<P> Malic acid can be further reduced during the winemaking process through malolactic fermentation or MLF . In this process, bacteria convert the stronger malic acid into the softer lactic acid; formally, malic acid is polyprotic (contributes multiple protons, here two), while lactic acid is monoprotic (contributes one proton), and thus has only half the effect on acidity (pH); also, the first acidity constant (pK) of malic acid (3.4 at room temperature) is lower than the (single) acidity constant of lactic acid (3.86 at room temperature), indicating stronger acidity . Thus after MLF, wine has a higher pH (less acidic), and a different mouthfeel . </P> <P> The bacteria behind this process can be found naturally in the winery, in cooperages, which make oak wine barrels that will house a population of the bacteria or they can be introduced by the winemaker with a cultured specimen . For some wines, the conversion of malic into lactic acid can be beneficial, especially if the wine has excessive levels of malic acid . For other wines, such as Chenin blanc and Riesling, it produces off flavors in the wine (such as the buttery smell of diacetyl) that would not be appealing for that variety . In general, red wines are more often put through MLF than whites, which means a higher likelihood of finding malic acid in white wines (though notable exceptions, such as oaked Chardonnay, are often put through MLF). </P> <P> A much milder acid than tartaric and malic, lactic acid is often associated with "milky" flavors in wine and is the primary acid of yogurt and sauerkraut . It is produced during winemaking by lactic acid bacteria (LAB), which includes three genera: Oenococcus, Pediococcus and Lactobacillus . These bacteria convert both sugar and malic acid into lactic acid, the latter through MLF . This process can be beneficial for some wines, adding complexity and softening the harshness of malic acidity, but it can generate off flavors and turbidity in others . Some strains of LAB can produce biogenic amines, such as histamine, tyramine and putrescine, which may be a cause of red wine headaches in some wine drinkers . Winemakers wishing to control or prevent MLF can use sulfur dioxide to stun the bacteria . Racking the wine quickly off its lees will also help control the bacteria, since lees are a vital food source for them . The winemakers must also be very careful of what wine barrels and winemaking equipment to which the wine is exposed, because of the bacteria's ability to deeply embed themselves within wood fibers . A wine barrel that has completed one successful malolactic fermentation will almost always induce MLF in every wine stored in it from then on . </P> <P> While very common in citrus fruits, such as limes, citric acid is found only in very minute quantities in wine grapes . It often has a concentration about 1 / 20 that of tartaric acid . The citric acid most commonly found in wine is commercially produced acid supplements derived from fermenting sucrose solutions . These inexpensive supplements can be used by winemakers in acidification to boost the wine's total acidity . It is used less frequently than tartaric and malic due to the aggressive citric flavors it can add to the wine . When citric acid is added, it is always done after primary alcohol fermentation has been completed due to the tendency of yeast to convert citric into acetic acid . In the European Union, use of citric acid for acidification is prohibited, but limited use of citric acid is permitted for removing excess iron and copper from the wine if potassium ferrocyanide is not available . </P>

When do you add citric acid to wine