Staling bread is a perfect example of reversion to the mean. Bakery engineers say that the typical soft bread contains 32-38 percent moisture. If the bread is left unwrapped and exposed to the elements, it will become hard when it drops to about 14 per cent moisture.Why does the bread get still and lose the moisture?
All the food technologists don't fully understand all the causes, a process called 'retrogradation' occurs, in which internal changes take place in the starch structure.
Although breads are formulated to have a softer crumb portion than crust area, during retrogradation some of the crumb moisture migrates to the crust, which results in the softening of the crust and hardening of the crumb.
A portion of the starch in the flower undergoes a gradual change known as 'crystallization' which results in a gradual firming of the bread.
Some of the ingredients in the dough, such as enzymes and monoglycerides, act too slow up the rate of retrogradation, but the process is inevitable and will occur quickly if the bread is unwrapped and exposed to air.
Hard starches such as crackers are crisp because they are baked with an extremely low moisture level, usually 2-5 per cent. When they soften, their internal structure doesn't change like staling hard breads. As they are exposed to the ambient air crackers absorb the air's moisture. Hard crackers will be perceived as soft once the moisture level reaches 9 per cent.