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Lactic acid is also the most common acidic ingredient of fermented milk items. It is used for tanning leather, dying wool, as a food preservative (by lowering pH), and for flavoring foods such as: It can be found in plant material, blood, muscle, and in soil. Special types of bacteria are used to create lactic acid when they are fed carbohydrates.

structural formula for lactic acid:

    H   H
     \ /
 H -- C        O -- H
       \      /
        C -- C == O
       / \    
 H -- O   H