Wine Fermentation 101
By Ed Kraus
I thought it would be interesting to run through the fermentation process and try to describe a little bit more clearly what is actually taking place when you are producing a fermentation.
It is not necessary to understand all the ins and outs of a fermentation to make wine--particularly if you are following a good recipe with solid directions. But none the less, having a more intimate understanding of the fermentation process can only make you a more accomplished winemaker if nothing else.
What Is Really Going On
In very general terms a wine fermentation occurs when yeast consumes sugar and converts it into approximately half alcohol and half CO2 gas (carbonation) by weight.
For example, if you had five gallons of juice that had 10 pounds worth of sugar in it, and you fermented all of that sugar with yeast, you would end up with 5 gallons of juice that has roughly 5 pounds of alcohol in it.
The other five pounds of sugar would dissipate into the air as CO2 (carbonic) gas. So in fact the five gallon batch would become five pounds lighter than it was before the fermentation started.
Realize that the breakdown of alcohol verses gas would not be exactly half and half, but usually it would be very close. Some variances do occur depending on external factors such as the amount of available air, nutrients as well as the type of yeast used. But, rest assured that it would be within 46% one way or another.
It is important to note here that the 10 pounds of sugar that was in the five gallon batch may not have come all from sugar you added, but partially from the fruit as well. And in some cases, such as when making a wine from grapes, there may be no sugar required at all. In these cases enough sugar is already in the fruit itself to produce a wine with 11 or 12 percent alcohol.
A wine fermentation has two distinct stages: primary and secondary--also sometimes described as aerobic and anaerobic fermentations.
* The Primary Fermentation will typically last for the first three to five days. On average, 70 percent of the fermentation activity will occur during these first few days. And in most cases, you will notice considerable foaming during this time of rapid fermentation.
The primary fermentation is also called an aerobic fermentation because the fermentation vessel is allowed to be opened to the air. This air plays an important roll in the multiplication of the yeast cells.
Here's how important. The little packets of yeast that is generally called for in a five gallon wine recipe will typically be multiplied up to 100 to 200 times during the few days of primary/aerobic fermentation. Without air this multiplying stage is hindered. That is why it is important that you do not use an air-lock during the first few days of a fermentation and allow the fermentation to be open to air.
Alcohol is being produced during the primary fermentation as well, but a significant portion of the yeast's energy is being devoted to reproducing itself.
* The Secondary Fermentation is when the remaining 30 percent of of fermentation activity will occur. Unlike the typical four to seven days the primary fermentation takes, the secondary fermentation will usually last anywhere from one to two weeks depending on the amount of nutrient and sugars still available.
So as you can start to see, the secondary fermentation is much slower with less activity at any given time. You will also notice the activity becoming slower and slower with each passing day.
The secondary fermentation is an anaerobic fermentation which means that air exposure is to be kept to a minimum. This can easily be done by attaching an air-lock to the fermentation vessel.
It is this reduction in air exposure during the secondary fermentation that entices the yeast to forget about multiplying and start giving its energy completely to making alcohol.
* Temperature plays an extremely vital role in the fermentation process. If the fermentation temperature is too cool, the yeast may not be invigorated enough to ferment. It will simply remain in the juice, dormant.
If the fermentation temperature is too warm, the yeast may ferment fine, but the flavor of the wine will usually suffer. This is because of the increased production of unwanted enzymes by the yeast and the possible growth of micro-organisms that thrive in warmer temperatures.
The optimum temperature for a fermentation is 72 degrees, but anywhere between 70 and 75 will do fine.
* Throughout the fermentation process you will need to transfer the wine off the sediment into a clean container. This is a process that is referred to as "racking" in most wine making books.
This should be done at the end of the primary fermentation or when the Specific Gravity reading on your hydrometer reaches approximately 1.030. It should also be racked after the secondary fermentation as well as right before bottling the wine.
* It is also important to understand that once the wine's fermentation activity has stopped that it also needs to be given time to clear as well before bottling. Yeast is a silty substance that can take up to 2-4 additional weeks to clear up once the fermentation has stopped.
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Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.