Bentonite is a type of super absorbent clay that is usually added prior to fermentation. This volcanic-ash clay can absorb many times its own weight in other compounds. Bentonite is mixed vigorously with water before being added to the must to prevent the clay from caking together and sitting as a lump at the bottom of your fermenter. The bentonite will quickly settle out into a fine dust at the bottom of the fermenter and remain until fermentation begins.
A quick word about Carbon Dioxide (CO2) and Nucleation Sites: During fermentation, the yeast produces CO2 which tries to escape the wine. This escape occurs at rough or jagged points and not along the smooth sides of a glass carboy or bucket. These rough areas are called nucleation sites, and you’ve seen them before even if you didn’t realize it. Some beer glasses have tiny dotted logos or shapes etched into the bottom of the glass. These allow the carbon dioxide to gather and raise to the surface of the beer, helping to maintain the foamy head of a beer and bringing aroma compounds to the surface. Another example would be dropping a Mentos in a bottle of Coke. The surface of the Mentos is rough and when dropped into the Coke the supersaturated CO2 in the Coke quickly forms at the many nucleation sites on the Mentos and rushes towards the surface of the bottle, creating the giant burst of gas and liquid from the bottle.
Bentonite benefits from nucleation sites because as CO2 is released into the wine by the yeast, the gas comes out of solution on the bentonite causing it to plume upwards into the wine and continually circulate. This process happens during most of the active fermentation, and is why it is recommended to stir the wine after primary fermentation to remove CO2 and help the bentonite finish its job. Bentonite gathers many floating compounds by absorption and the weight of these new compounds causes the bentonite to fall back to the bottom of the fermenter. While the bentonite falls it gathers stray proteins, dead yeast cells, and other wine-clouding compounds. This is why bentonite is much more effective when used prior to fermentation. After the production of CO2 ends the bentonite quickly falls to the bottom of the fermenter and does not capture any loose proteins or other compounds that are still suspended in the wine. Bentonite is probably the most widely used fining due to the fact that it settles out completely after fermentation leaving no flavor, aroma or color behind in the finished wine. You will find bentonite in almost every wine kit.
Chitosan and Kieselsol
I’ve lumped these two fining agents together because many times they are sold and used in pairs. Chitosan is a positively charged fining made from chitin, a compound that makes up the exoskeleton of crustaceans, such as shrimp, crabs, and lobsters. Kieselsol is a negatively charged fining made from silicon dioxide (silica) which is mostly found in quartz. These finings usually come together under a brand name, such as DualFine. First kieselsol is added and gently stirred into the wine after fermentation and then a day later the chitosan is added and stirred. This combination of the negatively and positively charged finings collect all of the suspending particles in your wine and can typically clear up a wine very quickly. I realize that these compounds may seem like they have a strange origin and are not something you want to consume, but rest assured they work by removing haze causing elements in wine and then settle to the bottom of the fermenter. When the wine is transferred out of the vessel, these compounds stay behind with the lees (the yeast and other junk that settles out on the bottom of the fermenter.)
There are many other clarifying agents available for home winemaking. Some are used widely and some are more specific to certain types of wine, but bentonite, chitosan, and kieselsol see use in almost every wine. Next time we will discuss some of the other general and style-specific types of finings.
Austin has been brewing beer commercially and at home for the past decade. Feel free to contact him with any questions or comments at email@example.com