Making a starter culture to increase the quantity of yeast pitched into a particular wine is a great way to assure consistent results.
The Wyeast Activator (125 mL) smack-pack is designed to directly inoculate 5 gallons of must. An Activator contains approximately 100 billion cells which will deliver slightly less than 6 million cells per mL in a 5 gallon batch of wine. Higher volumes of must require additional packages of yeast or making a starter culture.
Professional winemakers typically inoculate must at 3-6 million cells per mL in their fermentations.
Calculate the size of your starter by using Wyeast's Pitch Rate and Growth Calculator.
The two main factors associated with controlling the level of cell growth in fermentations are sterol content in cell membranes and amount of sugar available.
In most standard fermentations, the sterol content in the cultures is the limiting factor in cell growth. Sterols (See Oxygenation section) are only synthesized during the early stages of fermentations and are diluted every time a cell buds. When sterol levels reach a certain point in the cell membrane, the cell will stop budding. If very high pitch rates are used, as is the case with most starters, the culture can exhaust the sugar source prior to depleting the sterol contents. The depletion of sugar will cause the culture to stop growing.
The optimal media for cell growth and health require using a juice based media, fortified with nutrients. Gravity should be kept near 1.040 and cultures should be grown at 70° F (21° C).
2 L (2 qt.) Sterile juice (Grape, Pear, Apple, or Diluted Must*)
½ tsp Wyeast Wine Nutrient Blend
Pour juice and nutrient into a sanitized flask or jar with loose lid or foil.
Shake well and add yeast culture.
Allow to ferment 18-24 hours. Shake/agitate frequently.
Take gravity reading to confirm 75% attenuation prior to adding to main ferment.
** If must is used, dilute 1:1 with sterile water
Timing of Starter:
Because starters are inoculated at high cell densities, growth is usually maximized within 24-36 hours. The gravity of the starter should always be checked prior to inoculation into must to assure proper cell growth . Cultures should be used immediately, or refrigerated for up to 1 week before using. Cell viability will decrease rapidly if culture are left at ambient temperatures for extended time.
Stirring and O₂:
Agitation aids in removing inhibitive CO₂ from suspension as well as possibly adding small amounts of oxygen. Stirring or shaking the starter periodically or using a stir plate will improve cell growth. The use of stir plates has been shown to increase cell growth 25-50% over a non-stirred starter.
Small additions of oxygen periodically throughout the growth of a starter will replenish sterols and improve cell yield.
It is important to understand that creating a starter can increase the risk of infection by undesirable organisms. If non-sterile must is used, then there are other organisms present that could multiply to unacceptable levels. Use of either heat sterilized or sterile filtered must or juice is recommended. Use of a broad spectrum yeast nutrient is recommended for best results.