There are some other forms of organic acids which do not belong in the fatty series; that is, they do not contain the same general proportions of carbon and hydrogen. One of these is oxalic acid (C2H2O4) which is found in certain plants, such as sorrel, and is an active poison. Oxalic acid is used in the household for taking iron-rust out of cloth.
Lactic, malic and tartaric acids
Lactic acid (C3H6O3) is the acid of sour milk. Malic acid (C4H6O5) is found in many fruits such as apples, apricots, currants, pears, plums, prunes, etc. Tartaric acid (C4H6O6) is found principally in grapes. It is one of the constituent elements in the sediment found in wine casks, and is the active principle in cream of tartar. The latter is a potassium salt of tartaric acid.
Citric acid
Citric acid (C6H8O7) is one of the most important of the organic acids from the standpoint of the food chemist. It is the active principle of citrus-fruits, such as grapefruit, lemons, limes, oranges, etc. Lemons contain as high as five per cent of this acid. Citric acid is often used to make lemonade, and if pure citric acid is used, the manufactured product is equal to the original, except from a sentimental standpoint of having the genuine. The danger is, as in the case of adulterated vinegar, that the manufacturer may be tempted to use cheaper mineral acids instead of citric acid.
The other above-named groups of organic compounds which are formed from the three elements carbon, hydrogen, and oxygen—(f) carbohydrates and (g) fats—are very important to the food chemist. These will be considered in detail in Lesson IV. See pages [107]-[125].
ORGANIC NITROGENOUS COMPOUNDS
If to the three elements carbon, hydrogen, and oxygen, the element nitrogen is added, it still further increases the number of possible compounds that may be formed upon the base of the wonderful carbon atom. With this additional nitrogen factor, a new and a distinct quality is obtained.
The elements that make life possible
The chief characteristic of the element nitrogen is the ease with which its compounds change their chemical form. To quote the chemist, "the compounds of nitrogen are very unstable." Nearly all explosives are nitrogenous compounds. When this element, nitrogen, is combined with the wonderful variety of compounds formed by carbon, we have not only a great many intimately related yet distinct substances, but compounds which readily change from one form to another. These are the distinctive qualities or conditions necessary, from a chemical standpoint, to make the processes of life possible. Protoplasm, which is the basis of all life, is formed by an intimate mixture of a number of complex chemical compounds, the chief elements of which are carbon, hydrogen, oxygen, and nitrogen.