Give examples to prove that microbes release gases during metabolism.

Explanation

(i) Puffing up of dough, during making of dosa, idli or bread, is due to production of `CO_2` gas produced during the fermentation process.
(ii) Appearance of large holes in "Swiss cheese" is due to `CO_2` produced during fermentation by the bacterium Propionibacterium sharmanii.
(iii) During secondary treatment of sewage in sewage treatment plants, a number of gases like methane, `H_2S` and `CO_2` are produced during microbial digestion of organic compounds.
(iv) Methanogens (e.g.,ethanobacterium) present in the rumen of compound stomach of ruminants and dung of cattle are being used in biogas plants to produce biogas as energy source.