There are a number of factors to consider if you want to prevent mushy vegetable ferments. Here are a few:

Salt.

It is possible to ferment vegetables without salt, but salt does help to improve the overall flavor of the ferment while keeping it crisp. If you’re not using salt, you might want to consider it.

Temperature.

Pectin-digesting enzymes tend to come in high numbers when we ferment at high temperatures. This can cause the vegetables to break down and become mushy.

Tannins.

Tannins help to firm up the vegetables with their astringency. Use grape, oak, horseradish, or black tea leaves with your ferments to prevent mushiness caused by factors other than the two listed above.