what causes moisture on the plate underneath a piece of toast fresh out of the oven?

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I assume it has something to do with the toast changing the temperature of the air around it, and the air’s ability to hold moisture changing, in turn. But if I’m remembering my mediocre science education correctly, hot air can hold more humidity, so is it caused by the displacement of the (relatively) cool air around the toast, losing its humidity? I’m perplexed.

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There’s water in hot bread, too? Cold plate gathers condensation?

The toast still has moisture in it after it comes of the oven, and it’s hot, so the moisture is rapidly evaporating, in all directions. When the hot water vapour coming off the toast hits the cool plate, it quickly condenses back to liquid.

Two things I have found that help: One is just putting a piece of paper towel under the toast. This provides a little insulation and also absorbs some of the moisture. The other is heating up the plate. The warmer the plate, the less condensation forms.

Edit: I wanted to clarify, the condensation forms because of the rapid transfer of heat from the water vapour to the plate, and because there’s a surface for the droplets to form on.

When you put your hot, freshly toasted bread on a cool plate, there is still moisture in the middle of the piece of toast that continues to evaporate. The cool plate causes some of that vapor to condense, causing a soggy toast bottom.

Neat question but –

Bread does contain a fair bit of water as moisture BUT it also chemically binds water into it’s starch molecules as it ages. So for example, stale bread isn’t just “dry bread”, it’s bread that’s had it’s water combined with starch molecules, which changes the properties of the bread into what we call “stale”.

Have you ever wondered why lightly toasted bread seems to become *unstale*? That’s because the heat of toasting breaks the water out of the starch and returns to being moisture in the bread, making it soft and fresh again.

So your mechanism is correct, the hot bread is releasing water and it’s condensing on the colder plate. The condensation also accumulates instead of evaporating because it can’t evaporate, there’s a piece of toast on top of it.

The toasting of bread drives moisture from the surface as it heats up. When the surface is hot and dry enough it starts to brown so you get the classic toast look. This is why two day old bread toasts better than fresh.
If while it is hot, you put it on a cool surface the moisture still being driven off will condense on the cool surface. This can ruin a nice piece of toast by making one side soggy. Try standing the pieces on their end while cooling so you end up with toast that is firm and crisp on the outside and moist in the centre. Yum.