During the primary fermentation, it is extremely important to keep your yeast population nice and happy. Without adequate air supply and nutrient availability, yeast can produce unwanted bi-products that can destroy what would otherwise be a good wine.
Air
In the first week or so, a little air is generally good thing for your wine. A yeast cell can respirate both aerobically (with air), and anaerobically (without air). Anaerobic respiration is the normal process of yeast converting sugar into alcohol and CO2. Aerobic respiration occurs in the presence of oxygen and will convert alcohol and oxygen into CO2 and water. This process encourages healthy budding and multiplication of the yeast. If the wine is too starved of oxygen, sulfur compounds can begin to accumulate which will create hydrogen sulfide when ingested by the yeast. Hydrogen sulfide produces a gassy, rotten egg or swamp gas smell. It is also readily oxidized and eliminated if some air is introduced, but evolve into mercaptan (burnt rubber/garlic smell) if gone untreated. Mercaptan is one of the most difficult defects to eliminate from a wine.
When a young wine is too starved of oxygen and does create compounds like hydrogen sulfide, this is referred to as "reductive". A little reduction will generally work itself out as the wine encounters oxygen during pressing or punch downs... by the way, checkout the new smartwinemaking "punch buddy" punch down tool!
Nutrient
Yeast needs nitrogen and a variety of amino acids to function happily. Yeast Assimable Nitrogen (YAN) is often measured before the start of a fermentation to help guide nitrogen additions. Nitrogen is generally added in the form of diammonium phosphate (DAP). Many products exist to supplement the nitrogen addition with amino acids and yeast hulls. One such product is Fermaid K, which I have had great experience with.
Re-Hydration nutrients can also be used to help ensure that the yeast cells have the nutrient needed. Go-Ferm is commonly used with a yeast starter before adding to the must.
In the absense of sufficient nutrient, the yeast population will generally become stressed and create hydrogen sulfide or acetic acid. A stressed yeast population can also stall out before consuming all of the available sugars. It is important to feed the yeast early, and to stop feeding the yeast as the fermentation approaches 1/3 to 1/2 completion. Any residual nutrient or sugars leftover after fermentation can be used by undesirable bacteria during the aging period.
Temperature
Each yeast strain has an optimal temperature range which is listed on the packet, or available from the manufacturer. This is usually in the 60°F to 80°F range, but can be higher or lower. A fermentation that drifts above or below the recommended window can become stressed and create all the bad things that a lack of air or nutrient can create. To nudge the temperature up on a small fermentation, I like to use a seed heater or a space heater. To nudge it down, I will add frozen two liter bottles or gallon jugs. It is important to monitor temperature daily. The yeast can generate a good bit of heat when they really get roaring!
I should mention the stylistic implications of fermentation temperature. Cooler fermentations will retain more of the fruity aromatics, while warmer fermentations can be more intense and rich, but less aromatic. Choose a yeast that works well in the temperature profile that you plan to ferment within.
Hydration
Rather than sprinkle the yeast directly onto the must, I will generally hydrate the yeast and make a large yeast starter. This is relatively simple. Add the recommended go ferm dose to warm water, usually around 105°F. Add your yeast to the mix. Once things get bubbling away, add some of the wine must to the solution (I usually just about double the volume with each addition). Every 10 or 15 minutes, add some more wine must. When things are bubbling good and the temperature is within 10°F of your wine must, go ahead and pour the yeast starter on top. No need to mix it in, until you see signs fermentation. Make sure the must is warm enough to keep things going. It is usually a good idea to start a wine must around 70 or 75°F, then adjust the temperature to your planned profile once you see active bubbling.
Follow these simple steps, and you should have relatively few yeast related problems. If you have any specific tips or tricks, mention in the comments below and be sure to check out our Youtube Channel!
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