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.

Primary Fermentation

The beginning stages of primary fermentation.

A Primary Fermentation Bucket

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.

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.

Secondary fermentation often takes place in glass carboys.

A Secondary Fermentation Carboy

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.

  • Maria

    Good morning Matt

    From what I understand (so far) the primary fermentation is done in an open bucket (without the lid) since the air plays an important role in the multiplication of yeast cells, it is an aerobic process; after 3-5 days you rack it into a carboy and place the airlock, the process becomes anaerobic, in this phase air exposure should be kept to a minimum. I got this information from

    http://www.eckraus.com/wine-making-101/

    Maria

    • http://WinemakersAcademy.com/ Matt Williams

      Hello Maria!

      You are correct, oxygen plays a very important role in the fermentation process up until the yeast has reached a healthy population. Once that has been achieved if we do not restrict yeasts access to oxygen it will not produce the alcohol we are looking for.

      Primary fermentations tend to be quite vigorous so as the yeast population grows and the volumes of carbon dioxide produced increase you reach a point where the carbon dioxide produces a protective blanket over the wine thus limiting its exposure to oxygen.

      Check out The Epic Rise and Tragic Fall of a Yeast Empire for more information on the yeast life cycle. This goes into the role of oxygen, why we use airlocks, and more. Understanding how the population grows, struggles, and then dies is very important to making great wines. Everything we do in the beginning is to facilitate this process.

      Cheers!

      Matt

  • willowbel .

    Hi matt Im a first time wine maker here and have just batched my first mulberry wine..firstly I did my primary fermentation with a lid and airlock as I had learned that the airlock can help determine when the primary fermentation is finished and ready for the second phase. So I did a SG reading on day two of fermentating and it read at 1.090 the second reading I did was at day 5 at that read 1.020 so I racked of the wine mashed the berries and added in the extra liquid into a new barrel and placed the lid back on with the airlock the instructions said to leave it like this for 6 months and racking 2 times in between, after this you can bottle the wine where it has to be left for a futher 6 months to allow the wine to clear..so if I do a sg reading after the 6 months before I go to bottle it and my sg reading is at 1.000 what will this mean, does it mean the fermentation is complete and how will the wine go clear as it is still cloudy?

    • http://WinemakersAcademy.com/ Matt Williams

      Hello willowbel! First of all, congrats on starting your wine making journey. I really hope you enjoy this first batch.

      As for your question, an isolated specific gravity reading of 1.000 won’t tell you much except how much. What is more useful is comparing two specific gravity readings to each other.

      Two identical readings taken a couple weeks apart means fermentation is over. If there is any difference between the two readings your wine is still fermenting. Make sure you’re correcting your specific gravity readings for temperature differences though or you could get a false indication that things are still fermenting.

      Here’s an article on temperature correction that you may find useful <a href="http://winemakersacademy.com/specific-gravity-temperature-correction-calculator/&quot; target="_blank"Specific Gravity Temperature Correction Calculator.

      Wines can and do clear on their own. It takes longer than using a clarifier. Most wines clear in three to six months though if you have the patience :)

      I hope this helps. Keep those questions coming if you’ve got any!