It is possible you might find mold growing on the surface of the sauerkraut, but don’t panic! Mold typically forms only when the cabbage isn’t fully submerged or if it’s too hot in your kitchen. The sauerkraut is still fine (it’s still preserved by the lactic acid) — you can scoop off the mold and proceed with fermentation. This said, it’s still important to use your best judgement when fermenting. If something smells or tastes moldy or unappetizing, trust your senses and toss the batch.