ethanol is released by the yeast cells as a waste product. Eventually, if the yeast cells are healthy and fermentation is allowed to run to the completion, all fermentable sugars will be used up by the yeast with only the unfermentable pentose leaving behind a negligible amount of residual sugar. Other compounds in wine produced by yeast While the production of alcohol is the most noteworthy by-product of yeast metabolism from a winemaking perspective, there are a number of other products that yeast produce that can be also influence the resulting wine. This includes glycerol which is produced when an intermediate of the glycolysis cycle (dihydroxyacetone) is reduced to "recharge" the NADH enzyme needed to continue other metabolic activities. This is usually produced early in the fermentation process before the mechanisms to reduce acetaldehyde into ethanol to recharge NADH becomes the cell's primary means of maintaining redox balance. As glycerol contributes increased body and a slightly sweet taste without increasing the alcohol level of the wine, some winemakers try to intentionally favor conditions that would promote glycerol production in wine. This includes selecting yeast strains that favor glycerol production (or allowing some wild yeast like Kloeckera and Metschnikowia to ferment), increased oxygen exposure and aeration as well as fermenting at higher temperatures. Glycerol production is also encouraged if most available acetaldehyde is made unavailable by binding with bisulfite molecules in the wine, but it would take a substantial amount of sulfur dioxide addition (far beyond legal limits) to prolong glycerol production beyond just these very nascent stages of fermentation. Other by-products of yeast include: Methanol – Caused by the demethylation of pectins in the must by enzymes of the yeast. More commonly found in red wines than white but only in very small amounts between 20–200 mg/L. Fusel oils – Formed by the decomposition of amino acids by the yeast. This includes 2,3-Butanediol which is formed by yeast-consuming diacetyl, the compound that gives Chardonnay and other wines a "buttery" aroma, reducing it first to acetoin and then to the more neutral-smelling 2,3-Butanediol. Many beer and winemakers who have a wine with too much "butteriness" will often "pitch" fresh yeast cultures into the no longer fermenting tank so that the yeast will consume the diacetyl and reduce the aroma. Succinic acid – Like glycerol, this is often formed early in fermentation. Usually found in concentrations of 500–1200 mg/L, it is a