Popular Science Monthly/Volume 25/September 1884/The Chemistry of Cookery XV
|THE CHEMISTRY OF COOKERY.|
RESPECTING the rationale of the change that takes place in reheating stale bread, thereby renewing it and making it appear moist by actually driving away some of its moisture, the results of my investigations are as follow:
I find that, as bread becomes stale, its porosity appears to increase, and that, when renewed by reheating, it returns to its original apparently smaller degree of porosity. That this change can be only apparent is evident from the facts that the total quantity of solid material in the loaf remains the same, and its total dimensions are retained more or less completely by the rigidity of the crust. I say "more or less," because this depends upon the thickness and hardness of the crust, and also upon the completeness of its surrounding. Lightly-baked loaves shrink a little in dimensions in becoming stale, and partly regain the loss on reheating, but this difference only exaggerates the apparent paradox of varying porosity, as the diminished bulk of a given quantity of material displays increased porosity, and the increase of total dimensions accompanies the diminished porosity.
A reconciliation of this paradox may be obtained by careful examination of the structure of the crumb. This will show that the larger or decidedly visible pores are cells having walls of somewhat silky appearance. This silky luster and structure is, I have no doubt, due to a varnish of dextrin, the gummy nature of which I have already described. Now look a little more closely at this inner surface of the big blow-holes with the aid of a hand-lens of moderate power. It is not a continuous varnish of gum, but a network or agglomeration of gummy fibers and particles, barely touching each other.
My theory of the change that takes place as the bread becomes stale is that these fibers and particles gradually approach each other either by shrinkage or adhesive attraction, and thus consolidate and harden the walls of each of the millions of visible pores, i.e., the solid material of which the loaf is made up. In doing so they naturally increase the dimensions of these visible pores, while the invisible interstices or spaces between the minute fibers of the cell-walls are diminished by the approximation or adhesion of these fibers to each other.
This adhesion is probably aided by an oozing out or efflorescence of the vapor held by the fibers, and its condensation on their surfaces. This point, be it understood, is merely hypothetical, as the efflorescence is not visible.
"When the stale bread is again heated, a general expansion occurs by the conversion of liquid water into aqueous vapor, every grain of water thus converted expanding to seventeen hundred times its former bulk. As this happens throughout, i.e., upon the surface of every one of the countless fibers or particles, there must be a general elbowing in the crowd, breaking up the recent adhesion between these fibers and drawing them all apart in the directions of least resistance, i.e., toward the open spaces of the larger and visible pores, producing that apparent diminution of porosity that I have observed as the visible characteristic of the change.
This explanation of the change may be further demonstrated by cutting a loaf through the middle from top to bottom, and exposing the cut surfaces. In this case the bread becomes unequally stale, more so near the cut surface than within. The unequal pull due to the greater adhesive approximation of the fibers and small particles causes a rupture of the exposed surface of the crumb, which becomes cracked or fissured without any perceptible alteration of the size of the visible pores. If the two broken faces be now accurately placed together, the halves thus closely joined, firmly tied together, and placed for an hour in the oven, it will be seen on separating them that the chasms are considerably closed, though not quite healed. Careful examination of the structure of the inside, by breaking out a portion of the crumb, will reveal that loosening of the structure which I have described.
I should add that, in quoting the figures given by Boussingault in my last, I inadvertently omitted to reduce them from the French to the English thermometric scale: 130° to 150° centigrade is equal to 266° to 302° Fahr., which is considerably below the temperature required for starting the original baking.
"Popped corn" is a peculiar example of starch-cookery. Here a certain degree of porosity is given to an originally close-compacted structure of starch by the simple operation of explosive violence due to the sudden conversion into vapor of the water naturally associated with the starch. The operation is too rapid for the production of much dextrin.
As most of my readers doubtless know, peas, beans, lentils, and other seeds of leguminous plants are more nutritious, theoretically, than the seeds of grasses, such as wheat, barley, oats, maize, etc. I was glad to see at the Health Exhibition a fine series of the South Kensington cases displaying in the simplest and most demonstrative manner the proximate analyses of the chief materials of animal and vegetable food. I refer to them now because they do not receive the attention they deserve. On the opening day there was, out of all the crowd, only one other besides myself bestowing any attention upon them. I soon learned in conversation with him that he is a reader of "Knowledge." These cases show one pound of wheat, oats, potatoes, peas, etc., etc., on trays; by the side of these are bottles, containing the quantity of water in the one pound, and other trays, with the other constituents of the same quantity, the starch, gluten, casein, the mineral matter, etc., thus displaying at a glance the nutritive value of each so far as chemical analysis can display it. Those Irishmen and others, who think I have been too hard upon the potato, will do well to take its nutritive measure thus, and compare it with that of other vegetable foods.
They will see that all the leguminous seeds, the ground-nuts, etc., have their nitrogenous constituents displayed under the name of "casein." The use of this term is rather confusing. In many modern books it does not appear at all in connection with the vegetable kingdom, but is replaced by "legumin." Liebig regarded this nitrogenous constituent of the leguminous seeds, almonds, etc., as identical with the casein of milk, and it was a pupil and friend of Liebig's—the late prince consort—who devised and originally supervised this graphic method of displaying the chemistry of food.
I will not here discuss the vexed question of whether the analyses of Liebig, identifying legumin with casein, or rather those of Dumas and Cahours, who state that the vegetable casein is not of the same composition as animal casein, are correct.
The following figures display my justification for thus lightly treating the discussion:
The first column shows the results of Dumas for animal casein; the second those of Dumas and Cahours for legumin; the third those of Jones for the same; and the fourth those of Rochleder; all as quoted by Lehmann. Here it will be seen that the differences upon which Dumas and Cahours base their supposed refutation of the identity of the animal with the vegetable principle are much smaller than the differences differences, I suspect, are all due to the difficulty of isolating the substances in question, especially of the vegetable substance, which is so intimately mixed with the starch, etc., in its natural condition that complete separation is of questionable possibility.the results of different analyses of the latter. These
This will be understood by the following description of the method of separation as given by Miller ("Elements of Chemistry," Vol. Ill): "Legumin is usually extracted from peas or from almonds, by digesting the pulp of the crushed seeds in warm water for two or three hours. The undissolved portion is strained off by means of linen, and the turbid liquid allowed to deposit the starch which it holds in suspension; it is then filtered and mixed with dilute acetic acid. A white flocculent precipitate is thus formed, which must be collected on a filter and washed."
This is but a mechanical process, and its liability to variation in result will be learned by anybody who will repeat it, or who has separated the gluten of flour by similar treatment.
Practically regarded in relation to our present subject, casein and legumin may be considered as the same. Their nutritive values are equal and exceptionally high, supposing they can be digested and assimilated. One is the most difficult of digestion of all the nutritive constituents of vegetable food, and the other enjoys the same distinction among those of animal food. Both primarily exist in a soluble form; both are rendered solid and insoluble in water by the action of acids; both are precipitated as a curd by rennet and both are rendered soluble after precipitation or are retained in their original soluble form by the action of alkalies. They nearly resemble in flavor, and John Chinaman makes actual cheese from peas and beans.
These facts, coupled with what I have already said concerning cheese and its cookery, will doubtless lead my readers to expect something concerning pease-pudding and potash in my next.
"Pease-padding hot, pease-pudding cold,
I leave to Mr. Clodd the historical problem of determining whether this notable couplet is of Semitic, Aryan, Neolithic, or Palæolithic origin. Regarded from my point of view it expresses a culinary and chemical principle of some importance, and indicates an ancient practice that is worthy of revival.
I have lately made some experiments on the ensilage of human food, whereby the cellular tissue of the vegetable may be gradually subjected to that breaking up of fiber described in No. 28. One of the curious achievements of chemical metamorphoses that is often quoted as a matter for wonderment is that of converting old rags into sugar by treating them with acid. The wonderment of this is diminished, and its interest increased, when we remember that the cellulose or woody fiber of which the rags are composed has the same composition as starch, and thus its conversion into sugar corresponds to the every-day proceedings described in No. 30. All that I have read and seen in connection with the recent ensilage experiments on cattle-fodder indicates that it is a process of slow vegetable cookery, a digesting or maceration of fibrous vegetables in their own juices which loosens the fiber, renders it softer and more digestible, and not only does this, but, to some extent, converts it into dextrin and sugar.
I hereby recommend those gentlemen who have ensilage-pits and are sufficiently enterprising to try bold experiments, to water the fodder, as it is being packed down, with dilute hydrochloric acid or acetic acid, which, if I am not deluded by plausible theory, will materially increase the sugar-forming action of the ensilage. The acid, if not over-supplied, will find ammonia and other bases with which to neutralize itself.
Such ensilage will correspond to that which occurs when we gather Jersey or other superlatively fine pears in autumn as soon as they are full grown. They are then hard, woody, and acid, quite unfit for food, but by simply storing them for a month, or two, or three, they become lusciously soft and sweet, the woody fibers are converted into sugar, the acid neutralized, and all this by simply fulfilling the conditions of ensilage, viz., close packing of the fiber, exclusion of air by the thick rind of the fruit, plus the other condition which I have just suggested, viz., the diffusion of acid among the well-packed fibers of the ensilage material.
In my experiments on the ensilage of human food I have encountered the same difficulty as that which has troubled graziers in their experiments, viz., that small-scale results do not fairly represent those obtained with large quantities. There is, besides this, another element of imperfection in my experiments respecting which I am bound to be candid to my readers, viz., that the idea of thus extending the principle was suggested in the course of writing this series, and, therefore, a sufficient time has not yet elapsed to enable me (with much other occupation) to do practical justice to the investigation.
I find that oatmeal-porridge is greatly improved by being made some days before it is required, then stored in a closed jar, brought forth and heated for use. The change effected is just that which theoretically may be expected, viz., a softening of the fibrous material, and a sweetening due to the formation of sugar. This sweetening I observed many years ago in some gruel that was partly eaten one night and left standing until next morning, when I thought it tasted sweeter, but, to be assured of this, I had it warmed again two nights afterward, so that it might be tasted under the same conditions of temperature, palate, etc., as at first. The sweetness was still more distinct, but the experiment was carried no further.
I have lately learned that my ensilage notion is not absolutely new. A friend who read my Cantor lectures tells me that he has long been accustomed to have seven dishes of porridge in his larder, corresponding to the days of the week, so that next Monday's breakfast was cooked the Monday before, and so on, each being warmed again on the day fixed for its final execution, and each being thus seven days old. He finds the result more digestible than newly-made porridge. The classical nine days' old pease-pudding is a similar anticipation, and I find, rather curiously, that nine days is about the limit to which it may be practically kept before mildew—moldiness—is sufficiently established to spoil the pudding. I have not yet tried a barrel full of pease-pudding or moistened pease-meal, closely covered and powerfully pressed down, but hope to do so.
Besides these we have a notable example of ensilage in sour-kraut—a foreign luxury that John Bull, with his usual blindness, denounces, as a matter of course. "Horrid stuff," "beastly mess," and such-like expressions, I hear whenever I name it to certain persons. Who are these persons? Simply Englishmen and Englishwomen who have never seen, never tasted, and know nothing whatever of what they denounce so violently, in spite of the fact that it is a staple article of food among millions of highly-intelligent people. Common sense (to say nothing of that highest result of true scientific training, the faculty of suspending judgment until the arrival of knowledge) should suggest that some degree of investigation should precede the denunciation.
In the cases of the sour-kraut and the ripening pear there is acid at work upon the fiber, which, as I have before stated, assists in the conversion of such indigestible fiber into soluble and digestible dextrin and sugar; but the demand for the solution of the vegetable casein or legumin, which has such high nutritive value and is so abundant in peas, etc., is of the opposite kind. Acids solidify and harden casein, alkalies soften and dissolve it. Therefore the chemical agent suggested as a suitable aid in the ensilage or slow cookery, or the boiling or rapid cookery, of leguminous food is such an alkali as may be wholesome and compatible with the demands for nutrition.
Now, the analyses of peas, beans, and lentils, etc., show a deficiency of potash salts as compared with the quantity of nitrogenous nutriment they contain; therefore I propose, as in the case of cheese-food, that we should add this potash in the convenient and safe form of bicarbonate, not merely add it to the water in which the vegetables may be boiled, and which water is thrown away (as in the common practice of adding soda when boiling greens), but add the potash to the actual pease-porridge, pease-pudding, lentil-soup, etc., and treat it as a part of the food as well as an adjunct to the cookery. This is especially required when we use dried peas, dried beans of any kind, such as haricots, dried lentils, etc.
I find that taking the ordinary yellow split-peas and boiling them in a weak solution of bicarbonate of potash for two or three hours, a partial solution of the casein is effected, producing pease-pudding, or pease-porridge, or puree (according to the quantity of water used), which is softer and more gelid than that which is obtained by similarly boiling without the potash. The undissolved portion evidently consists of the fibrous tissue of the peas, the gelatinous or dissolved portion being the starch, with more or less of casein. I say "more or less," because, at present, I have not been able to determine whether or not the casein is all rendered soluble. The flavor of the clear pea-soup, which I obtained by filtering through flannel, shows that some of the casein is dissolved; this is further demonstrated by adding an acid to the clear solution, which at once precipitates the dissolved casein. The filtered pea-soup sets to a stiff jelly on cooling, and promises to be a special food of some value, but, for the reasons above stated, I am not yet able to speak positively as to its practical value. The experience of any one person is not sufficient for this, the question being, not whether it contains nutritive material—this is unquestionable—but whether it is easily digested and assimilated. As we all know, a food of this kind may "agree" with some persons and not with others—i.e., it may be digested and assimilated with ease or with difficulty according to personal idiosyncrasies. The cheesy character of the abundant precipitate, which I obtain by acidulating this solution, is very interesting and instructive, regarded from a chemical point of view. The solubility of the casein is increased by soaking the peas for some hours, or, better still, a few days, in the solution of bicarbonate of potash.
Another question is opened by these experiments, viz.: What is the character and the value of the fibrous solid matter remaining behind after filtering out the clear pea-soup? Has the alkali acted in an opposite manner to the acid in the ripening pear? Is it merely a fibrous refuse only fit for pig-food, or is it deserving of further attention in the kitchen? Should it be treated with dilute acid—say a little vinegar—to break up the fiber, and thereby be made into good porridge? Other questions crop up here, as they have been cropping continually since I committed myself to the writing of these papers, and so abundantly that, if I could afford to set up a special laboratory, and endow it with a staff of assistants, there would be some years* work for myself and staff before I could answer them exhaustively, and doubtless the answers would suggest new questions, and so on ad infinitum. I state this in apology for the merely suggestive crudity of many of the ideas that I throw out in the course of these papers.
Before leaving the subject of peas, I must here repeat a practical suggestion that I published in "The Birmingham Journal" about twenty years ago, viz., that the water in which green peas are boiled should not be thrown away. It contains much of the saline constintuents of the peas, some soluble casein, and has a fine flavor, the very essence of the peas. If to this, as it comes from the saucepan, be added a little stock, or some Liebig's extract, a delicious soup is at once produced, requiring nothing more than ordinary seasoning. With care, it may form a clear soup such as just now is in fashion among the fastidious; but, prepared however roughly, it is a very economical, wholesome, and appetizing soup, and costs a minimum of trouble.
I must here add a few words in advocacy of the further adoption in this country of the French practice of using, as potage, the water in which vegetables generally (excepting potatoes) have been boiled. When we boil cabbages, turnips, carrots, etc., we dissolve out of them a very large proportion of their saline constituents—salts which are absolutely necessary for the maintenance of health; salts, without which we become victims of gout, rheumatism, lumbago, neuralgia, gravel, and all the ills that human flesh, with a lithic-acid diathesis, is heir to, i. e., about the most painful series of all its inheritances. The potash of these salts existing therein, in combination with organic acids, is separated from these acids by organic combustion, and is then and there presented to the baneful lithic acid of the blood and tissues, the stony torture-particles of which it converts into soluble lithate of potash, and thus enables them to be carried out of the system.
I know not which of the fathers of the Church invented fast-days and soup maigre, but could almost suppose that he was a scientific monk, a profound alchemist, like Basil Valentine, who, in his seekings for the aurum potabile, the elixir of life, had learned the beneficent action of organic potash salts on the blood, and therefore used the authority of the Church to enforce their frequent use among the faithful.—Knowledge.
- Shortly after the close of the Great Exhibition of 1851, when the South Kensington Museum was only in embryo, I had occasion to call at the "boilers," and there found the prince hard at work giving instructions for the arrangement and labeling of these analyzed food-products and the similarly displayed materials of industry, such as whalebone, ivory, etc. I then, by inquiry, learned how much time and labor he was devoting, not only to the general business of the collection, but also to its minor details.