'Secondary' fermentation?

A famous quote from Kurt Vonnegut from his book Breakfast of Champions sums up the process:

Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast.  They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne.

This scenario actually applies to the end of the fermentation process for making any kind of wine, or any alcoholic beverage for that matter.

The duration of primary fermentation depends on temperature, growth of the yeast culture, and obviously, the sugar content of the must. In general, the primary fermentation takes about a week. At the end, only residual sugar will be left in the wine since the majority was converted into alcohol and carbon dioxide. 

Visibly speaking, the amount of ‘bubbling’ or ‘fizzing’ happening in the fermentation vessel becomes minimal compared to the vigorous primary fermentation stage. The yeast are slowing down, or dying off, as they succumb to the alcohol or lack of nutritive resources. 

Oxygen was essential for yeast propagation during the primary fermentation. However, now that the yeast cells have converted all the sugar, they no longer produce the carbon dioxide blanket to prevent oxidation of the newly made alcohol. Unfortunately, at this marginal potency of alcohol (generally about 10 to 15% for wine) the wine could spoil as the alcohol turns to acetic acid (vinegar).  I will have more on the science (and microbiology – my favourite topic!) of this process in future post.

In some instances, the fermentation finale as it occurs in the air-locked vessel as described in the last winemaking post is referred to as a secondary fermentation.  The purpose of the air-lock is to allow carbon dioxide to escape and to prevent the entry of outside air which is rich in oxygen.

The wine is siphoned off the sludge, known as ‘lees’, that settles to the bottom of the fermentation vessel.  Called 'racking'.  If at all possible, it is best practice to top up your vessel to minimize the level of airspace or ('headspace').  Using a similar wine to the kind you are making ensures that you will not dilute your creation.


The wine generally sits for at least a month at a cooler temperature to allow the vast majority of fine particulate matter to settle out. However, even after this length of time, the wine may still appear cloudy. Additional steps are taken to polish the wine to clarity with organic or inorganic agents that accelerate precipitation.  This process is known as 'fining' and will be the subject of the next winemaking post.