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Why are shredded vegetables treated differently?

August 4, 2014

Even though the FARMcurious Fermenting Set generally doesn’t require a weight for fermentation, shredded vegetables are one of the notable exceptions.

Here’s why:

During normal fermentation in an airlock system like the FARMcurious Fermenting Set, carbon dioxide is created through the process of fermentation and it escapes through the vegetables into the top of the jar where it forces oxygen (and accompanying mold spores) out of the jar and through the airlock. Forcing out that oxygen is what keeps the airlock system clean and mold-free.

However, layers of shredded vegetables like you have with sauerkraut actually create a barrier making it difficult for carbon dioxide to escape. This means that the carbon dioxide bubbles will frequently force the cabbage and brine up (sometimes into your airlock) instead of forcing oxygen out of the top of the jar. Because the oxygen and its accompanying mold spores aren’t pushed out through the airlock, mold may grow on the top of your sauerkraut.

Because of this phenomenon, we recommend you use a weight on top of shredded fermentations to ensure that you don’t have mold issues. The weight will keep the vegetables from expanding upwards and the carbon dioxide bubbles will eventually find their way out and push oxygen out with them, keeping your fermentation nice and clean.

What can be used as a weight?

Some people use a boiled rock or a small glass item (a clean, label-free baby food jar or a small dish or expresso cup that will fit inside the jar) and these items work very well. You’ll want to make sure you use something that is lead-free. Any food safe item will be fine. A heavy vegetable also works  well, like a big chuck of turnip, rutabaga, etc. Please note that a beet will stain your fermentation which might be fun (or not)!

You can also find small porcelain weights designed specifically for mason jars here.


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  • You can also use a zip-top bag filled with brine as a weight. Freezer bags have the best seals but sandwich bags can be used in a pinch. Make a brine of 3Tbsp salt to 1qt water and fill the bag about half way. You don’t want it too full or you can’t compress the bag to fit it in the mouth of the jar. Squeeze the air out, seal the bag and insert into the jar, pressing down to bring the brine level up above the shredded vegetables. You don’t have to buy weights, and even if the bag leaks you’ve just added more brine.