For a long time I just took other gardeners' word for it when they'd say that Brussels sprouts, broccoli and other Brassicas (the genus from which all of these lovely and sometimes misunderstood vegetables hail) were sweeter after a frost.
I mean, it's easy and nice to plant things that can take a frost and don't need to be covered. It appeals to the lazy part of me that really doesn't want to have to go out to the garden before bed to throw sheets over plants and tuck in stray branches and such.
But that's as far as my understanding went. I knew that Brassicas could take a frost and were therefore awesome and worthwhile to grow in the frosty time of year when most other plants need all kinds of special care.
The fact that they also became sweeter after a good frost set in, well, that was a nice bonus. Especially when things like nasturtium, that are totally frost tender (ie. they totally croak when it's frosty), turn to absolute goo when the temps drop into the frosty range.
But why do Brassicas like kale and broccoli taste better after they're hit with frost?
See, when the temperatures drop, Brassicas go into survival mode and start producing more sugars that then lower the temperature that freezing begins in the water in their leaves so that they can survive the cold weather and not turn into a useless and gross pile of goo.
If you're curious about some of the tasty bits in the Sweeter After a Frost Brassica Genus:
They can even be sweet enough for dessert.