Yeast is a single celled micro-organism which reproduces asexually by budding that basically do all the hard work. It is a fungus. Think of this; in on fluid ounce of excitedly fermenting must there are at least six billion yeasts. Your job as the winemaker is to make sure that these little bustling yeasts are kept happy. Take care of them and they will yield to you. 

Caring for Your Wine Making Yeast? 

You may ask, “How do you care for and keep wine making yeast happy?” First thing is to know that although they can survive in both hot and cold temperatures, they simply do not like big fluctuations or extremes very well. Yeast generally like a slightly acidic environment, they need oxygen to reproduce and they really hate sterilizing agents. Wine making yeast enjoy eating and what they like to eat is sugar or a yeast nutrient. In the process, the sugar is digested and ends up as two things—carbon dioxide and ethyl alcohol. Once all the sugar is gone, the yeast stops growing and eventually dies. Sometimes, the yeast is killed from overeating. What I mean is, the yeast produces so much alcohol that it no longer stays alive in the vat, even if sugar is still present. Yeast can get full and fall into a non producing state, this is called lees. This usually happens when there is about 18% yeast. This is the time to rack wine of the lees and wash the mess down the drain. 

Hybridization of Wine Making Yeast 

Hybridization is the making of high performance yeast. This means that first we need to know that now all yeasts are the same. One strain may be rich in qualities such as; being a strong fermenter, being sugar and alcohol tolerant, low in nitrogen demand and cold tolerant. Now the down side is this same strain does not put off a good smell and produces an abundance of SO2. Now a totally different strain is highly aromatic and has a higher affinity for fructose, but the down side for this strain of wine making yeast is that it is low in sugar and alcohol tolerance and is temperature sensitive and has a high nitrogen demand. Through the process of hybridization, they blend the two strains but during the process they take out the bad qualities. This is how commercial yeasts are made. 

Wine Faults 

Defined by Wikipedia, a wine fault or defect is an unpleasant characteristic often resulting from poor wine making or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine but at insufficient concentrations to adversely affect it. In fact, depending on perception concentrations may impart positive characters to the wine. However when the concentration of these compounds greatly exceeds the sensory threshold, they replace or obscure the flavors and aromas that the wine should be expressing. Ultimately the quality of the wine is reduced, making it less appealing and sometimes undrinkable. 

Wine making yeast faults occur when some yeast such as Brettanomyces also known as “Brett” develops in the wine. This unwanted growth of yeast can result in consequent loss and wine faults. Brettanomyces is considered as a major contributor to wine damage or wine faults. "Brett" if allow to rule the wine, sometimes will give off odors such as barnyard, horse stable, antiseptic, bacon, spice, cloves, smoky, sweaty sadly, cheese and rancidity. None of these are something we would wish to smell in a glass of wine. 

Characteristics of Wine Making Yeast 

First off let me say that you should not use Baker’s or Brewer’s yeast. Always use wine yeast. Wine yeast is derived from the yeasts which occur naturally on the skin of grapes as they bloom. There are both general-purpose and specific varieties of wine yeast available in dried granulated form. 

Four desirable characteristics of wine making yeast: 

1) They ferment rapidly and vigorously. 

2) They have a tolerance to high levels of alcohol. 

3) They rapidly form firm sediment when fermentation is complete. 

4) They do not impart off-flavors to wine. 

Yeast Nutrients 

Yeast needs certain chemicals, in particular; nitrogen (N), phosphorus (P) and sulfur (S) for vigorous growth and effective fermentation. If this is a problem for you, you can add yeast nutrient preparation when preparing the must. The best nutrient is a mixture of ammonium phosphate and ammonium sulfate crystals. This is included in some wine yeast preparations; if not, simply dissolve one level teaspoon of the mixture in the cool must or yeast starter. The nutrient compound is destroyed by hot water.