Popular Science Monthly/Volume 25/June 1884/The Chemistry of Cookery XIII
|THE CHEMISTRY OF COOKERY.|
THE changes which occur when starch-granules are subjected to the action of water, at a temperature of 140°, have been described. If the heat is raised to the boiling-point, and the boiling continues, the gelatinous mass becomes thicker and thicker; and if there are more than fifty parts of water to one of starch a separation takes place, the starch settling down with its fifty parts of water, the excess of water standing above it. Carefully-dried starch may be heated to above 300° without becoming soluble, but at 400° a remarkable change commences. The same occurs to ordinary commercial starch at 820°, the difference evidently depending on the water retained by it. If the heat is continued a little beyond this it is converted into dextrin, otherwise named "British gum," "gommeline," "starch-gum," and "Alsace gum," from its resemblance to gum-arabic, for which it is now very extensively substituted. Solutions of this in bottles are sold in the stationers' shops under various names for desk uses.
The remarkable feature of this conversion of starch into dextrin is that it is accompanied by no change of chemical composition. Starch is composed of six equivalents of carbon, ten of hydrogen, and five of oxygen—C6H10O5, i. e., six of carbon and five of water or its elements. Dextrin has exactly the same composition; so also has gum-arabic when purified. But their properties differ considerably. Starch, as everybody knows, when dried, is white, and opaque and pulverent; dextrin, similarly dried, is transparent and brittle; gum-arabic the same. If a piece of starch, or a solution of starch, is touched by a solution of iodine, it becomes blue almost to blackness, if the solution is strong; no such change occurs when the iodine solution is added to dextrin or gum. A solution of dextrin when mixed with potash changes to a rich blue color when a little sulphate of per is added; no such effect is produced by gum-arabic, and thus we have an easy test for distinguishing between true and fictitious gum-arabic.
The technical name for describing this persistence of composition with changes of properties is isomerism and bodies thus related are said to be isomeric with each other. Another distinguishing characteristic of dextrin is that it produces a right-handed rotation on a ray of polarized light—hence its name, from dexter, the right.
The conversion of starch into dextrin is a very important element of the subject of vegetable cooking, inasmuch as starch-food can not be assimilated until this conversion has taken place, either before or after we eat it. I will therefore describe other methods by which this change may be effected.
If starch be boiled in a dilute solution of almost any acid, it is converted into dextrin. A solution containing less than one per cent of sulphuric or nitric acid is sufficiently strong for this purpose. One method of commercial manufacture (Payen's) is to moisten ten parts of starch with three of water, containing 150 of its weight of nitric acid, spreading the paste upon shelves, allowing it to dry in the air, and then heating it for an hour and a half at about 240° Fahr.
But the most remarkable and interesting agent in effecting this conversion is diastase. It is one of those mysterious compounds which have received the general name of "ferments." They are disturbers of chemical peace, molecular agitators that initiate chemical revolutions, which may be beneficent or very mischievous. The morbific matter of contagious diseases, the venom of snake-bite, and a multitude of other poisons, are ferments. Yeast is a familiar example of a ferment, and one that is the best understood. I must not be tempted into a dissertation on this subject, but may merely remark that modern research indicates that many of these ferments are microscopic creatures, linking the vegetable with the animal world; they may be described as living things, seeing that they grow from germs and generate other germs that produce their like. Where this is proved, we can understand how a minute germ may, by falling upon suitable nourishment, increase and multiply, and thus effect upon large quantities of matter the chemical revolution above named.
I have already described the action of rennet upon milk, and the very small quantity which produces coagulation. There appears to be no intercession of living microbia in this case, nor have any been yet demonstrated to constitute the ferment of diastase, though they may be suspected. Be this as it may, diastase is a most beneficent ferment. It communicates to the infant plant its first breath of active life, and operates in the very first stage of animal digestion.
In a grain of wheat, for example, the embryo is surrounded with its first food. While the seed remains dry above-ground there is no assimilation of the insoluble starch or gluten, no growth, nor other sign of life. But when the seed is moistened and warmed, the starch is changed to dextrin by the action of diastase, and the dextrin is further converted into sugar. The food of the germ thus gradually rendered soluble penetrates its tissues; it is thereby fed and grows, unfolds its first leaf upward, throws downward its first rootlet, still feeding on the converted starch until it has developed the organs by which it can feed on the carbonic acid of the air and the soluble minerals of the soil. But for the original insolubility of the starch it would be washed away into the soil, and wasted ere the germ could absorb it. The maltster, by artificial heat and moisture, hastens this formation of dextrin and sugar; then by a roasting heat kills the baby plant just as it is breaking through the seed-sheath. Blue-ribbon orators miss a point in failing to notice this. It would be quite in their line to denounce with scathing eloquence such heartless infanticide.
Diastase may be obtained by simply grinding freshly germinated barley or malt, moistening it with half its weight of warm water, allowing it to stand, and then pressing out the liquid. One part of diastase is sufficient to convert two thousand parts of starch into dextrin, and from dextrin to sugar, if the action is continued. The most favorable temperature for this is from 140° to 150° Fahr. The action ceases if the temperature be raised to the boiling-point.
The starch which we take so abundantly as food appears to have no more food-value to us than to the vegetable germ until the conversion into dextrin or sugar is effected. From what I have already stated concerning the action of heat upon starch, it is evident that this conversion is more or less effected in some processes of cookery. In the baking of bread an incipient conversion probably occurs throughout the loaf, while in the crust it is carried so far as to completely change most of the starch into dextrin, and some into sugar. Those of us who can remember our bread-and-milk may not have forgotten the gummy character of the crust when soaked. This may be felt by simply moistening a piece of crust in hot water and rubbing it between the fingers. A certain degree of sweetness may also be detected, though disguised by the bitterness of the caramel, which is also there.
The final conversion of starch-food into dextrin and sugar is effected in the course of digestion, especially, as already stated, in the first stage—that of insalivation. Saliva contains a kind of diastase, which has received the name of salivary diastase and mucin. It does not appear to be exactly the same substance as vegetable diastase, though its action is similar. It is most abundantly secreted by herbivorous animals, especially by ruminating animals. Its comparative deficiency in carnivorous animals is shown by the fact that, if vegetable matter is mixed with their food, starch passes through them unaltered.
Some time is required for the conversion of the starch by this animal diastase, and in some animals there is a special laboratory or kitchen for effecting this preliminary cookery of vegetable food. Ruminating animals have a special stomach-cavity for this purpose in which the food, after mastication, is held for some time and kept warm before passing into the cavity which secretes the gastric juice. The crop of grain-eating birds appears to perform a similar function. It is there mixed with a secretion corresponding to saliva, and is thus partially malted—in this case before mastication in the gizzard.
At a later stage of digestion, the starch that has escaped conversion by the saliva is again subjected to the action of animal diastase contained in the pancreatic juice, which is very similar to saliva.
It is a fair inference from these facts that creatures like ourselves, who are not provided with a crop or compound stomach, and manifestly secrete less saliva than horses or other grain-munching animals, require some preliminary assistance when we adopt graminivorous habits; and one part of the business of cookery is to supply such preliminary treatment to the oats, barley, wheat, maize, peas, beans, etc., which we cultivate and use for food.
Having described the changes effected by heat upon starch, and referred to its further conversion into dextrin and sugar, I will now take some practical examples of the cookery of starch-foods, beginning with those which are composed of pure, or nearly pure, starch.
When arrowroot is merely stirred in cold water it sinks to the bottom undissolved and unaltered. When cooked in the usual manner to form the well-known mucilaginous or jelly-like food, the change is a simple case of the swelling and breaking up of the granules described as occurring in water at the temperature of 140° Fahr. There appears to be no reason for limiting the temperature, as the same action takes place from 140° upward, to the boiling-point of water.
I may here mention a peculiarity of another form of nearly pure starch-food, viz., tapioca, which is obtained by pulping and washing out the starch-granules of the root of the manihot, then heating the washed starch in pans and stirring it while hot with iron or wooden paddles. This cooks and breaks up the granules and agglutinates the starch into nodules which, as Mr. James Collins explains ("Journal of Society of Arts," March 14, 1884), are thereby coated with dextrin, to which gummy coating some of the peculiarities of tapioca-pudding are attributable. It is a curious fact that this manihot-root, from which our harmless tapioca is obtained, is terribly poisonous. The plant is one of the large family of nauseous spurgeworts (Euphorbiaceæ). The poison resides in the milky juice surrounding the starch-granules, but, being both soluble in water and volatile, most of it is washed away in separating the starch-granules, and any that remains after washing is driven off by the heating and stirring which has to reach 240°, in order to effect the changes above described.
I suspect that the difference between the forms of tapioca and arrowroot has arisen from the necessity of thus driving off the last traces of the poison with which the aboriginal manufacturers were so well acquainted as to combine the industry of poisoning their arrows with that of extracting the starch-food from the same root. No certificate from the public analyst is demanded to establish the absence of the poison from any given sample of tapioca, as the juice of the manihot-root, like that of other spurges, is unmistakably acrid and nauseous.
Sago, which is a starch obtained from the pith of the stem of the sago-palm and other plants, is prepared in grains like tapioca, with similar results. Both sago and tapioca contain a little gluten, and therefore have more food-value than arrowroot.
The most familiar of our starch-foods is the potato. I place it among the starch-foods, as, next to water, starch is its prevailing constituent, as the following statement of average compositions will show: Water, 75 per cent; starch, 18·8; nitrogenous materials, 2; sugar, 3; fat, 0·2; salts, 1. The salts vary considerably with the kind and age of the potato, from 0·8 to 1·3 in full grown. Young potatoes contain more. In boiling potatoes, the change effected appears to be simply a breaking up or bursting of the starch-granules, and a conversion of the nitrogenous gluten into a more soluble form, probably by a certain degree of hydration. As we all know, there are great differences among potatoes, some are waxy, others floury; and these, again, vary according to the manner and degree of cooking. I can not find any published account of the chemistry of these differences, and must, therefore, endeavor to explain them in my own way.
As an experiment, take two potatoes of the floury kind; boil or steam them together until they are just softened throughout, or, as we say, "well done." Now leave one of them in the saucepan or steamer, and very much overcook it. Its floury character will have disappeared, it will have become soft and gummy. The reader can explain this by simply remembering what has already been explained concerning the formation of dextrin. It is due to the conversion of some of the starch into dextrin. My explanation of the difference between the waxy and floury potato is that the latter is so constituted that all the starch-granules may be disintegrated by heat in the manner already described, before any considerable proportion of the starch is converted into dextrin, while the starch of the waxy potatoes for some reason, probably a larger supply of diastase, is so much more readily convertible into dextrin that a considerable proportion becomes gummy before the whole of the granules are broken up—i.e., before the potato is cooked or softened throughout.
I must here throw myself into the great controversy of jackets or no jackets. Should potatoes be peeled before cooking, or should they be boiled in their jackets? I say most decidedly in jackets, and will state my reasons. From 53 to 56 per cent of the above-stated saline constituents of the potato is potash, and potash is an important constituent of blood—so important that in Norway, where scurvy once prevailed very seriously, it has been banished since the introduction of the potato, and, according to Lang and other good authorities, it is owing to the use of this vegetable by a people who formerly were insufficiently supplied with saline vegetable food.
Potash salts are freely soluble in water, and I find that the water in which potatoes have been boiled contains potash, as may be proved by boiling it down to concentrate, then filtering and adding the usual potash test, platinum chloride.
It is evident that the skin of the potato must resist this passage of the potash into the water, though it may not fully prevent it. The bursting of the skin only occurs at quite the latter stage of the cookery. The greatest practical authorities on the potato. Irishmen, appear to be unanimous. I do not remember to have seen a pre-peeled potato in Ireland. I find that I can at once detect by the difference of flavor whether a potato has been boiled with or without its jacket, and this difference is evidently saline.
These considerations lead to another conclusion, viz., that baked potatoes, and fried potatoes, or potatoes cooked in such a manner so as to be eaten with their own broth, as in Irish stew (in which cases the previous peeling does no mischief), are preferable to boiled potatoes. Steamed potatoes probably lose less of their potash juices than when boiled; but this is uncertain, as the modicum of distilled water condensed upon the potato and continually renewed may wash away as much as the larger quantity of hard water in which the boiled potato is immersed.
Those who eat an abundance of fruit, of raw salads, and other vegetables supplying a sufficiency of potash to the blood, may peel and boil their potatoes; but the poor Irish peasant who depends upon the potato for all his sustenance requires that they shall supply him with potash.
When traveling in Ireland (I explored that country rather exhaustively when editing the fourth edition of "Murray's Hand-book"), I was surprised at the absence of fruit-trees in the small farms where one might expect them to abound. On speaking of this, the reason given was that all trees are the landlord's property; that if a tenant should plant them they would suggest luxury and prosperity, and therefore a rise of rent; or, otherwise stated, the tenant would be fined for thus improving the value of his holding. This was before the passing of the Land Act, which we may hope will put an end to such legalized brigandage. With the abolition of rack-renting, the Irish peasant may grow and eat fruit; may even taste jam without fear and trembling; may grow rhubarb and make pies and puddings in defiance of the agent. When this is the case, his craving for potato-potash will probably diminish, and his children may actually feed on bread.
As regards the nutritive value of the potato, it is well to understand that the common notion concerning its cheapness as an article of food is a fallacy. Taking Dr. Edward Smith's figures, 760 grains of carbon and 24 grains of nitrogen are contained in one pound of potatoes; two and one half pounds of potatoes are required to supply the amount of carbon contained in one pound of bread; and three and one half pounds of potatoes are necessary for supplying the nitrogen of one pound of bread. With bread at three halfpence per pound, potatoes should cost less than one halfpenny per pound, in order to be as cheap as bread for the hard-working man who requires an abundance of nitrogenous food.
My own observations in Ireland have fully convinced me of the wisdom of William Cobbett's denunciation of the potato as a staple article of food. The bulk that has to be eaten, and is eaten, in order to sustain life, converts the potato-feeder into a mere assimilating machine during a large part of the day, and renders him unfit for any kind of vigorous mental or bodily exertion. If I were the autocratic Czar of Ireland, my first step toward the regeneration of the Irish people would be the introduction, acclimatizing, and dissemination of the Colorado beetle, in order to produce a complete and permanent potato-famine. The effect of potato-feeding may be studied by watching the work of a potato-fed Irish mower or reaper who comes across to work upon an English farm where the harvest-men are fed in the farm-house and where beer is not excessive. The improvement of his working powers after two or three weeks of English feeding is comparable to that of a horse when fed upon corn, beans, and hay, after feeding for a year on grass only.
The reader may have observed that the starch-foods already described are all derived from the roots or stems of plants. Many others might be named that are used in tropical climates where little labor is demanded or done, and but little nitrogenous food required. Having treated the cookery of the chief constituents of these parts of the plant, the fiber and the starch, I now come to food obtained from the seeds and the leaves.
Taking the seeds first, as the more important, it becomes necessary to describe the nitrogenous constituents which are more abundant in them than in any other part of the plant, though they also contain the starch and cell material, or woody fiber, as already stated.
In No. 29 of this series, page 65, I described a method of separating starch from flour by washing a piece of dough in water, and thereby removing the starch-granules, which fall to the bottom of the water. If this washing is continued until no further milkiness of the water is produced, the piece of dough will be much reduced in dimensions, and changed into a gray, tough, elastic, and viscous or glutinous substance, which has been compared to bird-lime, and has received the appropriate name of gluten. When dried, it becomes a hard, horny, transparent mass. It is insoluble in cold water, and partly soluble in hot water. It is soluble in strong vinegar, and in weak solutions of potash or soda. If the alkaline solution is neutralized by an acid, the gluten is precipitated.
If crude gluten obtained as above is subjected to the action of hot alcohol it is separated into two distinct substances, one soluble and the other insoluble. As the solution cools, a further separation takes place of a substance soluble in hot alcohol, but not in cold, and another soluble in either hot or cold alcohol. The first—viz., that insoluble in either hot or cold alcohol—has been named gluten-fibrin; that soluble in hot alcohol, but not in cold, gluten-casein; and that soluble in either hot or cold alcohol, gluten. I give these names and explain them, as my readers may be otherwise puzzled by meeting them in books where they are used without explanation, especially as there is another substance, presently to be described, to which the name of vegetable casein has also been applied. The gluten-fibrin is supposed to correspond with blood-fibrin, gluten-casein with animal casein, and gluten with albumen.—Knowledge.