Why do you use 70 percent alcohol and not 90 percent while microbial culture?

Answer 1

Ethanol creates pores in the cell membrane of the microbial cell by solubilizing the lipoproteins of the membrane. The activity of ethanol determines its concentration.

If we 90 percent ethanol, the amount of ethanol molecules available to act upon the cell is more and it attacks the cell membrane from all sides. The dissolved lipoprotein forms a kind of layer around the cell, due to which the alcohol is unable to act further and cause damage to the cell.

However, if we use 70 percent ethanol, the amount of ethanol molecules are few and it is not able to attack the cell from all sides. Thus, pores are created intermittently in the plasma membrane, the ethanol penetrates the cell and damages it completely. So, 70 percent ethanol is preferable to 90 percent when using microbial cultures.