Why shop-bought bread lasts longer than homemade

This is the distinction that makes the rest of the answer make sense, and almost every discussion of it runs the two together.

Going mouldy is microbial spoilage. It is addressed by preservatives, by packaging and by hygiene during production.

Staling is not spoilage at all. It is starch recrystallising as the loaf cools and continues to sit — a physical change that makes the crumb firm. A completely sterile loaf still stales, and refrigerating a loaf makes it stale FASTER even though it delays mould.

When people say shop bread lasts longer, they usually mean it stays soft. That is the staling question, and preservatives have nothing to do with it.

EMULSIFIERS slow the firming directly. They interact with starch in a way that slows the recrystallisation that hardens a crumb. This is the single largest reason a packaged loaf is still soft on day five, and it is a texture effect rather than a moisture effect — the loaf is not wetter, it is less firm.

ENZYMES, particularly heat-stable amylases, also reduce staling. They are increasingly preferred to chemical additives partly because they are perceived as more natural and fit clean-label standards.

PACKAGING controls moisture loss. A sealed plastic bag stops the crumb drying, at the cost of destroying the crust by keeping its water in — which is why packaged bread has a soft crust and a bakery loaf does not.

PRESERVATIVES and mould inhibitors extend the time before spoilage. This is the mechanism people assume is doing all the work, and it addresses the problem they were not asking about.

Most of the gap is not closeable at home and some of it is not worth closing.

The closeable part is storage. A cut loaf left out dries; wrapped in a bag it stays soft and loses its crust; frozen sliced it keeps almost indefinitely and toasts straight from frozen. Freezing is the one intervention that genuinely matches industrial shelf life, and it works because it stops the starch recrystallising rather than by adding anything.

The part not worth closing is that a crisp-crusted loaf is a loaf designed to be eaten the day it is baked. The crust that makes it good is the crust that goes soft in a bag. That is a trade, not a fault, and it is the same trade the packaging in a supermarket has already made in the other direction.