Other substances besides those of a carbohydrate character are included in the nitrogen-free extract. Chief among these are the organic acids, compounds which are found mostly in the fruits, although they appear in certain fermented products, such as sauerkraut and sour milk. The most important and well known of these are acetic acid, found in vinegar, citric acid in lemons, lactic acid in sour milk, malic acid in many fruits, such as currants and apples, and oxalic acid in rhubarb. Probably these acids are sometimes free, but the trend of opinion is that generally they are united with potassium or some other base, forming an acid salt. Excepting the fruits, only fermented foods contain acids to an appreciable extent. When milk sours, the sugar in it is changed to lactic acid under the influence of a ferment. In sauerkraut, various acids are formed at the expense of the carbohydrates that are in the material which is subjected to fermentation.