The science of food is truly interesting. Find out the reason behind the above phenomena by reading the following top rated answers by four internet users. You’re sure to learn something new.
Harder baked goods absorb moisture over time because they start out dry compared to the moisture in the air and become softer as a result.
Softer baked goods that already have moisture in them which is used chemically to make their gel-like structure instead lose moisture over time (since there’s less moisture in the air than in them). This makes them harder.
There are two things going on:
- Dehydration/ re-hydration
- Starch retrogradation
First, let me address dehydration/re-hydration. Soft baked items such as cake and cookies often contain egg, and are baked until the egg proteins set, but are not baked until all the water content escapes. The baking also activates the chemical leavening, puffing them up. (Cakes and cookies are not typically yeast-leavened; they typically use baking soda or baking powder, which release CO2 as they are heated and/or moistened.) As they “go stale”, they lose water content, leaving the egg protein and starch matrix hard and dry. Hard-baked items like matzos, fillo dough, and other such things are baked to dehydrate them well past the level of ambient moisture. They pick up moisture, and get soft. (Or, to be more precise, lose their crispiness; they don’t really ever go totally soft again.)
Secondly, “staleness” also involves starch retrogradation. Starch is not in its lowest energy state when it is cooked; a portion of it will revert to the uncooked state given the passage of time because it is energetically favorable. This reversion to the uncooked or less-cooked state is what constitutes some of the texture of “staleness” in baked goods. Retrograded starch is harder than the soft-cooked stuff, but softer than the hard-cooked stuff. Retrogradation can be reversed to a large extent by just re-heating the baked goods.
It has to do with the moisture of the food trying to match that of the room.
A pot of cream for example, is really high in moisture. As time passes, the water in it begins to evaporate to match that of the surrounding environment (and it’s the reason it dries from the edge in). A rusk is really dry and with time, moisture permeates it and makes it go “papery”.
Breads and cakes go “hard” because the moisture in the air actually crystallizes the structure. Cookies on the other hand are much drier and the absorption of water from the air adds moisture to their structure and causes it to re-bond with pliable connections. That’s why stale cookies are softer but pliable.
Anecdote: Jaffa cakes in the UK were the subject of controversy when it was argued that they were biscuits (cookies) and should be taxed higher than they were, as cakes were not taxed as luxuries like cookies were. The main thing that kept them from being taxed as cookies was that they staled hard rather than going soft, which is one of the defining differences.