Take off the stems of the jalapeño peppers.
Put the peppers, garlic cloves, salt, sauerkraut juice, and water into a food processor and blend into a paste. Careful not to get your eyes too close when you open the lid afterwards. The fume released can be quite strong and result in a burning sensation.
Pour the jalapeño paste into a glass jar, and screw on the lid.
Leave the jar in at room temperature away from sunlight for 1-2 weeks. The sauce will ferment, bubble, and fade in color. You can open the lid every other day to let it "burp."
Once the hot sauce is fermented, pour over a cheese cloth or a fine-mesh strainer. Squeeze or press out the liquid as much as you can.
Add apple cider vinegar and shake well.
Store the sauce in a glass container in the fridge. You can also save the leftover pulp and add it to sauces and dips.