With dibasic or polybasic acids, either one or more of the carboxyl H atoms may be replaced with an alcohol radical, so that both acid and neutral esters of all such acids are possible. Examples of all of these different types of derivatives of organic acids are frequently found in plant tissues.
The occurrence, properties, and functions of a particular type of glycerol, and other esters of organic acids, which are known as fats and waxes, are not taken into consideration in the following discussions, but reserved for a subsequent chapter dealing specially with them.
SOME COMMON ORGANIC ACIDS
Free organic acids, or their mineral salts or volatile esters, sometimes occur as separate and characteristic individual compounds in particular species of plants, or fruits; but much more commonly, two, three, or even more acids or their derivatives, are associated together.
Formic acid, H·COOH (H2CO2), occurs in free form and in considerable proportions in the leaves of several species of nettle, where it is responsible for the unpleasant effects of the "sting." It may be detected in small amounts in the vegetative parts of many, if not all, plants, especially during periods of rapid growth, and is probably one of the intermediate products in the photosynthesis of carbohydrates (see [Chapter III]).
Higher members of the formic acid series (as acetic, CH3·COOH; propionic, C2H5·COOH; butyric, C3H7·COOH; etc.) are often found in small quantities in the leaves of many plants and seem to be characteristically present in certain species. They are easily produced from carbohydrates by bacterial action and, hence, are always present in fermenting tissues, such as silage, sauerkraut, etc. Furthermore, the glycerol esters of higher members of this and other monobasic acid series are constituents of all natural fats and oils (see [Chapter X]).
Oxalic acid, HOOC·COOH (H2C2O4), is found in small amounts in nearly all plants and in relatively large proportions in those of Oxalis, rhubarb, etc. It occurs both as the free acid and as neutral, or acid, oxalates of calcium, potassium, and, perhaps, of magnesium and sodium. Solid crystals of insoluble calcium oxalate are often found in plant cells, and it has been shown that when so deposited the calcium cannot become again available for metabolic uses. It is stated, further, that such crystals form only when calcium is in excess in the plant sap; hence, the deposition of crystallized calcium oxalate seems to be a device for the avoidance of excessive calcium rather than excessive oxalic acid, in the plant juices.
Succinic acid, HOOC·CH2·CH2·COOH (H6C4O4), occurs in many fruits and vegetables, and is also found in some animal tissues. In fruits, it is usually associated with its derivatives, malic and tartaric acids.
Malic acid, HOOC·CH2·CHOH·COOH (H6C4O5), occurs in apples and in many small fruits, and in many vegetables. Acid calcium malate is now produced commercially as a by-product from the manufacture of syrups from fruit juices, and is used as a substitute for "cream of tartar" in the manufacture of baking powders.
Tartaric acid, HOOC·CHOH·CHOH·COOH (H6C4O6), is found in many fruits, but most characteristically in the grape, where it occurs as the mono-potassium salt. During the fermentation of grape juice into wine, this salt is deposited in considerable quantities in the bottom of the wine-casks. This crude product is collected and sold under the name "argols." From these argols, pure acid potassium tartrate is obtained by decolorization and recrystallization, and constitutes the "cream of tartar" of commerce.