample, on a clean glass  slide.

• Add a drop of methylene blue stain and allow it to stand for 2-3 min.

• Lower a coversip gently with the help of a needle on the material.

• Observe the slide under the low power of compound microscope

Observation : 

Minute rod shaped blue coloured cells of lactobacilli bacteria are found under the microscope.

What I learnt:

By adding a small quantity of curd or its whey to luke warm milk the lacto bacilli bacteria are introduced  to curdle the sample of milk. These bacteria ferment the milk sugar lactose into lactic acid and form curd.

Variation :

You could add few drops of your favourite food essence like pineapple or strawberry and the relevant  food colour to the sample of lukewarm milk before adding curd to it, to prepare flavoured curd.

Please refer to the link below - CBSE Class 8 Science Activity A Friend Indeed