How come when I leave a cracker, cookie, or a potato chip on the counter, it gets soft/soggy, but if I leave a piece of bread, or a piece of candy, on the same counter it dries up and gets hard?
It’s like the old joke - you put something hot in a thermos and it keeps it hot - you put something cold in a thermos and it keeps it cold - how does the thermos know?
I know how the thermos does it - but what’s with crackers vs. bread?
Isn’t it obvious? A cracker is dry and gets humidity from the atmosphere while bread is moist and loses humidity to the atmosphere. Everything ends up with the same temperature.
Bread softness comes from moisture retained in it during baking. Bread dries out because its ingredients do not tend to hold onto the moisture.
Crackers are dry and have little moisture. The ingredients in crackers tend to absorb moisture from the air.
Are the ingredients in crackers and bread really that different? I’d expect the flour in both to absorb some water, and if the bread had any egg and/or sugar in it (like a challa), it would hold even more moisture. Preservatives, natural or otherwise, can include hygroscopic ingredients, as well.
Yet bread gets positively crispy when exposed to air and then stays that way, and crackers sometimes get so limp they bend. Why, if bread has all the water-attracting ingredients of crackers and more besides?
I suspect it has something to do with the structure of each, which differs more than the ingredients, but I can’t quite work out a hypothesis in more detail than that.
I still think it comes down to the fact that crackers start out dry (and we prefer them that way) and bread starts out moist, they’re just moving toward equilibrium.
Crackers are usually alot more salty then bread. IANA chemist but that might be why crackers hold on to water better.
It appears that the staling process for bread is not just a matter of losing moisture–apparently the structure of the starch etc. actually changes (IANAFoodChemist; I just read this somewhere). So there may be more moisture in stale bread than you think there is. Crackers start out with less moisture than bread, so the staling process for crackers additionally involves absorbing moisture.
Good thing I looked it up before I corrected you, I was thinking hydrophilic!
But I think the key word in your post is -sometimes-. We notice the bread dry out when the relative humidity is low; we notice the cracker get limp when the relative humidity is high. As for why the cracker gets limp but the bread doesn’t? I think it’s because the cracker, being more dense, has more of its molecules affected by a certain small quantity of water.
I’m banking on the confirmation bias for the most part.
Good catch! Yes, not the same thing; in making cosmetics or cooking, it’s important to know the difference:
Hydrophilic = “likes water”; is water soluble
Hygroscopic = absorbs water out of the air and holds on to it.
Yeah, I’m thinking density, surface to mass ratio or some physcial characteristic is playing a role here. I really want to ask Alton Brown, to be honest.
Also certainly true. Although with a 3 year old and an in home childcare biz, I see a LOT of dessicated bread crusts in a week! 
Not all breads are the same, either. I buy unsliced loaves. Because it takes me a week or so to go through a loaf, I usually get “olive oil bread”, which lasts a week easily. Compare that to a French loaf that is rock hard in 48 hours.
Great question. The idea that the two move towards equilibrium and pass one another, for no specific reason, doesn’t make sense. The idea that crackers are saltier could be very important, because including salt in a recipe will make the end result equilibrate with the atmosphere in a wetter state, other things being equal.
Three more possible explanations:
Bread feels more dried out, and crackers feel more soggy, because we are used to their normal states, which are soft and crackling dry, respectively. So, if you carefully analyzed the end results you wouldn’t still think the cracker was wetter and the bread was drier. But, I don’t like this one much - surely we are aware enough to see past this one.
And, bread is leavened, whereas crackers aren’t. Bread has some kind of toughness as a result, and winds up stronger in the dried state than crackers do. This is similar to how crackers are easier to crumble into crums than bread is. I’m not sure this is true, but the leavening is certainly a difference. Melba toast might be another good test case, if it is chemically closer to bread but physically configured more like a cracker.
Finally, all wheat flour is not the same. I heard something recently about how different some pasta flours are from bread flours, which are different from cake flours. There aren’t that many kinds for sale in the grocery store, but a mass producer of crackers or bread could have all kinds of reasons to choose a particular and distinctive flour.
Ooh, hang on, you might be onto something. Perhaps it’s not the leaving itself, but the gluten? Bread has a lot more gluten in it than crackers - perhaps something in the gluten chemical or structure contributes to this perceived effect?
Anecdotaly, my daughter’s gluten free bread is very stiff and crumbly, feeling stale even when fresh, until it’s heated. When it’s not heated, it forms crumbs much easier than wheat bread. I’ll leave a slice out and see if it dries hard like a cracker or goes soggy.