There is a bit of confusion out there as to what the difference is between primary and secondary fermentation. Sometimes secondary fermentation is confused with a second fermentation and other times malolactic fermentation. Let’s set the record straight.
Before we talk about secondary fermentation lets start at the beginning with primary fermentation. This stage starts as soon as you add your yeast to the must. During this stage the yeast population is growing rapidly.
You know you’re in the primary stage because there’s a lot of visible activity. There’s often a lot of foam on top of the must and your airlock will be bubbling like crazy.
Why? The yeast population growing really fast because of the huge supply of sugar, nutrients, and oxygen they lucked into. It’s like a party in there. Everyone is hopped up on sugar and bouncing off the walls.
This is the most active and productive phase of fermentation. In fact up to 70% of the total amount of alcohol is produced during this stage which only lasts about three to five days. After that we move into secondary fermentation.
After a while things start to slow down. The oxygen has been depleted and the bulk of the sugar has been used up. Because of this the yeast population is no longer expanding. In fact life is getting hard for the yeast.
Alcohol levels have risen to the point that it is affecting the yeasts ability to reproduce and even survive. Many cells are dying off and collecting at the bottom of the fermenter. This is one of the reasons we have to rack the wine after primary fermentation is over. We don’t want to pick up any off flavors from the dead yeast.
Secondary fermentation lasts between a week to two weeks. Obviously this is a much slower stage in the process. Primary fermentation took three to five days and produced 70% of our alcohol while secondary fermentation takes up to two weeks just to get the last 30%.
The foam will disappear and you will see tiny bubbles breaking at the surface of your wine. Your airlock will now be bubbling every 30 seconds or so.
There is no identifying event that separates the primary stage from the secondary stage. When it happens depends on the grape varietal, sugar content, yeast strain, fermentation temperature, etc. In other words you just have to watch your airlock or the level of activity at the surface.
Secondary Fermentation is not a Second Fermentation
This is where it gets confusing. A second fermentation is where excess sugar not previously consumed by the yeast restarts alcoholic fermentation. Commonly this happens when a wine is back sweetened before all the yeast have died.
Some people mistakenly refer to malolactic fermentation as a second fermentation. I think it makes sense to differentiate between the two so that we’re speaking a common language. Malolactic fermentation is malolactic fermentation.
Second fermentations usually happen by accident except when making sparkling wines. Sparkling wines are bottled before the yeast is dead and a little unfermented grape must is added to give the yeast something to eat. In so doing the carbon dioxide produced is trapped in the bottle and we have bubbly. That’s the short version.