Making naturally-fermented pickles or sauerkraut is something of a gamble. You put food and salt and maybe some spices or water into a jar, and you wait for it to go bad. And then you hope that it goes bad in a good way. Just like when you're growing a sourdough starter or making yogurt or crème fraîche, you want the good bacteria to ward off the bad.
My mother made fermented pickles in an old pottery cookie jar. More than half the time, the pickles went bad. And once they started to go off, there was no saving them. The pickles would get soft and slimy, and they'd start to smell bad. She'd toss them and try another batch.
Kraut Kaps ($9.99) are designed to help prevent the bad bacteria from working their evilness on the pickles, while letting the good bacteria proliferate. It's a simple idea, really—an airlock on the top of a cap that fits any wide-mouth canning jar.
So does it work? I made two batches of fermented pickles in half-gallon jars, and both were successful. Now I have a batch of sauerkraut in process. It's bubbling like it's supposed to after a couple weeks, and I'm not seeing any sign of rotting.
Whether these products would have worked without the airlock is the question. I've successfully made pickles and sauerkraut without a gadget, but it requires a lot of attention, scraping off the scum on the top, adding water as it evaporated, and constantly looking for mold. Using the caps made the process a lot easier. I still checked the pickles on occasion, and there was a little evaporation, but I didn't need to check them every day. And for those with sensitive noses, not much aroma escapes the jars.
Remember that even with these caps, you can still introduce bacteria into the process with a jar that isn't quite clean enough, pickles that aren't washed well, or utensils or other ingredients that could introduce bugs. But the caps do keep nasties from coming in from the outside, so your chances of success are better.
The caps are a bit pricey, but if you do a lot of pickling, they could save you from having to throw out batches of pickles that have gone bad. Also, since the lids only fit wide-mouth canning jars, the batch sizes you can make are limited.
The caps were recently redesigned with silicone replacing parts that were previously made from rubber, so these new ones should last a bit longer.
Disclaimer: Testing samples were provided to Serious Eats.