Diacetyl can be produced by the LAB through metabolism of sugar or of citric acid. While citric acid is naturally present in grapes, it is in a very small amount with most of it coming from deliberate addition by the winemaker to acidify the wine. In the presence of both malic and citric acids, the LAB use both, but use the malic much more quickly, with the rate of citric use/diacetyl formation influenced by the particular bacterial strain (with most strains of "O. oeni" producing less diacetyl than "Lactobacillus" and "Pediococcis" species), as well as the redox potential of the wine. In wine conditions that have a low redox potential (meaning it is more oxidative such as in a barrel that is not fully topped up), more citric acid will be consumed and diacetyl formed. In more reductive conditions, such as in alcoholic fermentations where yeast populations are at their peak and the wine is heavily saturated with carbon dioxide, the formation of diacetyl is much slower. The yeasts also help keep levels low by consuming diacetyl and reducing it to acetoin and butylene glycol.