Читать книгу The Chemistry of Cookery - W. Mattieu Williams - Страница 4

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As this is one of the most rudimentary of the operations of cookery, and the most frequently performed, it naturally takes a first place in treating the subject.

Water is boiled in the kitchen for two distinct purposes: 1st, for the cooking of itself; 2nd, for the cooking of other things. A dissertation on the difference between raw water and cooked water may appear pedantic, but, as I shall presently show, it is considerable, very practical, and important.

The best way to study any physical subject is to examine it experimentally, but this is not always possible with everyday means. In this case, however, there is no difficulty.

Take a thin[1] glass vessel, such as a flask, or, better, one of the ‘beakers,’ or thin tumbler-shaped vessels, so largely used in chemical laboratories; partially fill it with ordinary household water, and then place it over the flame of a spirit-lamp, or Bunsen’s, or other smokeless gas-burner. Carefully watch the result, and the following will be observed: first of all, little bubbles will be formed, adhering to the sides of the glass, but ultimately rising to the surface, and there becoming dissipated by diffusion in the air.

This is not boiling, as may be proved by trying the temperature with the finger. What, then, is it?

It is the yielding back of the atmospheric gases which the water has dissolved or condensed within itself. These bubbles have been collected, and by analysis proved to consist of oxygen, nitrogen, and carbonic acid, obtained from the air; but in the water they exist by no means in the same proportions as originally in the air, nor in constant proportions in different samples of water. I need not here go into the quantitative details of these proportions, nor the reasons of their variation, though they are very interesting subjects.

Proceeding with our investigation, we shall find that the bubbles continue to form and rise until the water becomes too hot for the finger to bear immersion. At about this stage something else begins to occur. Much larger bubbles, or rather blisters, are now formed on the bottom of the vessel, immediately over the flame, and they continually collapse into apparent nothingness. Even at this stage a thermometer immersed in the water will show that the boiling-point is not reached. As the temperature rises, these blisters rise higher and higher, become more and more nearly spherical, finally quite so, then detach themselves and rise towards the surface; but the first that make this venture perish in the attempt—they gradually collapse as they rise, and vanish before reaching the surface. The thermometer now shows that the boiling-point is nearly reached, but not quite. Presently the bubbles rise completely to the surface and break there. Now the water is boiling, and the thermometer stands at 212° Fahr. or 100° Cent.

With the aid of suitable apparatus it can be shown that the atmospheric gases above named continue to be given off along with the steam for a considerable time after the boiling has commenced; the complete removal of their last traces being a very difficult, if not an impossible, physical problem.

After a moderate period of boiling, however, we may practically regard the water as free from these gases. In this condition I venture to call it cooked water. Our experiment so far indicates one of the differences between cooked and raw water. The cooked water has been deprived of the atmospheric gases that the raw water contained. By cooling some of the cooked water and tasting it, the difference of flavour is very perceptible; by no means improved, though it is quite possible to acquire a preference for this flat, tasteless liquid.

If a fish be placed in such cooked water it swims for a while with its mouth at the surface, for just there is a film that is reacquiring its charge of oxygen, &c., by absorbing it from the air; but this film is so thin, and so poorly charged, that after a short struggle the fish dies for lack of oxygen in its blood; drowned as truly and completely as an air-breathing animal when immersed in any kind of water.

Spring water and river water that have passed through or over considerable distances in calcareous districts suffer another change in boiling. The origin and nature of this change may be shown by another experiment as follows: Buy a pennyworth of lime-water from a druggist, and procure a small glass tube of about quill size, or the stem of a fresh tobacco-pipe may be used. Half fill a small wine-glass with the lime-water, and blow through it by means of the tube or tobacco-pipe. Presently it will become turbid. Continue the blowing, and the turbidity will increase up to a certain degree of milkiness. Go on blowing with ‘commendable perseverance,’ and an inversion of effect will follow; the turbidity diminishes, and at last the water becomes clear again.

The chemistry of this is simple enough. From the lungs a mixture of nitrogen, oxygen, and carbonic acid is exhaled. The carbonic acid combines with the soluble lime, and forms a carbonate of lime which is insoluble in mere water. But this carbonate of lime is to a certain extent soluble in water saturated with carbonic acid, and such saturation is effected by the continuation of blowing.

Now take some of the lime-water that has been thus treated, place it in a clean glass flask, and boil it. After a short time the flask will be found incrusted with a thin film of something. This is the carbonate of lime which has been thrown down again by the action of boiling, which has driven off its solvent, the carbonic acid. This crust will effervesce if a little acid is added to it.

In this manner our tea-kettles, engine-boilers, &c., become incrusted when fed with calcareous waters, and most waters are calcareous; those supplied to London, which is surrounded by chalk, are largely so. Thus, the boiling or cooking of such water effects a removal of its mineral impurities more or less completely. Other waters contain such mineral matter as salts of sodium and potassium. These are not removable by mere boiling, being equally soluble in hot or cold, aerated, or non-aerated water.

Usually we have no very strong motive for removing either these or the dissolved carbonate of lime, or the atmospheric gases from water, but there is another class of impurities of serious importance. These are the organic matters dissolved in all water that has run over land covered with vegetable growth, or, more especially, that which has received contributions from sewers or any other form of house drainage. Such water supplies nutriment to those microscopic abominations, the micrococci, bacilli, bacteria, &c., which are now shown to be connected with blood poisoning. These little pests are harmless, and probably nutritious, when cooked, but in their raw and growing state are horribly prolific in the blood of people who are in certain states of what is called ‘receptivity.’ They (the bacteria, &c.) appear to be poisoned or somehow killed off by the digestive secretions of the blood of some people, and nourished luxuriantly in the blood of others. As nobody can be quite sure to which class he belongs, or may presently belong, or whether the water supplied to his household is free from blood-poisoning organisms, cooked water is a safer beverage than raw water. I should add that this germ theory of disease is disputed by some who maintain that the source of the diseases attributed to such microbia is chemical poison, the microbia (i.e. little living things) are merely accidental, or creatures fed on the disease-producing poison. In either case the boiling is effectual, as such organic poisons when cooked lose their original virulent properties.

The requirement for this simple operation of cooking increases with the density of our population, which, on reaching a certain degree, renders the pollution of all water obtained from the ordinary sources almost inevitable.

Reflecting on this subject, I have been struck with a curious fact that has hitherto escaped notice, viz. that in the country which over all others combines a very large population with a very small allowance of cleanliness, the ordinary drink of the people is boiled water, flavoured by an infusion of leaves. These people, the Chinese, seem in fact to have been the inventors of boiled-water beverages. Judging from travellers’ accounts of the state of the rivers, rivulets, and general drainage and irrigation arrangements of China, its population could scarcely have reached its present density if Chinamen were drinkers of raw instead of cooked water. This is especially remarkable in the case of such places as Canton, where large numbers are living afloat on the mouths of sewage-laden rivers or estuaries.

The ordinary everyday domestic beverage is a weak infusion of tea, made in a large teapot, kept in a padded basket to retain the heat. The whole family is supplied from this reservoir. The very poorest drink plain hot water, or water tinged by infusing the spent tea-leaves rejected by their richer neighbours.

Next to the boiling of water for its own sake, comes the boiling of water as a medium for the cooking of other things. Here, at the outset, I have to correct an error of language which, as too often happens, leads by continual suggestion to false ideas. When we speak of ‘boiled beef,’ ‘boiled mutton,’ ‘boiled eggs,’ ‘boiled potatoes,’ we talk nonsense; we are not merely using an elliptical expression, as when we say, ‘the kettle boils,’ which we all understand to mean the contents of the kettle, but we are expounding a false theory of what has happened to the beef, &c.—as false as though we should describe the material of the kettle that has held boiling water as boiled copper or boiled iron. No boiling of the food takes place in any such cases as the above-named—it is merely heated by immersion in boiling water; the changes that actually take place in the food are essentially different from those of ebullition. Even the water contained in the meat is not boiled in ordinary cases, as its boiling-point is higher than that of the surrounding water, owing to the salts it holds in solution.

Thus, as a matter of chemical fact, a ‘boiled leg of mutton’ is one that has been cooked, but not boiled; while a roasted leg of mutton is one that has been partially boiled. Much of the constituent water of flesh is boiled out, fairly driven away as vapour during roasting or baking, and the fat on its surface is also boiled, and, more or less, dissociated into its chemical elements, carbon and water, as shown by the browning, due to the separated carbon.

As I shall presently show, this verbal explanation is no mere verbal quibble, but it involves important practical applications. An enormous waste of precious fuel is perpetrated every day, throughout the whole length and breadth of Britain and other countries where English cookery prevails, on account of the almost universal ignorance of the philosophy of the so-called boiling of food.

When it is once fairly understood that the meat is not to be boiled, but is merely to be warmed by immersion in water raised to a maximum temperature of 212°, and when it is further understood that water cannot (under ordinary atmospheric pressure) be raised to a higher temperature than 212° by any amount of violent boiling, the popular distinction between ‘simmering’ and boiling, which is so obstinately maintained as a kitchen superstition, is demolished.

The experiment described on pages 8 and 9 showed that immediately the bubbles of steam reach the surface of the water and break there—that is, when simmering commences—the thermometer reaches the boiling-point, and that however violently the boiling may afterwards occur, the thermometer rises no higher. Therefore, as a medium for heating the substances to be cooked, simmering water is just as effective as ‘walloping’ water. There are exceptional operations of cookery, wherein useful mechanical work is done by violent boiling; but in all ordinary cookery simmering is just as effective. The heat that is applied to do more than the smallest degree of simmering is simply wasted in converting water into useless steam. The amount of such waste may be easily estimated. To raise a given quantity of water from the freezing to the boiling point demands an amount of heat represented by 180° in Fahrenheit’s thermometer, or 100° Centigrade. To convert this into steam, 990° Fahr. or 550° Cent. is necessary—just five-and-a-half times as much.

On a properly-constructed hot-plate or sand-bath a dozen saucepans may be kept at the true cooking temperature, with an expenditure of fuel commonly employed in England to ‘boil’ one saucepan. In the great majority of so-called boiling operations, even simmering is unnecessary. Not only is a ‘boiled leg of mutton’ not itself boiled, but even the water in which it is cooked should not be kept boiling, as we shall presently see.

The following, written by Count Rumford nearly 100 years ago, remains applicable at the present time, in spite of all our modern research and science teaching:

‘The process by which food is most commonly prepared for the table—Boiling—is so familiar to everyone, and its effects are so uniform and apparently so simple, that few, I believe, have taken the trouble to inquire how or in what manner these effects are produced; and whether any and what improvements in that branch of cookery are possible. So little has this matter been an object of inquiry that few, very few indeed I believe, among the millions of persons who for so many ages have been daily employed in this process, have ever given themselves the trouble to bestow one serious thought upon the subject.

‘The cook knows from experience that if his joint of meat be kept a certain time immersed in boiling water it will be done, as it is called in the language of the kitchen; but if he be asked what is done to it, or how or by what agency the change it has undergone has been effected—if he understands the question—it is ten to one but he will be embarrassed. If he does not understand he will probably answer without hesitation, that “The meat is made tender and eatable by being boiled.” Ask him if the boiling of the water be essential to the process. He will answer, “Without doubt.” Push him a little further by asking him whether, were it possible to keep the water equally hot without boiling, the meat would not be cooked as soon and as well as if the water were made to boil. Here it is probable he will make the first step towards acquiring knowledge by learning to doubt.’

In another place he points to the fact that at Munich, where his chief cookery operations were performed, water boils at 209½° (on account of its elevation), while in London the boiling-point is 212°. ‘Yet nobody, I believe, ever perceived that boiled meat was less done at Munich than at London. But if meat may without the least difficulty be cooked with a heat of 209½° at Munich, why should it not be possible to cook it with the same degree of heat in London? If this can be done in London (which I think can hardly admit of a doubt), then it is evident that the process of cookery, which is called boiling, may be performed in water which is not boiling hot.’

He proceeds to say, ‘I well know, from my own experience, how difficult it is to persuade cooks of this truth, but it is so important that no pains should be spared in endeavouring to remove their prejudices and enlighten their understandings. This may be done most effectually in the case before us by a method I have several times put in practice with complete success. It is as follows: Take two equal boilers, containing equal quantities of boiling hot water, and put into them two equal pieces of meat taken from the same carcase—two legs of mutton, for instance—and boil them during the same time. Under one of the boilers make a small fire, just barely sufficient to keep the water boiling hot, or rather just beginning to boil; under the other make as vehement a fire as possible, and keep the water boiling the whole time with the utmost violence. The meat in the boiler in which the water has been kept only just boiling hot will be found to be quite as well done as that in the other. It will even be found to be much better cooked, that is to say tenderer, more juicy, and much higher flavoured.’

Rumford at this date (1802) understood perfectly that the water just boiling hot had the same temperature as that which was boiling with the utmost violence, but did not understand that the best result is obtained at a much lower temperature, for in another place he states that if the meat be cooked in water under pressure, so that the temperature shall exceed 212°, it will be done proportionally quicker and as well. My reasons for controverting this will be explained in the following chapters.