Direct from Winexpert on why you don't rack off the lees before adding the fining agent.
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Q: Why is it necessary to add the fining agents (package #4) before transferring the wine must off the sediment that has built up in the carboy bottom? Wouldn't it be more efficient for package #4 to be added after the sediment has been removed? It seems the clearing agent has to do more work to clear the wine by adding it with the sediment still in the carboy, especially when you're stirring this sediment up in the process.</A>
A: This one fools a lot of people, as it does seem at the outset that you'd want to get rid of the sediment first and then add the clearing agent, particularly when the wine in the carboy otherwise seems clear. The temptation is so great, many winemakers DO switch the steps themselves. This is not wrong - it's just less efficient, believe it or not.
The clearing, or fining, agents used in Winexpert's wine kits, whether it be chitosan or isinglass, both act more efficiently in clearing wine when they have a base of sediment to begin with. The sediment acts as a trigger mechanism which sends the finings into action in clearing out the mix of proteins, pigments, phenolics, dead yeast, etc.
Both the fining agents and the particles to clear out from the wine have either a positive or a negative charge. And just like in the movies, opposites attract. A negatively charged fining agent like bentonite will serve to bring together those particles having a positive charge, while positively charged fining agents like chitosan or isinglass will attract negatively charged particles. This process allows for the molecular weight structures of the particles to become larger: smaller particles join together to become larger particles, which in turn fall to the bottom of the carboy when their mass becomes great enough.
If the fining agents do not 'find' enough particles present in the wine must to join together into larger particles, the clearing process may stall, as there will not be enough small particles present to conglomerate into the larger particles which will fall out. Small particles on their own will remain suspended in the must, and the fining's efficiency is reduced.
This is why you must thoroughly stir the sediment when adding package #4, as it effectively mixes the fining agents and the particles together to start the clearing process.
Resist the urge to jump the gun on transferring, or racking, the wine! Trust the method behind the madness of Winexpert's instructions, and stir up that sediment with confidence!
I have made many of these type of kits and there is no need to filter as the fining agent will clear the wine brilliantly in 2 weeks. I like to let my white winessettle for 3 weeks before racking and bottling.
