With the furious bubbling behind us and a mostly still bucket of wine-to-be, it’s time to move on to secondary fermentation. The exponential growth phase of the yeast is done and you’re probably sitting at about 10% alcohol as the yeast chugs along anaerobically. This means that the yeast no longer require oxygen and in fact, the wine must be protected from oxygen exposure and the resulting oxidation.
A syphon hose is used to rack (transfer) the must into a carboy, leaving behind the lees. Care should be taken to avoid introduction of air into the must at this point. The simplest way to do this is to ensure that your syphon hose goes right to the bottom of your carboy to limit bubbles as the must is transferred.
Once the transfer is complete, the air must be kept out using an airlock. Most people fill their airlocks with sanitizer solution but vodka (such a waste!) and water also work fine. The slow carbon dioxide bubbles from fermentation will keep the airspace in the carboy free of oxygen. The airlock acts as a one-way valve letting out excess CO2 and preventing air from entering.
And now we wait… secondary fermentation will take a couple weeks to finish and then the wine will begin to clear. As the remaining yeast is starved of sugar (it’s all booze now), it will settle to the bottom and the wine will clarify a bit.
When even airlock has shown no sign of life for a few days, it’s time to rack the wine again. This phase is used to degas and stabilize the wine. The first step is to remove as much dissolved CO2 as possible to allow the following additives to work optimally and to ensure we don’t have a slight sparkling in our wine. Flip your long brew spoon upside down and use the handle to mix the wine furiously for a couple minutes. This is a good time to taste and consider backsweetening if desired. Next comes potassium metabisulfite (kills yeast and bacteria) and potassium sorbate (prevents microbial reproduction), collectively termed stabilization. Mix vigorously after each addition to dissolve them thoroughly.
Clarifying comes next. Some people prefer to simply leave the wine in the carboy for an extended period of time and allow it to age and clear naturally. The less patient among us add clarifying agents to speed the process. My preferred clarifier is the very common kieselsol and chitosan combo. It is vital that the kieselsol is added first with thorough mixing before adding the chitosan. These agents work much like bentonite, using electrical charge attractions to grab and pull down dissolved proteins that cause haze. For more information, check out clarifying agents.
After a about a week, the wine should be clear and ready to bottle. I like to do one final racking before bottling to ensure my racking wand can’t suck up the goo in the bottom of the carboy during bottling. Your bottles should be clean and sanitized with a weak solution of potassium metabisulfite (1/2 teaspoon in 500mL water). There is no need to rinse the bottles with water, just shake out the excess sulfite before filling them. Cork your bottles with natural or synthetic corks that have been soaked in the same potassium metabisulfite solution.
You’re done! Now comes the hardest part of al… waiting for the wine to age sufficiently for drinking. As a rule, I wait 4 months for white wines and 8 months at a minimum for reds. Of course the longer you can wait, the nicer the wine will be.