What gives a yeast strain its particular competitive rating? Is it the yeasts' ability to dominate a fermentation, or is it a yeasts' ability to literally "kill" or retard other yeasts?
Even with the best sanitation, the clusters of grapes bring in from the field and equipment a whole host of microorganisms, both friendly and unfriendly. There is a regular slugfest going on in the must during the first day or so of the fermentation and may the best yeast win. With care you can make it possible for your added yeast to dominate the competition by adding as early as possible with at least 4 X 106 per ml of properly rehydrated selected yeast. Adding nutrients throughout the first half of the fermentation gives added insurance that the select strain will be vigorous to the end.
Nature has provided all of its creatures with competitive factors that give them the ability to compete and survive in order to pass on the gene pool. We are familiar with camouflage, speed, strength and proliferation. Nature has provided wine yeast, Saccharomyces, with several competitive factors for it to survive in the hostile fermentation environment: Acid tolerance, Alcohol production and tolerance, function with or without oxygen and competitive factor for some of the strains.
Wine yeast can grow well over a wide range of temperature and pH. This gives it a very competitive edge over most bacteria and mold. Yeast have cell walls and a metabolism that allows it to function even below 3,0 pH. Most bacteria, especially health related bacteria, cannot survive below pH 4.5.
Alcohol production and tolerance:
Most selected wine yeast strains can produce alcohol above 15% while most spoilage yeast strains cease to grow at 3 - 5% alcohol. Wine yeast can rapidly produce alcohol in this range, eliminating these competitors and then go on to enjoy the remainder of the fermentation without worry.
Function with or without oxygen:
Yeast are unique creatures in that they have metabolic pathways that allow them function with or without oxygen. They prefer to have a small amount of oxygen but can reproduce 4 - 6 times without oxygen. They obtain their energy from the sugars more efficiently in the presence of oxygen and produce more biomass and CO2. Without oxygen they are less efficient and may even run out of gas as the alcohol level climbs at the end of the fermentation.
The mitochondria of some yeast strains have the ability to produce a small 1100 MW protein that is guided through the cell wall and attaches itself to the cell wall of another strain of yeast disrupting the magnesium metabolism resulting in the death of the cell. This tiny protein was first observed in 1964 and was thought to be restricted to this one strain. Several years later a survey was made of many wine culture collections and the ability to produce this disruptive protein was found in 20 - 80% of the strains. It seemed to be more prevalent in the warm regions and less prevalent in cold climates. A further study found that all genus of yeast have some strains that produce the competitive factor. The factor from one genus cannot cross over and kill yeast from another genus. There are five or more competitive factors: K1, K2, K3 etc. Ninety-five percent (95%) of the strains of wine yeast can make the K2 competitive factor and five percent (5%) can produce the K1 competitive factor. Yeast strains are divided into three categories regarding the competitive factor:
1. Competitive positive, which is a yeast strain that produces a competitive factor protein and is immune to this protein produced by other strains.
2. Competitive neutral, which is a yeast strain that does not produce a competitive factor protein and is immune to the competitive factor proteins produced by other yeast.
3. Competitive sensitive, which is a yeast strain that does not produce a competitive factor protein and is sensitive to the competitive factor proteins from other strains.
The competitive factor functions best at >4.0 pH and is almost inactivated at low pH. It is wise to be aware of the competitive factor when selecting a yeast for primary and secondary fermentation and if you are adding a second yeast to restart a sluggish or stuck fermentation, but do not rely on it to take the place of good sanitation.