Did a bit of looking into this a while ago.
Straight petrol contains no water. The moisture content necessary for combustion is drawn from the atmosphere.
Bio-fuel (ethanol) is able to absorb water and carry water.
Unlike petrol, ethanol is highly soluble in water. When the water content of the E5 blend (i.e. fuel with a 5% ethanol content; E10 = a 10% ethanol component) reaches a critical level the ethanol component and associated water will separate from the E5 blend and form an ethanol/water phase.
This will accumulate at the bottom of a tank leaving petrol (without the ethanol component) in the upper layer and is known as phase separation. On a motorcycle this effect will result in the carburettor drawing virtually neat ethanol.
If phase separation occurs the process is essentially irreversible, there is no straightforward means of emulsifying - reblending - the ethanol back into the petrol.