How hardworking microbes ferment cabbage into kimchi

Ingredients, temperature, time all affect the microbial activity that fuels fermentation — and how fermented foods taste. As kimchi ferments, the flavors will change over time. (Meredith Rizzo for NPR)

I love fermented foods. I love that you can leave something out on the counter, in the refrigerator — forget about it, even — and it just gets better.

But why do some foods improve with age, while others spoil? I wanted to know how it happens.