Before enzymes can attack starch, it must be gelatinized. This has traditionally been done by pressure cooking.
 
Potatoes, for example, are heated to 150°C at a pressure of five atmospheres. When the pressure is released suddenly, the cell walls of the potatoes literally explode, releasing the starch. In this case, the enzymes are added to the mash tun after cooking, but in other cases a highly heat-stable alpha-amylase can be used in the cooker itself.
 
The old non-pressure cooking method has been gaining in popularity, especially in smaller distilleries. Instead of temperatures around 150°C, the maximum temperature is from 60°C to 95°C. There are obvious energy savings and there is no need to invest in pressure vessels.