Popular Science Monthly/Volume 23/October 1883/The Chemistry of Cookery V
THE process of frying follows next in natural order to those of roasting and grilling. A little reflection will show that in frying the heat is not communicated to the food by radiation from a heated surface at some distance, but by direct contact with the heating medium, which is the hot fat commonly, but erroneously, described as "boiling fat."
As these papers are intended for intelligent readers who desire to understand the philosophy of the common processes of cookery, so far as they are understandable, this fallacy concerning boiling fat should be pushed aside at once.
Generally speaking, ordinary animal fats are not boilable under the pressure of our atmosphere (one of the constituent fatty acids of butter, butyric acid, is an exception; it boils at 314° Fahr.). Before their boiling-point, i. e., the temperature at which they pass completely into the state of vapor, is reached, their constituents are more or less dissociated or separated by the repulsive agency of the heat, new compounds being in many cases formed by recombinations of their elements.
When water is heated to 212° it is converted completely into a gas, which gas returns to the fluid state without any loss on cooling below 212°. In like manner if we raise an essential-oil, such as turpentine, to 320°, or oil of peppermint to 340°, or orange-peel oil to 345°, or patchouli to 489°, and other such oils to various other temperatures, they pass into a state of vapor, and these vapors, when cooled, recondense into their original form of liquid oil without alteration. Hence they are called "volatile oils," while the greasy oils which can not thus be distilled (in which animal fats are included) are called "fixed oils."
A very simple practical means of distinguishing these is the following: Make a spot of the oil to be tested on clean blotting-paper. Heat this by holding it above a spirit-lamp flame, or by toasting before a fire. If the oil is volatile, the spot disappears; if fixed, it remains as a spot of grease until the heat is raised high enough to char the paper, of which charring (a result of the dissociation above-named) the oil partakes.
But the practical cook may say, "This is wrong, for the fat in my frying-pan does boil, I see it boil, and I hear it boil." The reply to this is, that the lard, or dripping, or butter that you put into your frying-pan is oil mixed with water, and that it is not the oil but the water that you see boiling. To prove this, take some fresh lard, as usually supplied, and heat it in any convenient vessel, raising the temperature gradually. Presently, it will begin to splutter. If you try it with a thermometer you will find that this spluttering-point agrees with the boiling-point of water, and if you use a retort you may condense and collect the splutter-matter, and prove it to be water. So long as the spluttering continues, the temperature of the melted fat, i. e., the oil, remains about the same, the water-vapor carrying away the heat. When all the water is driven off, the liquid becomes quiescent, in spite of its temperature, rising from 212º to near 400º, then a smoky vapor comes off and the oil becomes darker; this vapor is not vapor of lard, but vapor of separated and recombined constituents of the lard, which is now suffering dissociation, the volatile products passing off while the non-volatile carbon ( i. e., lard-charcoal) remains behind, coloring the liquid. If the heating be continued, a residuum of this carbon, in the form of soft coke or charcoal, will be all that remains in the heated vessel.
We may now understand what happens when something humid— say a sole—is put into a frying-pan which contains fat heated above 212º. Water, when suddenly heated above its boiling-point, is a powerful explosive, and may be very dangerous, simply because it expands to 1,728 times its original bulk when converted into steam. Steam engine boilers and the boilers of kitchen-stoves sometimes explode simply by becoming red-hot while dry, and then receiving a little water which suddenly expands to steam.
The noise and spluttering that are started immediately the sole is immersed in the hot fat, are due to the explosions of a multitude of small bubbles formed by the confinement of the suddenly expanding steam in the viscous fat, from which it releases itself with a certain degree of violence. It is evident that, to effect this amount of eruptive violence, the temperature must be considerably above the boiling point of the exploding water. If it were only just at the boiling-point, the water would boil quietly. As we all know, the flavor and appearance of a boiled sole or mackerel are decidedly different from those of a fried sole or mackerel, and it is easy to understand that the different results of these cooking processes are to some extent due to the difference of temperature to which the fish is subjected.
The surface of the fried fish, like that of the roasted or grilled meat, is "browned." What is the nature, the chemistry of this browning?
I have endeavored to find some answer to this question, that I might quote with authority, but no technological or purely chemical work within my reach supplies such answer. Rumford refers to it as essential to roasting, and provides for it in the manner already described, but he goes no further into the philosophy of it than admitting its flavoring effect.
I must therefore struggle with the problem in my own way as I best can. Has the gentle reader ever attempted the manufacture of "hard-bake," or "toffy," or "butter-scotch," by mixing sugar with butter, fusing the mixture, and heating further until the well-known hard, brown confection is produced? I venture to call this fried sugar. If heated simply without the butter it may be called baked sugar. The scientific name for this baked sugar is caramel.
The chemical changes that take place in the browning of sugar have been more systematically studied than those which occur in the constituents of flesh when browned in the course of ordinary cookery. Believing them to be nearly analogous, I will state, as briefly as possible, the leading facts concerning the sugar.
Ordinary sugar is crystalline, i. e., when it passes from the liquid to the solid state it assumes regular geometrical forms. If the solidification takes place undisturbed and slowly, the geometric crystals are large, as in sugar-candy; if the water is rapidly evaporated with agitation, the crystals are small, and the whole mass is a granular aggregation of crystals, such as we see in loaf-sugar. If this crystalline sugar be heated to about 320º it fuses, and without any change of chemical composition undergoes some sort of internal physical alteration that makes it cohere in a different fashion. (The learned name for this is allotropism, and the substance is said to be allotropic, other conditioned; or dimorphic, two-shaped.) Instead of being crystalline the sugar now becomes vitreous, it solidifies as a transparent amber-colored glass-like substance, the well-known barley-sugar, which differs from crystalline sugar, not only in this respect, but has a much lower melting-point; it liquefies between 190º and 212º, while loaf-sugar does not fuse below 320. Left to itself, vitreous sugar returns gradually to its original condition, loses transparency, and breaks up into small crystals. In doing this, it gives out the heat which during its vitreous condition had been doing the work of breaking up its crystalline structure, and therefore was not manifested as temperature.
This return to the crystalline condition is retarded by adding vinegar or mucilaginous matter to the heated sugar, hence the confectioners' name of "barley-sugar," which, in one of its old-fashioned forms, was prepared by boiling down ordinary sugar in a decoction of pearl barley.
The French cooks and confectioners carry on the heating of sugar through various stages bearing different technical names, one of the most remarkable of which is a splendid crimson variety, largely used in fancy sweetmeats, and containing no foreign coloring-matter, as commonly supposed. Though nothing is added, something is taken away, and this is some of the chemically-combined water of the original sugar, in the parting with which not only a change of color occurs, but also a modification of flavor, as anybody may prove by experiment.
When the temperature is gradually raised to 420º, the sugar loses two equivalents of water, and becomes caramel—a dark-brown substance, no longer sweet, but having a new flavor of its own. It further differs from sugar by being incapable of fermentation. Its analogies to the crust of bread and the "brown" of cooked animal food will be further discussed in my next.
In my last I described the dissociation of sugar by heat and the formation of caramel, to illustrate by simple example the "browning" of other kinds of food. I might have added, in connection with this cookery of sugar, an historical connection with one of the lost arts of the kitchen—viz., the "spinning" of sugar. Within the reach of my own recollection no evening party could pretend to be stylish unless the supper-table was decorated with a specimen of this art—a temple, a pagoda, or something of the sort done in barley-sugar. These were made by raising the sugar to 320º, when it fused and became amorphous, or vitreous, as already described. The cook then dipped a skewer into it, the melted vitreous sugar adhered to this and was drawn out as a thread, which speedily solidified by cooling. While in the act of solidification it was woven into the desired form, and the skillful artist did this with wonderful rapidity. I once witnessed with childish delight the spinning of a great work of art by a French cook in St. James's Palace. It was a ship in full sail, the sails of edible wafer, the hull a basket-work of spun sugar, the masts of massive sugar-sticks, and the rigging of delicate threads of the same. As nearly as I can remember, the whole was completed in about an hour.
But to return from high art to chemical science. The conversion of sugar into caramel is, as already stated, attended with a change of flavor: a kind of bitterness replaces the sweetness. This peculiar flavor, judiciously used, is a powerful adjunct to cookery, and one which is shamefully neglected in our ordinary English domestic kitchens. To test this, go to one of those Swiss restaurants originally instituted in this country by that enterprising Ticinese, the late Carlo Gatti, and which are now so numerous in London and our other large towns; call for macaroni al sugo; notice the rich, brown gravy, the "sugo." Many an English cook would use half a pound of gravy beef to produce the like, but the basis of this is half an ounce of sugar, or even less; the sugar is browned by heating, not quite up to the caramel state. Burnt onion may contribute, but this is only another form of caramel with more savory properties.
While engaged upon your macaroni, look around at the other dishes served to other customers. Instead of the pale slices of meat spread out in a little puddle of pale, watery liquid, that are served in English restaurants of corresponding class, you will see dainty morsels, covered with rich, brown gravy, or surrounded by vegetables immersed in the same. This sugo is greatly varied according to the requirements, by additions of stock-broth, tarragon-vinegar, ketchup, etc., etc., but burnt sugar, or burnt onions, or burnt something is the basis of it all, sugar being the cheapest.
To further test the flavoring properties of browning, take some eels cut up as usual for stewing; divide into two portions; stew one brutally—by this I mean simply in a little water—serving them with this water as a pale gravy or juice. Let the second portion be well fried, fully browned, then stewed, and served with brown gravy. Compare the result. Make a corresponding experiment with a beefsteak. Cut it in two portions: stew one brutally in plain water; fry the other, then stew it and serve brown.
Take a highly-baked loaf, better one that is black outside; scrape off the film or crust that is quite black, i. e., completely carbonized, and you will come to a rich brown layer, especially if you operate upon the bottom crust. Slice off a thin shaving of this, and eat it critically. Mark its high flavor as compared with the comparatively insipid crumb of the same loaf, and note especially the resemblance between this flavor and that of the caramel from sugar, and that of the browned eels and browned steak. A delicate way of detecting the flavor due to the browning of bread is to make two bowls of bread and milk in the same manner, one with the crust, the other with the crumb of the same loaf. I am not suggesting these as examples of better or worse flavor, but as evidence of the fact that much flavor of some sort is generated. It may be out of place, as I think it is, in the bread and milk, or it may be added with much advantage to other things, as it is by the cook who manipulates caramel and its analogues skillfully.
The largest constituent of bread is starch. Excluding water, it constitutes about three fourths of the weight of good wheaten flour. Starch differs but little from sugar in composition. It is easily converted into sugar by simply heating it with a little sulphuric acid, and by other means of which I shall have to speak more fully hereafter when I come to the cookery of vegetables. When simply heated, it is converted into dextrin or "British gum," largely used as a substitute for gum-arabic. If the heat is continued a change of color takes place; it grows darker and darker until it blackens just as sugar does, the final result being nearly the same. Water is driven off in both cases, but in carbonizing sugar we start with more water, sugar being starch plus water or the elements of water. Thus the brown material of bread-crust or toast is nearly identical with caramel.
I have often amused myself by watching what occurs when toast and-water is prepared, and I recommend my readers to repeat the observation. Toast a small piece of bread to blackness, and then float it on water in a glass vessel. Leave the water at rest, and direct your attention to the under side of the floating toast. Little threadlike streams of brown liquid will be seen descending in the water. This is a solution of the substance which, if I mistake not, is a sort of caramel, and which ultimately tinges all the water.
Some years ago I commenced a course of experiments with this substance, but did not complete them. In case I should never do so, I will here communicate the results attained. I found that this starch caramel is a disinfectant, and that sugar caramel also has some disinfecting properties. I am not prepared to say that it is powerful enough to disinfect sewage, though at the time I had a narrow escape from the Great-Seal Office, where I thought of patenting it for this purpose as a non-poisonous disinfectant that may be poured into rivers in any quantity without danger. Though it may not be powerful enough for this, it has an appreciable effect on water slightly tainted with decomposing organic matter.
This is a very curious fact. We do not know who invented toast and-water, nor, so far as I can learn, has any theory of its use been expounded, yet there is extant a vague, popular impression that the toast has some sort of wholesome effect on the water. I suspect that this must have been originally based on experience, probably on the experience of our forefathers or foremothers living in country places where stagnant water was a common beverage, and various devices were adopted to render it potable.
Gelatine, fibrine, albumen, etc.—i. e., all the materials of animal food—as already shown, are composed, like starch and sugar, of carbon, hydrogen, and oxygen, with, in the case of these animal substances, the addition of nitrogen; but this does not prevent their partial carbonization (or "caramelizing," if I may invent a name to express the action which stops short of blackening). Animal fat is a hydrocarbon which may be similarly browned, and, if I am right in my generalization of all these browning processes, an important practical conclusion follows, viz., that cheap soluble caramel made by skillfully heating common sugar, is really, as well as apparently, as valuable an element in gravies, etc., as the far more expensive coloring-matter of brown meat-gravies, and that our English cooks should use it far more liberally than they usually do.
Its preparation is easy enough; the sugar should be gradually heated till it assumes a rich brown color and has lost its original sweetness. If carried just far enough, and not too far, the result is easily soluble in hot water, and the solution may be kept for a long time, as it is by cooks who understand its merits. In connection with the idea of its disinfecting action, I may refer to the cookery of tainted meat or "high" game. A hare that is repulsively advanced when raw, may by much roasting and browning become quite wholesome, and such is commonly the case in the ordinary cooking of hares. If it were boiled or merely stewed (without preliminary browning) in this condition, it would be quite disgusting to ordinary palates.
A leg of mutton for roasting should be hung until it begins to become odorous; for boiling it should be as fresh as possible. This should be especially remembered now that we have so much frozen meat imported from the antipodes. When duly thawed it is in splendid condition for roasting, but is not usually so satisfactory when boiled. I may here mention incidentally that such meat is sometimes unjustly condemned on account of its displaying a raw center when cooked. This arises from imperfect thawing. The heat required to thaw a given weight of ice and bring it up to 60º, is about the same as demanded for the cookery of an equal quantity of meat, and therefore, while the thawed portions of the meat is being cooked, the frozen portion is but just thawed, and remains quite raw.
A much longer time is demanded for thawing—i. e., supplying 142º of latent heat—than might be supposed. To ascertain whether the thawing is completed, drive an iron skewer through the thickest part of the joint. If there is a core of ice within, it will be distinctly felt by its resistance.
Before leaving the subject of caramel, I should say a few words about French coffee, or "coffee as in France," of which we hear so much. There are two secrets upon which depend the excellence of our neighbors in the production of this beverage: First, economy in using the water; second, flavoring with caramel. As regards the first, it appears that English housewives have been demoralized by the habitual use of tea, and apply to the infusion of coffee the popular formula for that of tea, "a spoonful for each person and one for the pot."
The French after-dinner coffee-cup has about one third of the liquid capacity of a full-sized English breakfast-cup, but the quantity of solid coffee supplied to each cupful is more than equal to that ordinarily allowed for the larger English measure of water.
Besides this, the coffee is commonly though not universally flavored with a specially and skillfully prepared caramel, instead of the chiccory so largely used in England. Much of the so-called "French coffee" now sold by our grocers in tins is caramel flavored with coffee, rather than coffee flavored with caramel; and many shrewd English housewives have discovered that, by mixing the cheapest of these French coffees with an equal quantity of pure coffee, they obtain a better result than with the common domestic mixture of three parts coffee and one of chiccory.
A few months ago a sample of "coffee-finings" was sent to me for chemical examination, that I might certify to its composition and wholesomeness. I described it in my report as "a caramel, with a peculiarly rich aroma and flavor, evidently due to the vegetable juices or extractive matter naturally united with the saccharine substance from which it is prepared." I had no definite information of the exact nature of this saccharine substance, but have good reason to assume that it was a by-product of sugar-refining.
Neither the juice of the beet-root nor the sap of the sugar-cane consists entirely of pure sugar dissolved in pure water. They both contain other constituents common to vegetable juices, and some peculiar to themselves. These mucilaginous matters, when roughly separated, carry down with them some sugar, and form a sort of coarse sweet wort, capable, by skillful treatment, of producing a rich caramel such as I received.
I tested its practical merits by making an infusion of pure coffee of fine quality, dividing this into two parts, adding to one a small quantity of the caramel, and leaving the other half unmixed. I found the infusion greatly improved in flavor by the admixture, and recognized the peculiarity which characterizes the coffee prepared by Gatti and his compatriots, whose numerous establishments are doing so much for the promotion of temperance in this country. The aroma of this particular caramel is peculiarly fine, and the greater part of it is soluble in boiling water; thus I was able to mix it by merely adding to the coffee as we add sugar.
I have used my best eloquence in trying to persuade the manufacturers to sell it separately, but have not yet succeeded. They seem to have had painful experience of the gastronomic bigotry of Englishmen, who refuse to eat or drink anything that is not hallowed by the sanction of their great-grandmothers, unless it is surreptitiously introduced by means of some device approaching as nearly as possible to a commercial swindle.
Returning to the subject of frying, we encounter a good illustration of the practical importance of sound theory. A great deal of fish and other kinds of food are badly and wastefully cooked in consequence of the prevalence of a false theory of frying. It is evident that many domestic cooks (not hotel or restaurant cooks) have a vague idea that the metal plate forming the bottom of the frying-pan should directly convey the heat of the fire to the fried substance, and that the bit of butter or lard or dripping put into the pan is used to prevent the fish from sticking to it, or to add to the richness of the fish by smearing its surface.
The theory which I have suggested (see No. XIII, page 818) is that the melted fat cooks by convection of heat, just as water does in the so-called boiling of meat. If that is correct, it is evident that the fish, etc., should be completely immersed in a bath of melted fat or oil, and that the turning over demanded by the greased-plate theory is unnecessary. Well-educated cooks understand this distinctly, and use a deeper vessel than our common frying-pan, charge this with a quantity of fat sufficient to cover the fish, which is simply laid upon a wire support, or frying-basket, and left in the hot fat until the browning of its surface, or of the flour or bread-crumbs with which it is coated, indicates the sufficiency of the cookery.
At first sight this appears extravagant, as compared with the practice of greasing the bottom of the pan with a little dab of fat; but any housewife who will apply to the frying of sprats, herrings, etc., the method of quantitative inductive research, described and advocated by Lord Bacon in his "Novum Organum Scientarum," may prove the contrary.
"Must I read the 'Novum Organum,' and buy another dictionary, in order to translate all this?" she may exclaim in despair. "No!" is my reply. This Baconian inductive method, to which we are indebted for all the triumphs of modern science, is nothing more nor less than the systematic and orderly application of common sense and definite measurement to practical questions. In this case it may be applied simply by frying a weighed quantity of any particular kind of fish—say sprats—in a weighed quantity of fat used as a bath; then weighing the fat that remains and subtracting the latter weight from the first, to determine the quantity consumed. If the frying be properly performed, and this quantity compared with that which is consumed by the method of merely greasing the pan-bottom, the bath-frying will be proved to be the more economical as well as the more efficient method.
The reason of this is simply that much or all of the fat is burned and wasted when only a thin film is spread on the bottom of the pan, while no such waste occurs when the bath of fat is properly used. The temperature at which the dissociation of fat commences is below that required for delicately browning the surface of the fish itself, or of the flour or bread-crumbs, and therefore no fat is burned away from the bath, as it is by the overheated portions of a merely greased frying-pan; and, as regards the quantity adhering to the fish itself, this may be reduced to a minimum by withdrawing it from the bath when the whole is uniformly at the maximum cooking temperature, and allowing the fluid fat to drain off at once. When cooked on the greased plate, one Bide is necessarily cooling, and the fat settling down into the fish, while the other is being heated from below.