Originally By Tony Ackland
How Much Yeast to Use in FermentationsJack advises ...
Higher than 4 grams per gallon will get you some sulfur flavors that can be hard to get rid of, so only use the 100 grams of dry yeast per 5 gallons (20 litres) rule for a pure sugar mash that is destined to be carbon polished and turned into vodka or a "base spirit" for liqueurs, etc.
If buying that much dry yeast is a problem, you can make a starter. Make a small "mini batch" of your mash - using the same ingredients at the same concentration (no less than 500ml no more than 2,000ml for a 5 gal/20liter batch) and put it into a sanitized glass flask, bottle, jug, etc. Do this one or two days before you plan to make the main (5 gal/20L) batch. Add the small (typically 5 gram) packet of yeast to the starter, and when it is at high kraeusen, add it to the main batch. Yeast "learns" to feed on sugars when it wakes up from that little packet- it takes yeast seven generations to learn how to digest a different kind of sugar- therefore you MUST make your starter out of the same stuff you are going to make the main mash out of (this is why waking up your yeast in orange juice is a bad idea). Also, yeast is sensitive to sugar concentrations- so the starter MUST be the same strength or weaker than the main batch in order to prevent osmotic pressure from causing the formation of mutant yeast cells (a big cause of off flavors).
The temperature the yeast is used at also can cause the flavor to degrade. Most whiskey mashes use an ale yeast- the ideal temperature range is 60 to 70 degrees F. Lower temps will slow down the yeast- if sanitation is good- this is not a problem. If a higher temp is reached - the yeast will undergo "stress" reactions that cause excessive ester and higher alcohol formation- this will result in a solvent- like flavor that can carry over into the finished spirit. Lager yeasts tend to form a lot of sulfur compounds at the begining of the ferment- during the lagering stage the yeast reabsorbs these sulfur compounds, leaving a crisp clean lager flavor in the beer- since you don't want to store a whiskey mash for 2 months in the fridge- it's best to use an ale yeast.
When you are fermenting wine (for brandy or drinking)- it is best to use 2 grams of dry yeast per gallon and no more (two of the five gram packets per 5gal/20L batch). It's true that you would think to use 4 grams per gallon since the alcohol is so high (typically 10% or more)- but, with wine, in order to preserve the delicate aroma of the fruit you are fermenting, you need to have a slow, cool (60-70F) ferment to prevent the CO2 from driving off all of the more delicate flavors. A fast ferment in a wine will find the CO2 "scrubbing" the delicate flavors out, leaving you with a bland acidic wine that tastes pretty rough.
The very high cell concentrations typically cause a reduction in yeast growth. This makes the yeast that is pitched is the yeast that is responsible for the ferment- if the yeast viability is below 90%, stuck ferments may occur. Otherwise, the profile of the flavors that yeast makes is typically a mix of compounds made during both the aerobic and anaerobic phases- with the aerobic phase suddenly gone- some very odd smells occur (sulfur compounds), that, thanks to the stress of fermenting without any time to adapt (the lag phase), the yeast is damaged, and unable to reabsorb any of the esters and sulfur compounds when they go dormant (during the settling out and lagering phase-if any). The high cell count also makes fining and filtering more difficult.
Overall, underpitching is more of a concern than overpitching. Underpitching causes a long lag time that can allow bacterial infection to take hold, overpitching can cause off flavors to develop that can be removed with a long lagering/secondary ferment, and alot of copper exposure.
As a general rule, you use 400ml of yeast solids per hectoliter of wort (for a lager yeast), and half of that for ale yeast (granted, this is at 12degrees plato). For dry yeast, 2 to 4 grams per gallon of wort is best- 2 grams for standard beer, 4 rgams for doppelbocks, barleywines, etc. For an active yeast starter, the actively fermenting starter should comprise 10% of the volume of the mash/wort. It should also be of the same sugars/composition and at the same concentration (err on the side of a weaker starter, rather than a stronger one- yeast can go from "rags to riches", but not the reverse.)
Here are a few guidelines for proper yeast use in any ferment: