So, this morning I made my first wine ever, from a CellarCraft kit that included grape skins (fancy!). And lo, I, too, was surprised by the addition of bentonite at the beginning.
While I was looking up what the heck "chitosan" and "kieselsol" are (which were also included in the kit), I stumbled upon this:
Bentonite is unique in that it can be added to your wine either before or after fermentation. Most kit wines include Bentonite in the add pack, with instructions on how to mix it into the must before adding yeast. If you just dump the dry Bentonite into the must or wine, it will instantly turn into a clump of white mud and go straight to the bottom. It needs to be hydrated in a cup of hot water, and whipped into a slurry — preferably with a blender — and stirred into the must.
When added pre-fermentation, it first settles to the bottom of the grape must. But when the turbulence of fermentation begins, CO2 gas bubbles form in the must and grab onto the Bentonite. The bubble will lift the Bentonite up to the top of the fermenting must, attracting positively charged solids as it rises. The bubble will burst when it reaches the surface, and the Bentonite particle will fall to the bottom again, still gathering positively charged solids as it sinks. This up and down circulation of Bentonite during fermentation clears your wine as it ferments. The self stirring action of fermentation allows the Bentonite to collect the dead yeast and other particles as they are being produced, so that by the time you are ready to rack from the primary, the Bentonite will have gathered a nice sediment of unwanted solids and dead yeast on the bottom.
In addition, Bentonite helps keep wine stable during fermentation. When making kit wines, I have found that not adding the Bentonite on day one, as per instructions, sets the stage for an overly vigorous fermentation that tends to foam over and make a mess — even to the point of pushing the loose cover off the primary pail.
... which is pretty much what y'all are saying above. But printed in a magazine at one point! So it must be true.
I bet bentonite would help with something like White Labs WLP004 Irish Ale yeast, which, for me at least, always makes crazy amounts of foam that tend to defeat my best efforts at blow-off contraptions.
Is there something about beer yeast that makes it more likely to stall than wine yeast? Because if bentonite is included in wine kits made for noobs like me, then stalling must not be in the realm of common wine kit problems. But with beer yeast, I don't know.
Edited to add: BTW, the yeast included in the kit was EC-1118, which is a high attenuator, and laughs in the face of sticking. So maybe that's it.
Do they include EC-1118 in all kits? I know it from meadmaking, but was kinda surprised that they'd put it in a red wine kit. Maybe that's part of the promise of "6 weeks to bottle".