Brining vs. Dry-Salting
Vegetable fermentation is normally done by one of two methods:
- brining (submerging whole or chopped vegetables in brine)
- dry salting (mixing chopped vegetables with salt and letting osmotic magic draw fluid from the vegetables to create a brine)
I'm pretty sure you're describing a brining process. Kraut is normally a dry salted application. Sandor Katz's "The Art of Fermentation" is a bible for this sort of thing. See Chapter 5 for a discussion of this; he lists kraut and kimchi as the classic examples of dry salting. You might want to try a more traditional dry-salting process. Katz's Wild Fermentation site has a weight-based dry-salting recipe.
Having said that: it won't guarantee you get no mold.
If you don't want to see mold, you've surely picked the wrong hobby.
The Futility of Preventing Mold
I get that you're asking how to prevent mold, but the real question is: can you prevent mold?
Maybe you could with with an industrial-grade setup, but I'm not even sure of that. I've been making kraut and many other vegetable ferments for almost a decade now, and I can tell you this: mold happens. It doesn't mean you did something wrong.
From The Art of Fermentation section "Surface Molds and Yeasts" (p. 103 in my hard-cover printing):
"An inevitable aspect of [fermentation] technique is the edge, where (in an open vessel) the surface of the liquid... comes into contact with oxygen-rich air. The meeting... encourages rich biodiversity, where molds and yeasts frequently develop. Surface growth is common and normal; it should be removed, bit is not cause for alarm and it does not ruin your fermenting vegetables."
Having said that—I'm with you. Mold grosses me out. I try to minimize it.
In my experience, the wider the vessel's neck, the more mold and kahm (yeast) develop. I have good results with 2-quart mason jars, which have a low neck-to-volume ratio. That minimizes the "edge" that Katz describes. For large batches, that means multiple 2-quart jars, but the control is worth it to me.
I also check the surface of my ferments every 2-3 days and skim off any developing kahm or mold. Every time I check, I'm exposing the surface to more mold and yeast spores... but if I catch it when it's the size of a pinhead, it won't grow to the size of a quarter.
I've also had much better success since I started using actual fermentation weights and vented lids for my mason jars. Any will probably do, but I've been using this weight and this lid.