I made a toaster waffle for myself this morning. Growing impatient, I popped it out before it was all the way done. As I was buttering it, I noticed parts of the waffle were still cold. Since there was already butter and syrup on it, I couldn’t put it back in the toaster. I threw it in the microwave for 20 seconds and it came out floppy instead of crispy. What gives?
In: Technology
Toasters heat bread from the outside. The heat gets concentrated on the outer layer of bread. Water evaporates from this outer layer, which lets it get even hotter (the presence of water keeps stuff below water’s boiling point), to the point that it browns and crisps up. It also helps that the bread is placed vertically between grills (or laid horizontally on a grill, if it’s a toaster oven) that allow air to circulate around the bread, so water vapor can escape.
Microwaves (1) heat bread from the inside (sort of) and (2) they don’t (typically) allow air to circulate around the bread. Let’s start with number 1. Microwave ovens heat stuff by sending electromagnetic waves (microwaves, specifically) into the stuff. These waves make the atoms inside the stuff jiggle faster (faster-jiggling atoms are hotter – that is basically what heat, or at least temperature, is). Microwaves can penetrate some distance into food. How far depends on the food, and it’s not super far. You won’t heat up the inside of a frozen chicken before the outside has thawed. But they do go deep enough into a slice of bread to basically be heating the bread evenly.
As the bread is heated, water is evaporating from it, but because the slice of bread is resting on a plate (most likely), it can’t actually escape from the bottom. Instead, it condenses onto the plate below, and this liquid water gets reabsorbed into the bread. And even the top surface of the slice is staying moist, as water from the interior of the slice is getting forced out. So, no crisping can occur as the surface doesn’t get hot enough. Meanwhile, the (relatively) mild and moist heat is causing the starches in the bread to (re-)gelatinize. You know that moist, soft texture that freshly baked bread has? That’s because the fresh, warm bread still has gelatinized starch. As the bread cools, the starches harden a bit and so you get a drier, firmer texture. If you reheat bread, you will re-gelatinize the starches (somewhat) and return it to this softer, freshly baked-like texture, which for a slice of bread will make it rather floppy. Meanwhile, the water that condensed onto the plate may also get absorbed into the crust of the bread, which turns soggy when it gets wet.
If you keep heating the bread longer in the microwave, and especially if you allow for better air circulation (e.g. by standing the slice on its end some how), you will find that the bread does get crispy, and may even burn. Only, it (probably) won’t get the same nice consistency as a well toasted slice. Because the bread is heating all the way through, water evaporates everywhere too, and so the slice will dry out and harden as you are basically boiling the bread from the inside until most of the water is gone. Whereas, in a toaster, the interior of the slice will stay somewhat soft and moist (at least if you don’t take it too far) because the heat is applied from the outside.
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