Chapter IX

ORGANIC ACIDS, ACID SALTS, AND ESTERS

Organic acids, either in free form, or partially neutralized with calcium, potassium, or sodium, forming acid salts, or combined with various alcohols in the form of esters, are widely distributed in plants. They occur in largest proportions in the fleshy tissues of fruits and vegetables, where they are largely responsible for the flavors which make these products attractive as food for men and animals. But organic acids and their salts are also found in the sap of all plants, and undoubtedly play an important and definite part in the vital processes of metabolism and growth.

CHEMICAL CONSTITUTION

All organic acids contain one (or more) of the characteristic acid group, —COOH, or , known as "carboxyl." This group is monovalent, and in the simplest organic acid, formic acid (H₂CO₂), it is attached to a single hydrogen atom, thus, H·COOH. In all other monobasic acids, it is attached to some other monovalent group, usually an alkyl radical, i.e., a radical derived from an alcohol and containing only carbon and hydrogen (as methyl, CH₃, ethyl, C₂H₅, butyl, C₄H₉, acryl, C₂H₃, etc.). Hence, the general formula for all monobasic organic acids is R·COOH, the R representing any monovalent radical. In the simplest dibasic acid, oxalic (H₂C₂O₄), two carboxyl groups are united to each other, thus, HOOC·COOH; but in the higher members of the series, the two characteristic acid groups are united through one or more —CH₂— groups, or their oxy-derivatives (as HOOC·CH₂·COOH, malonic acid; HOOC·CH₂·CH₂·CH₂·COOH, glutaric acid; HOOC·CHOH·CH₂·COOH, malic acid, etc.). Polybasic acids, containing three or more carboxyl groups, linked together through one or more alkyl carbon atoms, are also possible, and a few typical ones (as

are found in fruits and other plant tissues.

The H atom of the COOH group may be replaced by metals, in exactly the same way as it is replaceable in inorganic acids, producing either neutral or acid salts, depending upon whether all or only a part of the acid H atoms are replaced by the basic element.

Similarly, the acid H atom of either an organic or an inorganic acid may be replaced by the alkyl group of an alcohol, producing "ethereal salts," or "esters."

Thus, with nitric acid;

NO₂OH (HNO₃) + C₂H₅OH = NO₂OC₂H₅ (C₂H₅NO₃) + H₂O

Nitric acid Ethyl alcohol Ethyl nitrate

And, with acetic acid;

CH₃·COOH (H₄C₂O₂) + C₂H₅OH = CH₃·COOC₂H₅ + H₂O

Acetic acid Ethyl acetate

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.

Citric Acid, , occurs in large proportions in lemons, and associated with malic acid in strawberries, cherries, currants, etc. It is also found in small quantities in the seeds of the common leguminous vegetables, beans, peas, etc.

Tannic acid occurs widely distributed in the plant kingdom as a constituent of the special type of glucosides known as tannins, whose properties and functions have already been discussed (see Chapter VII).

PHYSIOLOGICAL USES OF ORGANIC ACIDS

No conclusive evidence concerning the rôle of organic acids in plant, or animal, growth, has yet been produced. There can be no doubt that the hypothetical carbonic acid and its acid and normal salts have a significant effect in regulating the acidity or alkalinity of plant juices, or body fluids, and so determining the nature of the enzymic activities and colloidal conditions of the biological systems (see Chapters XIV and XV). It is probable that other organic acids, such as formic, acetic, oxalic, and succinic acids, in plants and sarco-lactic acid, in animal tissues, perform similar regulatory rôles; but there seems as yet to be no indication as to why different acids should be used for this purpose by different species, or organisms; or as to the methods by which they perform their specific functions, whatever these may be.

In plants, the organic acids are usually in solution in the sap. When the plant ripens, they generally disappear, either being neutralized by calcium, or other bases, and deposited as crystals in the leaves or stems, or else used up in the synthesis of other organic compounds. Small proportions of these acids are usually present in mature seeds, and the percentage increases materially during germination, indicating that they play an important rôle in insuring the proper conditions for the conversion of the reserve food of the seed into soluble materials available for the nutrition of the young growing plant.

BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC.

The occurrence of organic acids, or their derivatives, which have pronounced odors or flavors, in the flesh surrounding the seeds of fruits, in the endosperm of vegetable seeds, or in the tubers, etc., of perennial plants, thus making them attractive as food for animals and men, undoubtedly serves to insure a wider distribution of the reproductive organs of these plants; a fact which has unquestionably had a marked influence upon the survival of species in the competitive struggle for existence during past eras and in the development and cultivation of different species by man. Indirect evidence that the proportion of these attractive compounds present in certain species may have been considerably increased by the processes of "natural selection" in the past is furnished by the many successful attempts to increase the percentage of such desirable constituents in fruits or vegetables by means of artificial selection of parent stocks by skillful plant breeders.