Popular Science Monthly/Volume 24/February 1884/The Chemistry of Cookery IX
|THE CHEMISTRY OF COOKERY.|
I NOW come to a very important constituent of animal food, although it is not contained in beef, mutton, pork, poultry, game, fish, or any other organized animal substance. It is not even proved satisfactorily to exist in the blood, although it is somehow obtained from the blood by special glands at certain periods. I refer to casein, the substantial basis of cheese, which, as everybody knows, is the consolidated curd of milk.
It is evident at once that casein must exist in two forms, the soluble and insoluble, so far as the common solvent, water, is concerned. It exists in the soluble form, and completely dissolved in milk, and insoluble in cheese. When precipitated in its insoluble or coagulated form, as the curd of new milk, it carries with it the fatty matter, or cream, and therefore, in order to study its properties in a state of purity, we must obtain it otherwise. This may be done by allowing the fat-globules of the milk to float to the surface, and then remove them—i. e., by separating the cream as by the ordinary dairy method. We thus obtain in the skimmed milk a solution of casein, but there still remains some of the fat. This may be removed by evaporating it down to solidity, and then dissolving out the fat by means of ether, which leaves the soluble casein behind. The adhering ether being evaporated, we have a fairly pure specimen of casein in its original or soluble form.
This, when dry, is an amber-colored, translucent substance, devoid of odor, and insipid. This insipidity and absence of odor of the pure and separated casein is noteworthy, as it is evidently the condition in which it exists in milk, but very different from that of the casein of cheese. My object in pointing this out is to show that in the course of the manufacture of cheese new properties are developed. Skim milk—a solution of casein—is tasteless and inodorous, while cheese, whether made from skimmed or whole milk, has a very decided flavor and odor.
If we now add some of our dry casein to water, it dissolves, forming a yellowish, viscid fluid, which, on evaporation, becomes covered with a slight film of insoluble casein, which may be readily drawn off. Some of my readers will recognize in this description the resemblance of a now well-known domestic preparation of soluble casein, condensed milk, where it is mixed with much cream, and in the ordinary preparation also much sugar. The cream dilutes the yellowness, but does not quite mask it, and the viscidity is shown by the strings which follow the spoon when a spoonful is lifted. If a concentrated solution of pure casein is exposed to the air it rapidly putrefies, and passes through a series of changes that I must not tarry to describe, beyond stating that ammonia is given off, and some crystalline substances, such as leucine, tyrosine, etc., very interesting to the physiological chemist, but not important in the kitchen, are formed.
A solution of casein in water is not coagulated by boiling; it may be repeatedly evaporated to dryness and redissolved. Upon this depends the practicability of preserving milk by evaporating it down, or "condensing." This condensed milk, however, loses a little; its albumen is sacrificed, as everybody will understand who has dipped a spoon in freshly-boiled milk and observed the skin which the spoon removes from the surface. This is coagulated albumen.
If alcohol is added to a concentrated solution of casein in water, a pseudo-coagulation occurs; the casein is precipitated as a white substance like coagulated albumen, but, if only a little alcohol is used, the solid may be redissolved in water; if, however, it is thus treated with strong alcohol, the casein becomes difficult of solution, or even quite insoluble. Alcohol added to solid soluble casein renders it opaque, and gives it the appearance of coagulated albumen. The alcohol itself dissolves a little of this.
The characteristic coagulation of casein, or its conversion from the soluble to the insoluble form, is produced rather mysteriously by rennet. Acids precipitate it from an aqueous solution, producing an apparent coagulation, but it is not a true and complete coagulation like that effected by the rennet, for on neutralizing the acid precipitant with an alkali or metallic oxide the casein again dissolves. Excepting in the cases of acetic and lactic acids (vinegar and the acid of sour milk), which precipitate pure casein, the acid precipitates appear to be a compound of casein with the acids, and the casein is set free in its original state when the acid goes over to the alkali or basic metallic oxide. The action of rennet in the coagulation of casein is still a chemical mystery, especially when we consider the smallness of the quantity of coagulating agent required for the rapid and complete conversion.
A calf has four stomachs, the fourth being that which corresponds to ours, both in structure and functions. It is lined with a membrane, from which are secreted the gastric juice and other fluids concerned in effecting the conversion of food into chyme. A weak infusion made from a small piece of this "mucous membrane" will coagulate the casein of two or three thousand times its own quantity of milk, or the coagulation may be effected by placing a small piece of the stomach (usually salted and dried for the purpose) in the milk, and warming it for a few hours.
Many theoretical attempts have been made to explain this action of the rennet. Simon and Liebig supposed that it acts primarily as a ferment, converting the sugar of milk into lactic acid, and that this lactic acid coagulates the casein; but Selmi has shown that alkaline milk may be coagulated by rennet in the course of ten minutes, and that after the coagulation it still has an alkaline reaction. This is the case whether fresh naturally alkaline milk is used, or milk that has been artificially rendered alkaline by the addition of soda.
Casein, when thoroughly coagulated by rennet, then purified and dried, is a hard and yellowish horn-like substance. It softens and swells in water, but does not dissolve therein, nor in alcohol nor weak acids. Strong mineral acids decompose it. Alkalies dissolve it readily, and, if concentrated, decompose it on the application of heat. When moderately heated, it softens, and may be drawn into threads, and becomes elastic; at a higher temperature it fuses, swells up, carbonizes, and develops nearly the same products of distillation as the other protein compounds.
I have good and sufficient reasons for thus specifying the properties of this constituent of food. I regard it as the most important of all that I have to describe in connection with my subject—the science of cookery. It contains (as I shall presently show) more nutritious material than any other food that is ordinarily obtainable, and its cookery is singularly neglected, is practically an unknown art, especially in this country. We commonly eat it raw, although in its raw state it is peculiarly indigestible; and in the only cooked form familiarly known among us here, that of a Welsh rabbit, or rare-bit, it is too often rendered still more indigestible, though this need not be the case.
Here, in this densely populated country, where we import so much of our food, cheese demands our most profound attention. The difficulties and cost of importing all kinds of meat, fish, and poultry, are great, while cheese may be cheaply and deliberately brought to us from any part of the world where cows or goats can be fed, and it can be stored more readily and kept longer than other kinds of animal food. All that is required to render it, next to bread, the staple food of Britons, is scientific cookery.
If I shall be able, in what is to follow, to impart to my fellow-countrymen, and more especially countrywomen, my own convictions concerning the cookability, and consequent improved digestibility, of cheese, these papers will have "done the state some service!"
In my last I referred generally to the high nutritive value of cheese. I will now state particulars. First, as regards the water. Taking muscular fiber without bone, i.e., selected best part of the meat, beef contains on an average 722 per cent of water; mutton, 732; veal, 742; pork, 694; fowl, 734; while Cheshire cheese contains only 303, and other cheeses about the same. Thus, at starting, we have in every pound of cheese rather more than twice as much solid food as in a pound of the best meat, or comparing with the average of the whole carcass, including bone, tendons, etc., the cheese has an advantage of three to one.
The following results of Mulder's analysis of casein, when compared with those by the same chemist of albumen, gelatine, and fibrin, show that there is but little difference in the ultimate chemical composition of these, so far as the constituents there named are concerned:
We may therefore conclude that, regarding these from the point of view of nitrogenous or flesh-forming, and carbonaceous or heat-giving constituents, these chief materials of flesh and of cheese are about equal.
The same is the case as regards the fat. The quantity in the carcass of oxen, calves, sheep, lambs, and pigs varies, according to Dr. Edward Smith, from 16 per cent to 31·3 per cent in moderately-fatted animals, while in whole-milk cheeses it varies from 21·68 per cent to 32·31 per cent, coming down in skim-milk cheeses as low as 6·3. Dr. Smith includes Neufchâtel cheese, containing 18·74 per cent among the whole-milk cheeses. He does not seem to be aware that the cheese made up between straws and sold under that name is a ricotta, or crude curd of skim-milk cheese. Its just value is about threepence per pound. In Italy, where it forms the basis of some delicious dishes (such as budino di ricotta, of which anon), it is sold for about twopence per pound or less.
There is a discrepancy in the published analyses of casein which demands explanation here, as it is of great practical importance. They generally correspond to the above of Mulder within small fractions, as shown below in those of Scherer and Dumas:
In these the one hundred parts are made up without any phosphate of lime, while, according to Lehmann ("Physiological Chemistry," vol. i, p. 379, Cavendish edition), "casein that has not been treated with acids contains about six per cent of phosphate of lime; more, consequently, than is contained in any of the protein compounds we have hitherto considered."
From this it appears that we may have casein with, and casein without, this necessary constituent of food. In precipitating casein for laboratory analysis, acids are commonly used, and thus the phosphate of lime is dissolved out; but I am unable at present to tell my readers the precise extent to which this actually occurs in practical cheese-making where rennet is used. What I have at present learned only indicates generally that this constituent of cheese is very variable; and I hereby suggest to those chemists who are professionally concerned in the analysis of food, that they may supply a valuable contribution to our knowledge of this subject by simply determining the phosphate of lime contained in the ash of different kinds of cheese. I would do this myself, but, having during some ten years past forsaken the laboratory for the writing-table, I have neither the tools nor the leisure for such work; and, worse still, I have not that prime essential to practical research (especially of endowed research), a staff of obedient assistants to do the drudgery.
The comparison specially demanded is between cheeses made with rennet and those Dutch and factory cheeses the curd of which has been precipitated by hydrochloric acid. Theoretical considerations point to the conclusion that in the latter much or even all of the phosphate of lime may be left in solution in the whey, and thus the food-value of the cheese seriously lowered. We must, however, suspend judgment in the mean time.
In comparing the nutritive value of cheese with that of flesh, the retention of this phosphate of lime nearly corresponds with the retention of the juices of the meat, among which are the phosphates of the flesh.
These phosphates of lime are the bone-making material of food, and have something to do in building up the brain and nervous matter, though not to the extent that is supposed by those who imagine that there is a special connection between phosphorus and the brain, or phosphorescence and spirituality. Bone contains about eleven per cent of phosphorus, brain less than one per cent.
The value of food in reference to its phosphate of lime is not merely a matter of percentage, as this salt may exist in a state of solution, as in milk, or as a solid very difficult of assimilation, as in bones. That retained in cheese is probably in an intermediate condition not actually in solution, but so finely divided as to be readily dissolved by the acid of the gastric juice.
I may mention, in reference to this, that, when a child or other young animal takes its natural food in the form of milk, the milk is converted into unpressed cheese, or curd, prior to its digestion.
Supposing that on an average cheese contains only one half of the six per cent of phosphate of lime found, as above, in the casein, and taking into consideration the water contained in flesh, the bone, etc., we may conclude generally that one pound of average cheese contains as much nutriment as three pounds of the average material of the carcass of an ox or sheep as prepared for sale by the butcher; or, otherwise stated, a cheese of twenty pounds weight contains as much food as a sheep weighing sixty pounds as it hangs in the butcher's shop.
Now comes the practical question. Can we assimilate or convert into our own substance the cheese-food as easily as we may the flesh-food?I reply that we certainly can not if the cheese is eaten raw; but have no doubt that we may if it be suitably cooked. Hence the paramount importance of this part of my subject. A Swiss or Scandinavian mountaineer can and does digest and assimilate raw cheese as a staple article of food, and proves its nutritive value by the result; but feebler bipeds of the plains and towns can not do the like.
I may here mention that I have recently made some experiments on the dissolving of cheese by adding sufficient alkali (carbonate of potash) to neutralize the acid it contains, thus converting the casein into its original soluble form as it existed in the milk, and have partially succeeded both with water and milk as solvents; but before reporting these results in detail I will describe some of the practically established methods of cooking cheese that are so curiously unknown or little known in this country.
In the fatherland of my grandfather, Louis Gabriel Mattieu, one of the commonest dishes of the peasant who tills his own freehold and grows his own food is a "fondevin" (I can not explain the etymology of the word, and spell it only by ear, never having seen it in print or writing). This is a mixture of cheese and eggs, the cheese grated and beaten into the egg as in making omelets, with a small addition of new milk or butter. It is placed in a little pan like a flower-pot saucer, cooked gently, served as it comes off the fire, and eaten from the vessel in which it is cooked. I have made many a hearty dinner on one of these, plus a lump of black bread and a small bottle of genuine but thin wine; the cost of the whole banquet at a little auberge being usually less than sixpence. The cheese is in a pasty condition, and partly dissolved in the milk or butter. I have tested the sustaining power of such a meal by doing some very stiff mountain-climbing and long fasting after it. It is rather too good—over-nutritious—for a man only doing sedentary work.
A diluted and delicate modification of this may be made by taking slices of bread, or bread and butter, soaking them in a batter made of eggs or milk—without flour—then placing the slices of soaked bread in a pie-dish, covering each with a thick coating of grated cheese, and thus building up a stratified deposit to fill the dish. The surplus batter may be poured over the top; or, if time is allowed for saturation, the trouble of preliminary soaking may be saved by simply pouring all the batter thus. This, when gently baked, supplies a delicious and highly nutritious dish. We call it cheese-pudding at home, but my own experience convinces me that we make a mistake in using it to supplement the joint. It is far too nutritious for this; its savory character tempts one to eat it so freely that it would be far wiser to use it as the Swiss peasant uses his fondevin, i. e., as the one and only dish of a good wholesome dinner.
I have tested its digestibility by eating it heartily for supper. No nightmare has followed. If I sup on a corresponding quantity of raw cheese, my sleep is miserably eventful.—Knowledge.