The gas phase particles (co2) in the beer are moving much faster after shaken. They are constantly trying to reach equilibrium. When you open the container after shaking it, the faster moving particles quickly shoot out of the liquid.
If the beer is very cold, it will slow down these particles and be less flat than if it was warm.
If you wait a while after shaking, the particles will reach equilibrium again and it will be safe to open the container. You can also try to increase the pressure inside the can by squeezing the sides very very hard. Higher pressures will also result in slower moving particles.
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