Isolation of microbial flora of fermented milk

Aim

To isolate and enumerate bacteria of fermented milk.

Principle

Fermentation with certain microorganisms is necessary in the preparation of foods such as cheese, curd, yoghurt etc. Milk is fermented with lactic acid bacteria to make curd. Thus, the presence of microorganisms in food though considered harmful in some cases, it is definitely beneficial in other cases.

Materials Required

Curd, sterile tubes, nutrient agar, petri plates, Bunsen burner, pipettes, L-rod.

Procedure

1. Sterile nutrient agar plates containing 20 ml of medium were prepared.

2. Curd sample was diluted by serial dilution technique to obtain dilutions of 10^-4, 10^-5, 10^-6, 10^-7.

3. The plates were labelled corresponding to the dilutions.

4. Using sterile pipettes 0.1 ml from each dilution was placed in respective nutrient agar plates.

5. The L-shaped glass rod was sterilized with alcohol followed by flaming. The rod was cooled and gently placed on the surface of agar.

7. Petri plate was rotated in both clockwise and anticlockwise direction to uniformly spread the sample over agar surface. The plates were incubated in inverted position for 24 - 48 hours at 37⁰ C.

8. The number of colonies were counted and the total microorganisms per ml of original sample was calculated.

Observation

Each of the dilution plates were observed for colonies of bacteria. The number of colonies were counted.

Result

The number of organisms per ml was found to be  2.6 x 10⁷

 

Observation (on left side)

 Enumeration of Bacteria from Curd

Organism	Dilution	Number of Colonies	Number of Organisms per ml
BACTERIA	10-5	26	26 x 105    = 2.6 x 107 0.1
	10-6	4	4 x 106    = 4 x 107 0.1

 

The number of organisms per ml of curd can be calculated by applying the formula,

Number of organisms per ml of curd = Number of colonies ×dilution factor

                                                                           Amount plated