Popular Science Monthly/Volume 21/August 1882/The Chemistry of Sugar
|THE CHEMISTRY OF SUGAR.|
By Professor HARVEY W. WILEY.
THE annual consumption of sugar by the people of the United States amounts to more than forty pounds per capitum. This gives as a total the enormous quantity of two billion pounds per annum. The cost of this commodity may be safely placed at eight cents a pound. The total value of the sugar consumed each year, therefore, is one hundred and sixty million dollars. Sugar is a theme of general and pecuniary interest, which is a sufficient excuse for an article on its chemistry.
Sugar is a general name applied to a class of bodies composed of carbon, oxygen, and hydrogen, having a more or less sweet taste, and exercising a rotatory power on the plane of polarized light.
In chemical composition the sugars may be regarded as a combination of water with carbon, and they belong therefore to that class of bodies which are known as carbo-hydrates. Starch, wood-fiber, and various sorts of gums are bodies nearly allied in chemical composition to sugar.
Sugar is chiefly a product of vegetable growth, and is found in some part or other of a large number of substances.
Sometimes it is found in the root, as in the beet and sweet-potato. Again, it occurs in the fruit, as in the grape and water-melon. At other times it is stored in the juices of the plant, as in the maple-tree and the sugar-cane.
In whatever position it occurs, it is always diluted with water, and mixed with various gums and albuminous bodies peculiar to the plant containing it.
The manufacture and refining of sugar consist in separating it from these impurities, and evaporating the water until the crystallizing point is reached, or a sirup is produced.
I have said that sugar is of vegetable origin. This must be construed to mean the sugars of commerce and common consumption. The animal organism possesses a glycogenic function in common with plants.
The amount of sugar, however, produced by the animal organism, with the exception of that from the milk-glands, is inconsiderable in a state of health. In certain forms of disease, however, as in diabetes mellitus, the amount of sugar produced in the body may be immensely increased.
Sugar may also be made by chemical means from the bodies already mentioned, such as starch, cellulose, gum, etc.
In this sketch I will mention only the more important sugars. For the purposes of a popular classification the sugars may be arranged as follows:
1. Cane-sugar, or sucrose.
2. Grape-sugar, or glucose.
3. Milk-sugar, or lactose.
4. Starch-sugar, or amylose.
Cane-sugar, in a commercial sense, is by far the most important of these bodies.
It has never been formed by chemical synthesis, and the chief sources from which it is derived are the sugar-cane, the sugar-beet, and the sugar-maple. When pure it is a white crystalline body easily soluble in water and having an intensely sweet taste. The molecule of cane-sugar consists of forty-five atoms, distributed as follows, viz., twelve atoms of carbon, twenty-two of hydrogen, and eleven of oxygen. By the hundred this is equal to 42·11 per cent of carbon; 51·46 per cent oxygen, and 6·43 per cent hydrogen. Its expression by symbols is C12H22O11. The amounts of carbon and hydrogen in cane-sugar are determined by igniting it with oxide of copper or other substance rich in oxygen. By this process the carbon is converted into carbon dioxide (carbonic acid), and the hydrogen into water, each of which substances is easily collected and weighed.
Indeed, this process is used for estimating carbon and hydrogen in all sugars. The oxygen is usually estimated by difference.
It has thus been shown that oxygen and hydrogen exist in sugar in the exact proportions necessary to form water. A materialistic definition of sugar would be forty parts of fine charcoal mixed with fifty-eight parts of pure water. Yet charcoal and water mixed in the above proportions are far from being sweet; a good illustration of the difference between chemical union and mechanical mixture. Pure cane sugar, when left to itself, has no tendency to change. When diluted with water, however, and brought in contact with a nitrogenous body it undergoes fermentation, and yields at first alcohol, carbonic dioxide, and other products, as will be shown subsequently.
Pure cane-sugar has the power of twisting the plane of polarized light to the right. For the purple ray or transition tint this torsion amounts to 73·8°, and for the monochromatic sodium-flame to 66·67°. These numbers represent its specific rotatory power.
I will give an explanation of what these terms and numbers signify further on. From this property the chemist is able to determine the amount of pure sugar in any sample submitted to him for examination, and containing no other optically active body. For, if, using the sodium-flame, he should find the rotation to be 33·34°, he would know at once that the sample contained only fifty per cent of sugar, and so on for other numbers. When cane-sugar is heated with an acid, or for a long time to a high temperature without one, it suffers a peculiar change which is called inversion. Inverted sugar has almost lost its power of crystallization, and has changed its deportment toward polarized light. It consists now of two distinct kinds of sugar, one of which turns the plane of polarized light to the left and is called lævulose, while the other turns it to the right and is called dextrose.
At ordinary temperatures, however, the lævo-rotatory power of inverted sugar is much greater than the other. This preponderance of lævo-rotatory power increases as the temperature falls, and diminishes as it rises. At 88° Cent., these two powers are equal, and the sugar exerts no influence whatever on polarized light.
These twin constituents of inverted sugar can be separated with lime, which forms with the lævulose a compound less soluble than with the dextrose, and from which the latter is separated by pressure. The lime compound is then decomposed by oxalic acid and the lævulose set free.
Honey is composed of left-handed or invert-sugar, some grape sugar, and more or less cane-sugar. After keeping for some time the cane-sugar is all converted into the invert variety. When cane-sugar is heated to a temperature of about 200° C, it undergoes a transformation, and a part of it is changed into an aromatic substance called caramel. This change consists essentially in the loss of two molecules of water. Caramel is used chiefly in the manufacture of candies, for flavoring: whiskies, brandies, etc.
A pleasing and instructive experiment which any one can try will show that sugar is made of charcoal and water. A strong solution of sugar is placed in a glass until the bottom is well covered. Strong sulphuric acid (oil of vitriol) is now added, and the whole stirred with a glass rod until it swells up and turns black. The sulphuric acid has a very strong liking for water, which it steals from the sugar molecules, leaving only the carbon.
Another interesting experiment is to burn the carbon in sugar with the oxygen in the chlorate of potassium, the lire being kindled with a drop of sulphuric acid. For this experiment four parts of sugar are carefully mixed in an old saucer with five parts of chlorate of potassium, and the mass then touched with a glass rod which has been dipped into strong sulphuric acid. The chemical action produced by the sulphuric acid makes heat enough to ignite the whole mass, and the carbon of the sugar is thus burned out.
Grape-sugar, as its name implies, is found in grapes and some other substances. This name is also sometimes incorrectly given to one, and often all the sugars made from starch or derived from fruits. It is dextro-gyratory, having for the sodium ray a specific rotatory power of about 52·5°. It is fermentable, and is not changed by heating with dilute acids. It crystallizes, but with less facility than cane-sugar, and is much less sweet to the taste. There are other varieties of sugar which possess the same properties as grape-sugar. To these the general name, dextrose or glucose, has been given. Any sugar, whatever be its source, which, in a dry state, has the formula C6H12O6, and a specific rotatory power of 52·5° to the right, is entitled to the name dextrose.
Dextrose is also the final product of long-continued boiling of starch with an acid.
Starch-sugar, or amylose, is a mixture of various products, chief among which are dextrose, dextrine, and maltose.
For a full discussion of this sugar I refer to my paper in this magazine for June, 1881.
Milk-sugar, or lactose, is found in milk, and is not important commercially. It is used mostly as a vehicle for administering-medicines. In composition it is identical with cane-sugar, but differs from it greatly in both chemical and physical properties.
Optically it is nearly related to dextrose, its specific rotatory power being only a little greater. It is much less soluble and much less sweet than cane-sugar.
Both lactose and dextrose, where freshly dissolved from the crystalline state, have a rotatory power nearly double the normal. This peculiarity is called "birotation." Milk-sugar ferments when mixed with yeast, but not so readily as grape-sugar or dextrose. The fermented milk forms a mild alcoholic beverage much prized in some countries.
Most sugars readily combine with lime, and with the alkalies, and also with many of the ordinary salts. Cane-sugar especially combines easily with bases almost like an acid, forming salts which are called sucrates.
Many metallic compounds help the crystallization of the sugars, and such salts have been used in the refining of sugar for this purpose. Owing to the difficulty, however, of removing these compounds completely, the practice has been generally abandoned.
The action of sugars on copper compounds is of especial interest, because it is used as a means of estimating the quantity of sugar present in a substance.
Alkaline copper solutions, when heated with most sugars, have their copper reduced to the form of a suboxide (Cn2O). Of the sugars which act in this way, I may mention grape-sugar, lactose, dextrose, and maltose. Pure cane-sugar does not act upon copper solutions until after it has been converted into invert-sugar. Dextrine or starch gum is likewise inactive. The copper solution generally employed for the estimation of sugar contains the copper in the form of a tartrate, with some sulphate of sodium and an excess of sodium hydrate in the mixture. It is called "Fehling's solution."
The specific rotatory power of a sugar is its property of twisting the plane of polarized light either to the right or left. The instrument used to determine this is called a polariscope, or saccharimeter. The instrument in more common use has an ordinary oil or gas lamp as the source of light. By quartz plates this light is modified in character so as to produce a tint most sensible to change. This is called the transition tint, or teinte de passage. It is a purplish color, which on the one side changes to blue, and on the other to a rose-red. In the last few years instruments using a monochromatic light are coming into use, and they have some advantages over the other kind. The one-color light is produced by passing the rays from a sodium-flame through a crystal of bichromate of potassium, by which a pure yellow is obtained.
The field of view in these instruments has only half its area filled by a quartz plate. When the instrument is adjusted to zero the quartz semi-disk offers no opposition to the passage of the light. Interposing, however, a tube containing a sugar solution, one half the field is darkened. The analyzer is then turned until the field is equally illuminated again, and the angle through which it has been moved is read on a divided circle and vernier.
Since the rotation is less for the sodium-ray than for the transition tint, the two are distinguished by different symbols. For the former the symbol [a]D is used, and for the latter [a] or [a]j. The one-color saccharimeter is especially to be recommended for those who may be subject to any degree of color-blindness. A discussion of the optical principles involved in circular polarization would be out of place here.
Fermentation is a peculiar decomposition which sugars suffer under the influence of a nitrogeneous germ called the "ferment."
Cane-sugar, under the influence of a mucous sporule, undergoes "mucous fermentation," and is converted into a gum and a kind of sugar called "mannite." Neither acid nor alcohol is produced by this process.
Lactic fermentation takes place under the influence of an organism called penicilium glaucum. The chief product of this fermentative is lactic acid. In the case of a dextrose it may be represented by the following equation:
Milk-sugar undergoes this fermentation most readily, first absorbing a molecule of water and then breaking up into four molecules of lactic acid. If the process is allowed to go on, the lactic acid will break up into butyric acid, carbonic dioxide, and hydrogen. If the Torula aceti take the place of the germ named above, cane-sugar especially will yield acetic instead of lactic acid.
The vinous is by far the most important of the fermentations to which sugars are subjected.
Ordinary yeast is the nitrogenous body which seems best suited to develop this change.
Cane-sugar, before undergoing vinous fermentation, absorbs a molecule of water and is changed by an active principle of the yeast into invert-sugar. The chief products of vinous fermentation are alcohol and carbonic dioxide. Less important products are succinic acid, glycerine, cellulose, and fat.
All the sugar, with the exception of about four per cent., is converted into the two products first named. By an equation, the process may be represented as follows:
|C12H22O11 H2O 4C2H6O 4CO2.|
The peculiar fungus which is most active in the vinous fermentation is saccharomyces cerevisiæ; but there is much about the process which is yet obscure.
In the conversion of starch into sugar by diastase or acids, and the conversion of sugar into alcohol by fermentation, we have the rationale of that vast industry carried on by distillers and brewers. If the process of vinous fermentation is continued, the alcohol is converted chiefly into acetic acid (vinegar).
Decolorization.—Even a brief account of the chemistry of sugar would not be complete without an allusion to the methods employed to remove the coloring-matters naturally present in all sugars.
Sulphurous acid is sometimes employed for bleaching, but the carbon obtained by heating blood, bones, and other animal substances in closed retorts is by far the most efficacious means of decolorizing known. The pure white sugars and light-colored sirups of commerce are decolorized with this animal char. We may say that this decolorization is effected by oxidation of the coloring-matter, and yet the phenomenon does not appear to be wholly one of oxidation. Like the process of fermentation, it is difficult to explain it in full.
The solutions of sugar of a proper degree of dilution are passed, often under pressure, through successive filters of animal charcoal until their color is fully discharged.
Since a high temperature tends both to render sugar of a deeper color, and if it be sucrose to invert it, the evaporation of the sirup is carried on in vacuum-pans, whereby it is effected much more rapidly and without impairing the power of crystallization in the finished product.
From the multitude of facts connected with the chemistry of sugar I have endeavored to select those which are most important and of most interest to the general reader. Every intelligent man can not be a specialist in every department of science, but he can easily acquire a general idea of the progress of science. It is certainly a part of a liberal education to know something of the chemistry of common things.
This country ought to make its own sugar. The sugar-fields of Louisiana, with wiser management and a more scientific agriculture, could be made to increase their yield tenfold. Along the more northern parts of the Union the climate and soil are well adapted to the culture of the sugar-beet. We should not be discouraged because a few attempts in this direction have not proved financially successful. Twenty-five years of failure in Europe have been followed by fifty years of success, until at the present time two fifths of all the cane sugar produced m the world are obtained from the sugar-beet. Last of all, the great Indian-corn-producing area of the country is peculiarly suited to the growth of the sorghum sugar-cane, and the production of crystallized sugar from this source is no longer a mere possibility. It has already been realized. Land which will produce forty bushels of corn per acre will yield from six hundred to a thousand pounds of sugar, and nearly one hundred gallons of sirup. In another decade, instead of having to import eleven twelfths of the sugar we consume, as we do now, we may hope to produce it all.