Question

Why do you pass the open end of a broth tube or a bottle through the flame after opening it and before recapping it?

Answer 1

To ensure that no unwanted microorganisms are at the opening and may fall into your culture

Answer 2

Aseptic techniques are used to prevent the contamination of the media and culture from the microbes present in the environment. Another reason for using the aseptic techniques is to prevent the microorganisms in lab to spread in the environment or infect the person working with them.

Passing the open end of a broth tube or a bottle through the flame after opening it and before recapping it because the flame of the Bunsen burner creates a current in the form of convection current and this forces the air inside out of the tube. The air borne contaminants are also prevented from entering the tube. The heat of Bunsen burner increases the temperature around the working area thus reducing the chances of air borne contaminants to damage the cultures.