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I accidentally stumbled upon this 'trick' a week ago when I forgot about a baguette that I bought for a party and never used. It was half-covered (loosely) by a plastic grocery bag. When I took it out after the week period, the top half (not covered by plastic) was rock hard, but the part inside the plastic bag was still relatively soft.

What biological mechanics are causing plastic grocery bags to keep bread fresh longer? This Q/A discusses storage methods, of which plastic bags are one, but doesn't actually explain why it works the way it does.

I also stumbled up this video where a guy mentions that he also keeps bread in plastic bags, so this isn't an isolated incident.

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Water, water-vapor or moisture can't penetrate the plastic bag. Nice tender bread contains a lot of moisture. That moisture, all of it, will evaporate into the relatively dry air in your kitchen if it can. Bread (especially a baguette) depleted of moisture becomes rock hard, as you know. If you cover your bread with a plastic bag, a little moisture still escapes into the air inside the bag, but it can't get out of the bag, and after that you get an equilibrium between soft bread and slightly moist-ish air, and your bread stays pretty nice for a little longer.

The down-side is if you keep it too long, you may get mold taking advantage of the moist conditions in the plastic bag, and you will get moldy bread. Mold doesn't tend to grow in a dry environment (rock-hard bread).

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    And this is one of the uses of a bread box -- but as you note, it'll go moldy much faster. (I also recommend swapping out if you're using a plastic bag ... so you don't end up trapping some mold spores to then inoculate the next loaf that goes in) – Joe Aug 24 '18 at 14:16
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Most people think that stale bread is 'hard dried out bread'. The above question and the answer given by Lorel C is an example of that less than correct perception. The 'rock hard' exposed half baguette in Chris' question has 'dried out'. It has probably only partially gone stale. Most of us know not to store bread in the refrigerator because it goes stale much quicker than at room temperatrure, regardeless of being in a plastic bag or other air-tight container. That refrigerated bread has not dried out, it has gone stale. What happens when bread goes stale is that the starch molecules have crystalized. Drying out and going stale are 2 separate and distinct processes. Drying out is a physical action (evaporatrion), going stale chemical action (crystalization).

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    This is like the definition of 'bread' ... although there may be a rather specific definition for some communities of discourse (eg, professional bakers), for others it's just 'no longer pleasant to eat'. When people refer to 'stale chips' (crisps in the UK), it's that they've gained moisture, nothing to do with crystallization. (and then there's the equestrian definition, which has nothing to do with food) – Joe Aug 24 '18 at 14:14
  • Fascinating discussion of the word "stale".... But am I missing something when I can't locate a single occurrence of that word in either Chris Cirefice's question or my answer to it? – Lorel C. Aug 24 '18 at 14:38
  • @Lorel C Correct, Chris did not use the word 'stale'. However, the original title question includes '...keep it fresh longer/'. I would define 'fresh' as an antonym for 'stale' in this content = 'keep it from going stale'. In cooking and food preparations there are things we intentionally dry out without making them stale: crutons and fresh breadcrumbs are 2 that spring to mind. – Cynetta Aug 24 '18 at 14:59

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