Many yeast cells are needed to complete fermentation, and from 50 to 200 million cells per milliliter of juice are present at the peak of fermentation. The number of active yeast cells used to inoculate a fermentation range from 2 to 10 million cells per milliliter. When fermentations are started with several million cells per milliliter, only a few generations of new cells are needed to reach the peak population. Most winemakers prefer large inoculations because the inoculated yeast strain quickly dominates any other micro organisms in the must.

The phenomena of killer yeasts were discovered in 1965. Much information about killer strains has been learned since then, and yeasts from more than seven different genera are now known to produce killer toxins. The killer yeasts produce toxins as either proteins or glycoproteins, and these toxins are fatal to other yeast strains. Not all yeasts strains are sensitive to killer toxins, and yeasts are currently classified as killer strains, sensitive strains, unaffected strains and competitive yeast strains. The sensitive yeast strains cannot survive when significant quantities of killer toxins are present in the must. The insensitive strains are affected by the presence of killer toxins. Competitive yeast strains are not killer strains, and they are insensitive to killer toxins.

Toxicity is measured observing the inhibition of growth of one yeast strain in the presence of another strain. Research shows the killer toxins are more effective at pH values between 4 and 5. However, enough toxicity remains at wine pH values (3 - 4) to still be lethal. Some researchers hoped killer yeasts could be used to maintain pure cultures. Unfortunately, this does not seem feasible because although killer toxins are lethal to some yeasts, they do not affect other strains of yeast. Please note that killer toxins have no adverse effects on wine consumers.

Yeasts in the fourth classification are interesting because they seem to have developed a different survival mechanism. These yeasts are called "competitive" yeasts because they acquire and retain the micro nutrients in the must. These strains are not killers. They compete by stripping the must of nutrients needed by other micro organisms. Prise de Mousse and Pasteur Champagne yeasts are examples of competitive yeast strains. These yeasts are vigorous, and they seldom produce stuck fermentations because of their competitive nature. Both yeasts are popular with winemakers.

Most of the killer yeast toxins have been studied and identified. Killer toxins were originally placed in ten categories. However, recent research shows some killer toxins do not fall in any of these ten categories, so a broader classification system for killer toxins will probably be needed. Current studies are concerned with the genetics of killer toxin production, and much research is being done. Killer yeast strains are now available commercially in active, dry form.