Fermentation and yeast are the heart of wine making. Without both, we’d just be drinking grape juice. And who wants that?
Yeast are everywhere. You can find them on your skin, floating in the air and if you have a cellar, there are yeast floating around there that can start a fermentation (referred to as a wild fermentation) if you leave juice uncovered for a period of time.
So, if yeast are everywhere, why do we buy special wine yeast? The short answer is because the yeast floating around in the air may or may not be good for making wine. Wild fermentations also take longer to get started compared to commercial yeasts. Buying commercial yeast guarantees us that the fermentation will start quickly (usually within 24 hours) and that the resulting wine will be good and will taste a certain way.
Pro Tip: Some people will tell you it’s ok to use bread yeast to ferment your wine. It’s not. Wine yeast is engineered for better flavors and tolerates higher alcohol levels. Bread yeast does not. As a wine maker, always buy and use commercial wine yeast.
Yeast works by consuming sugars in your juice and producing alcohol as the byproduct. But yeast doesn’t survive on sugar alone. Your yeast need some basic nutrients and certain conditions to survive (and thrive!):
Oxygen: Many wine recipes or instructions will tell you not to place a lid and airlock on your bucket for the first so many hours. Instead, cover the bucket with a cheese cloth in order to provide oxygen for the fermentation to get started.
Yeast need oxygen to stay healthy and propagate. When oxygen is limited or used up quickly, the yeast won’t be able to propagate for long. If this happens, your yeast will die off and fermentation will stop with sugar still remaining in your juice. This is referred to as a stuck fermentation.
Sugar: Sugars are the main source of food for your yeast. In most cases there will be plenty of sugar available in your juice.
Nutrients: In order to complete fermentation, your yeast need things such as diammonium phosphate (DAP), amino acids, vitamins and minerals and zinc, among others. The best way to ensure your juice has all the nutrients the yeast need is to use a quality yeast nutrient such as this one from LD Carlson, which you will dissolve in your juice prior to adding the yeast.
Pro Tip: There are many different yeast strains available. Each strain of yeast has certain temperature ranges that it works best in. Be sure to check your yeast for the temperature range that works best for it. Also remember that fermentation produces heat. During fermentation, the inside of a 6-gallon bucket of juice can be 5 (F) degrees warmer than the air outside of the bucket. Be sure to take this into account when determining the temperature range for your yeast.
Fermentation usually begins within 24 hours of putting the yeast into the juice (called pitching the yeast). During fermentation, the yeast will begin to consume the sugars and other consumables. The yeast will soon begin to produce alcohol as the byproduct. When you start to see bubbles, foam or both in your juice, you will know that fermentation is well underway. You might also hear a slight fizzing sound, sort of like the sound Coke makes when sitting in a glass.
There are several phases to fermentation:
Lag Phase: This begins with pitching the yeast. Yeast are adapting to the environment and begin working.
Log Phase: This is the growth phase. The yeast will be propagating (multiplying) and expanding rapidly.
Deceleration Phase: Sugars and nutrient become scarce. Yeast cells can no longer propagate. Fermentation will start to slow down.
Stationary Phase: While the yeast are still alive and consuming any remaining sugar there may be little signs of it. Alcohol levels are reaching the top end that the yeast can tolerate.
Decline Phase: Alcohol levels reach toxic level and no consumables remain. The remaining yeast can no longer live in the environment and die off.
These phases typically take 1-2 weeks to complete. A hydrometer (also see our article on wine making equipment) will tell you when your fermentation is complete. When the reading registers 0.990 (completely dry), all the sugar has been consumed.
A good fermentation and quality yeast are keys to producing a great wine. It’s essential to pay close attention to the fermentation temperature as well. Temperatures above the maximum range can kill your yeast and/or result in off flavors in your final wine. Temperatures below the minimum range can result in fermentation not starting.
Pro Tip: Remember, bread yeast is not an acceptable yeast to use when making wine.