Factors affecting the growth of a yeast population, and therefore the progress and efficiency of a typical fermentation are as follows: The sugar concentration can cause problems if it is too high, or too low. If it is too low, the yeast will respire if there is any oxygen present, producing carbon dioxide and water instead of ethanol. If it is too high, the osmotic pressure exerted on the yeast can delay the onset of fermentation. Approximately 50% of the total available sugar is used during the exponential growth phase, while the rest is fermented by the cells during the stationary phase. Sugar concentration may also influence the composition and concentration of aroma compounds produced by the yeast (see Section 2.2.2).