It’s important to have a basic knowledge of yeast to know when you should take certain steps in wine making. Understanding the life cycle will be a big help.
When you first tear open the packet of yeast it is dormant. Once it’s sprinkled on the must it enters into the reproduction (lag phase). Once introduced to a high-nutrient must the individual yeast cells begin to acclimate to the environment. The oxygen, sugar, and other nutrients in the must are just what the doctor ordered for reproduction to begin. The yeast rapidly reproduce in a process known as “budding”. This is why you should stir in oxygen the first day or two of primary fermentation. Oxygen is very important at this point in fermentation. Note that the yeast can multiply many times in a single day.
They will continue to multiply until the nutrient supply is exhausted and/or the byproducts of the metabolism reduces the yeasts ability to reproduce.
At this point the yeast enters the alcohol production (stationary phase). This happens when reproduction and decay start to equal out and the yeast colony becomes stable or stationary. It’s important to note that while budding the yeast need oxygen however once they enter the alcohol making stage they no longer need oxygen and oxygen can even be considered bad for the process at this time. This is because with oxygen present the yeast will continue to multiply using sugar. You can be making yeast you really don’t need and using sugar in the process. This means less sugar to turn to alcohol. During this phase the yeast eats the sugar turning it to alcohol and CO2. The CO2 production is very rapid at this point and as it escapes it will keep oxygen off the surface of the must. Once this escaping CO2 slows it is time to place your wine under an airlock to keep oxygen out.
Once most of the sugar and nutrients are used up the byproducts of this process will start to build up and inhibit yeast growth. The yeast will now start to die off (death phase). The dead yeast cells will start to sink and release toxins further damaging the surviving yeast. These dead yeast cells form the sediment on the bottom of the secondary. During this last phase some of the yeast will go into self preservation mode and produce protective cysts and go dormant. These cysts are what can cause re-fermentation when oxygen, sugar and nutrients come together later on and provide a ripe environment for fermentation. This is why potassium metabisulphite and potassium sorbate are used, to stabilize wine, after completion of fermentation and before sweetening the wine . When these conditions come about the yeast will not reproduce (multiply) because the potassium sorbate will inhibit reproduction and the potassium metabisulphite will stun any dormant yeast coming to life. Also keeping the metabisulphite level above 20 ppm will inhibit an unwanted malolactic fermentation (MLF) from starting.
What you need to know:
First – Oxygen is very important to your yeast during the first 24 to 48 hours of fermentation. Stir vigorously a couple times a day.
Second – Limit oxygen to must after the first 48 hours. The must takes care of this for you, for a few days, by rapidly producing CO2 and releasing it which keeps the oxygen off the surface of the must. Once CO2 production slows it is very important to get the must under airlock to keep oxygen out.
Third – Nutrients are very important in wine making. The lack of proper nutrition will cause yeast to stress. Stressed yeast produce byproducts resulting in off tasting and sometimes very smelly wine. Also, for every gram of sugar that the yeast converts into these byproducts is one less gram of sugar it converts into alcohol.
Forth – Know the yeast you are using. Temperature, nutrient needs, and alcohol tolerance are all things you should know.
Happy Wine Making!
Country Fruit Wines 