Rock-hard cookies are not only hard to eat, but are usually associated with staleness. While cookies can be intentionally baked to be harder than usual, a cookie that has lost its original softness is likely to be stale. Stale bread gives similar tell-tale signs. What makes bread go stale though? It’s not mold or oxidation, which are common suspects of food gone bad.
The staleness of bread actually arises from the presence, or rather the lack of the presence of water. Bread is mostly made up of starch molecules, hence it being a significant source of carbohydrates. When you bake bread, the starch molecules weaken and allow moisture to enter the complex of chemical structures that make up bread. Water molecules are able to squeeze between the starch molecules. Starch granules weaken, giving most bread a soft and fluffy texture.
Once the bread is taken out of the oven, the slow journey to staleness begins, although it won’t be noticed for a while yet. The most significant factor in staleness is the recrystallization, also called retrogradation, of starch molecules; once cooling begins, which is the moment you take the bread out of the oven, the process that was used to bake the bread essentially reverses itself. The bread slowly drys itself out; as water molecules detach themselves from the network of starch molecules, the starch begins to recrystallize into their original shape.
Let’s go into a more detailed analysis of the chemistry behind all of this. Amylose and amylopecton are the two types of starch that are found in bread. When bread is baked in the oven, a process called gelatinization occurs. This occurs when starch is heated in water; hydrogen bonds in the starch granules break, allowing water to enter the granule and causing the granule to swell. Amylose molecules leave the starch granules, while the water molecules form hydrogen bonds with the amylopectin molecules. The swelling of the starch granules is the reason why bread “rises” in the oven.
Let’s see this in action: (the swelling becomes visible at around the 00:12 mark)
However, as stated before, this entire process begins to reverse itself once the bread is taken out of the oven. First, synersis causes amylose molecules to pull back together into the starch granule, squeezing out the water molecules previously inside. Then, retrogradation allows the amylose molecules to realign in a linear-chain pattern. This structure is kept rigid because hydrogen bonding occurs between the chains of amylose. As a result, the bread feels hard and is now stale. The following diagram should help in visualizing all this chemistry.
Figure 1: Gelatinization and Retrogradation of Starch Molecules
On a less chemistry-intense note, there is something pretty surprising about preventing bread from becoming stale: bread goes stale about 6 times faster in the refrigerator than at room temperature. You might ask – Isn’t the fridge supposed to keep things from going bad? Well, for most things, yes. However, the temperature inside the fridge is actually near the optimal temperature for retrogradation of starch molecules. So how you keep bread from going stale? If you do leave it on the kitchen counter at room temperature, mold growth will occur extremely fast, as you probably already know. What you can actually do is put the bread in the freezer. Freezing temperatures are too low for starch molecules to want to recrystallize, and mold growth is basically nonexistent. Just reheat the bread when you want to eat it, and it’ll be good as new!
So should you throw out any stale slices of bread you already have? No! Unless the bread feels like a piece of rock, it is highly likely that you can make it edible again. Just bake the bread in the oven again with water, and the effects of retrogradation should be almost completely reversed. Enjoy!
Author: Jonathan Yu
Sources:
Food Composition and Taste 