A Little Waiting is Good
When sweet wort or must is pitched with yeast, there is the characteristic lag phase before signs of fermentation are evident. This lag phase is an important part of the fermentation process. It may last from one to 24 hours depending on the quantity of yeast pitched, fermentation temperature, and oxygen content of the wort or must.
After the yeast is pitched, it begins to assimilate into is new environment. During this phase the yeast is involved in oxygen uptake and reproduction of new yeast cells. To make new yeast cells, lipids are required. Lipids make up the cell membrane. A necessary component of these membranes is sterols. To produce sterols, oxygen is required. Different yeast strains require different levels of oxygen to produce membrane sterols. Generally, the more flocculent strains require higher levels of dissolved oxygen.
If a wort or must is pitched with too much yeast, the yeast will begin to ferment without multiplying to the desired level of 50 million cells per mililiter. This leaves the pitching yeast as the main fermentation agent instead of new, healthy cells. Over pitching leads to low viability in succeeding generations and adds a yeasty flavor to the beer, wine, mead or cider. Conversely, if a wort or must is pitched with too little yeast, higher ester levels may be produced due to the higher yeast colony growth required. Underpitching can also leave the beer, wine, mead or cider exposed to potential growth from other micro-organisms. Home-fermenters rarely overpitch, but often underpitch yeast.