The Stock-Feeder's Manual Part 11

_History of b.u.t.ter._--The very general use of b.u.t.ter as an article of food is demonstrated by the familiar saying--"We should not quarrel with our bread and b.u.t.ter"; yet this article, now so commonly used throughout the greater part of Europe, was either unknown or but imperfectly known to the ancients. In the English translation of the Holy Scriptures the word b.u.t.ter does certainly frequently occur; but the Hebrew original is _chamea_, which, according to the most eminent Biblical critics, signifies cream, or thick, sour milk. In the 20th chapter of Job the following pa.s.sage occurs:--"He shall not see the rivers, the floods, the brooks of honey and b.u.t.ter." Now, we can conceive streams of thin cream, but we cannot imagine a river of b.u.t.ter. The oldest mention of b.u.t.ter is found in the works of Herodotus. In the description of the Scythians given by this ancient author, reference is made to their practice of violently shaking the milk of their mares, for the purpose of causing a solid fatty matter to ascend to its surface, which, when removed from the milk, they considered a delicious article of food. Hippocrates, who wrote a little later than Herodotus, describes, but in clearer language, the manufacture of b.u.t.ter by the Scythians; he also alludes to the preparation of cheese by the same people. The word, b.u.t.ter, does not occur in any of Aristotle's writings, and although mention is made of it in the works of Anaxandrides, Plutarch, and aelian, it is evident that they considered it only in the light of a curious substance, employed partly as an article of food, partly as a medicinal salve, by certain barbarous nations. About the second or third century, b.u.t.ter was but little known to the Greeks and Romans, and there is no reason to believe that it was ever generally used as an article of food by the cla.s.sic nations of antiquity; it is noteworthy, that the inhabitants of the south of Europe even at the present time use b.u.t.ter in very small quant.i.ties, which, indeed, is often sold for medicinal purposes in the apothecaries' shops in Italy, Spain, and Portugal. From the foregoing statements it is evident that the b.u.t.ter manufacture can lay no claim to a cla.s.sic origin; but that it took its rise in the countries of savage, of semi-civilised, and barbarous nations. It is probable that the Greeks were made acquainted with b.u.t.ter by the Thracians, Phrygians, and Scythians; and that the knowledge of this substance was conveyed to Rome by visitors from Germany. During the middle ages the practice of b.u.t.ter-making spread throughout Northern, Central, and Western Europe; but in many parts the commodity was very scarce and highly valued, notwithstanding its being almost, if not quite, in a semi-fluid state, instead of possessing the firm consistence of the b.u.t.ter of the present day.

_Irish b.u.t.ter._--b.u.t.ter is produced in such large quant.i.ties in Ireland that, after the home demand has been supplied, there remains a large excess--so considerable, indeed, as to const.i.tute one of the more important of our few commercial staples. The precise quant.i.ty of b.u.t.ter which, during late years, has been annually exported from Ireland is unknown. The greater part of the commodity is sent to trans-Channel ports; and, there being no duty on b.u.t.ter in the cross-Channel trade since 1826, we have no means of accurately estimating the amount of our exports to Great Britain. If, however, we refer to the statistics of our commerce for the period beginning in 1787, and ending in 1826, we shall find that the exportation of b.u.t.ter was enormous, and that a large proportion of that commodity consumed by the army and navy was supplied from the dairies of Ireland. During the three years ended on the 5th of January, 1826, the average annual amount of b.u.t.ter exported was as follows:--

cwts.

To Great Britain 441,226 To foreign countries 51,637

Of late years the exportation to foreign and colonial countries has fallen off; still the export trade is very considerable, probably amounting to 450,000 cwts. per annum. During the year 1867, the imports of foreign b.u.t.ter into Great Britain amounted to 1,142,262 cwts.

I have quoted the above statistics for the purpose of demonstrating the great importance of the b.u.t.ter trade to this country. Not only is a large proportion of the agricultural community pecuniarily interested in the production of this article, but the exportation is the chief cause of the commercial prosperity of a city, which, in point of population, ranks third in the kingdom. If b.u.t.ter, then, be an article of so much importance, it is obvious that the greatest care should be taken in its preparation, and that the efforts of both scientific and practical men should be directed towards the best mode of improving its quality. If the principles involved in the production of b.u.t.ter were thoroughly understood, and generally known, I believe that such terms as "seconds,"

"thirds," and "fourths," would speedily fall into disuse; that there would be only one kind of b.u.t.ter sent into the market; and that the article would always be of the best quality, in other words, "firsts."

_Composition of b.u.t.ter._--The composition and quality of b.u.t.ter depend to a great extent upon the condition of the milk or cream from which it is prepared, and on the skill and cleanliness of the dairy-maid. It consists essentially of fatty and oily matters, but it is always found in combination with casein (cheesy matter) and water. The following a.n.a.lyses, made by Mr. Way, late consulting chemist to the Royal Agricultural Society of England, shows its composition:--

INGREDIENTS PER CENT.

1. 2. 3.

Fatty matters 8270 7967 7912 Casein 245 338 337 Water 1485 1695 1751

No. 1 a.n.a.lysis shows the composition of a specimen obtained from the well-known Mr. Horsfall's dairy. It was made from raw cream. The other specimens were the produce of a Devons.h.i.+re dairy, and were prepared from scalded cream. In several specimens of well-made and unsalted Irish b.u.t.ter which I have a.n.a.lysed, I found the proportion of casein or cheesy matter never to exceed 1 per cent., whilst in the a.n.a.lysis above stated the centesimal amount is on the average more than 3 per cent.

The fatty matter is composed of two substances--one, a solid, termed _margarin_; the other fluid, and styled by chemists _elaine_. The solid fat is identical in composition with the solid fat of the human body.

The elaine is peculiar to milk, but it differs very slightly from _olein_, or fluid fat. The relative proportions of the fluid and solid fats vary with the seasons. According to Braconnot, the solid fat forms in summer 40 per cent. of the b.u.t.ter, but in winter the proportion rises to 65. This decrease in the proportion of the liquid fat in winter is the cause of the greater hardness of the b.u.t.ter in that season, which is often incorrectly attributed solely to the cold.

The cheesy and acid matters contained in b.u.t.ter are by no means essential; on the contrary, if it were quite free from them, it might be retained with little or no salt for a very long period without becoming rancid. The cheesy matter contains nitrogen; and nearly all the substances into which this element enters as a const.i.tuent are remarkably p.r.o.ne to decomposition. Yeast, and ferments of every kind--gunpowder, fulminating silver, chloride of nitrogen--and almost every explosive compound, contain this element. The cheesy matter is a very nitrogenous body, and in presence of air and moisture not only rapidly decomposes, or decays, itself, but induces by mere contact a like state of decomposition in other substances--such, for instance, as fat, sugar, and starch, which naturally have no tendency to change their state. Bearing the foregoing facts in mind, it is obvious that the chief precautions to be observed in the manufacture of b.u.t.ter are:--Firstly, to separate to as great an extent as practicable the casein from the b.u.t.ter; and, secondly, as in practice a small portion of the curd remains in the b.u.t.ter, to prevent it from undergoing any change--at least for a prolonged period. How these desiderata may best be accomplished I shall now proceed to point out.

_The b.u.t.ter Manufacture._--The theory of the process of churning is very simple. By violently agitating the milk or cream the little vesicles, or bags containing the b.u.t.ter, are broken, and, the fatty matter adhering, _lumps of b.u.t.ter_ are formed. The operation of churning also introduces atmospheric air into the milk, which, aided by the high temperature to which the fluid is raised, converts a portion of the _sweet_ sugar of milk into the _sour_ lactic acid. By the alteration produced in this way in the composition of the milk, it is no longer capable of holding the casein in solution, and the curd therefore separates.

The churn and other vessels in which the milk is placed cannot be kept too clean. No amount of labor bestowed on the scalding and scrubbing of the vessels is excessive. When wood is the material used in the milk-pans the utmost care should be taken in cleaning them, as the porous nature of the material favors the retention of small quant.i.ties of the milk. A simple was.h.i.+ng will not suffice to clean such vessels.

They must be thoroughly scrubbed and afterwards well scalded with _boiling_ water. Tin pans are preferable to wooden ones, as they are more easily cleaned, but in their turn they are inferior to gla.s.s vessels, which ought to supersede every other kind. Earthenware, lead, and zinc pans are in rather frequent use. The last-mentioned material is easily acted upon by the lactic acid of the sour milk, and is, therefore, objectionable. It is a matter of great importance that the dairy should not be situated near a pig-stye, sewer, or water-closet, the effluvia from which would be likely to taint the milk. It is surprising how small a quant.i.ty of putrescent matter is sufficient to taint a whole churn of milk; and as it has been demonstrated that the almost inappreciable emanations from a cesspool are capable of conferring a bad flavor on milk, it is in the highest degree important to remove from the churn and milk-pail every trace of the sour milk. I go further, it is even desirable that no one whose hands have a tendency to perspire should be allowed to manipulate in the dairy; and it should be constantly borne in mind that the dairy-maid's fingers and hot water should be on the most intimate visiting terms.

b.u.t.ter is made either from cream--sour and sweet--or from whole milk which has stood sufficiently long to become distinctly sour. It is a.s.serted by some makers that b.u.t.ter prepared from whole milk, or from scalded cream, contains a large proportion of curd. If this be true--which I greatly doubt--it is a serious matter, for such b.u.t.ter would speedily become rancid in consequence of the casein acting as a ferment. I believe that experience points to an exactly opposite conclusion. From the results of careful inquiries I feel no hesitation in a.s.serting that the b.u.t.ter should not be made from the cream, but from the _whole milk_. When made from the cream alone it is much more likely to acquire a bad taste, and is generally wanting in keeping qualities.

I have no doubt but that in the process of churning the whole milk there is a large amount of lactic acid formed, and a much higher temperature attained, than in the churning of cream; consequently, the separation of caseous matter must be more perfectly effected in the former than in the latter case. It is a mistake to think that there is very little casein in cream: out of 7 or 8 lbs. of thick cream only a couple of pounds of b.u.t.ter are obtainable; the rest is made up of water, casein, and sugar of milk. The yield of b.u.t.ter is greater when the whole milk is churned than when the cream alone is operated upon, and, what is of great importance, the quality of the b.u.t.ter is uniform during the whole year.

The labor of churning whole milk is, of course, much greater than if the cream alone were employed, but the increased yield and unvarying quality of the b.u.t.ter more than compensate for the extra expenditure of labor.

The proper temperature of the milk or cream is a point of great practical importance. If the fluid be too warm or too cold the b.u.t.tery particles will only by great trouble be made to cohere; and the quality of the b.u.t.ter is almost certain to be inferior. When the whole milk is operated on, the temperature should be from 55 to 60 degs. of Fahrenheit's thermometer; and if cream be employed the temperature should never exceed 55 degs. nor be lower than 50 degs. Hence it follows that in summer the dairy should be kept cooler, and in winter warmer, than the atmosphere. The temperature of milk is raised or lowered as may be found necessary, by the addition of hot or cold water--in performing which operations properly, a good thermometer is indispensable; one should always be kept in the dairy, and should be so constructed as to admit of being plunged into the milk. In some dairies the water, instead of being mixed with the milk, is put into a tub in which the churn is placed. There is a good kind of churn, which consists of two cylinders, the one within the other--the interval between them being intended for the reception of hot or cold water. The influence of temperature upon the production of b.u.t.ter has been placed beyond all doubt by numerous carefully-conducted experiments. Mr. Horsfall, a celebrated dairy farmer, in discussing this question, sums up as follows:--"By a series of carefully-conducted experiments at varying temperatures, I am of opinion that a correct scale of the comparative yield of b.u.t.ter at different temperatures might be arrived at; as thus: From a very low degree of temperature little or no b.u.t.ter; from a temperature of about 38 degs., 16 oz. from 16 quarts of milk; ditto, 45 degs., 21 oz. from 16 quarts of milk; ditto, 55 degs., 26 to 27 oz. from 16 quarts of milk."

This is a higher yield of b.u.t.ter than, I suspect, most dairymen get: but Mr. Horsfall's cows being of the best kind for milking, and well fed, the milk is, of course, rich in b.u.t.ter; and his experiments prove that even the richest milk will not throw up its b.u.t.ter unless at a certain temperature.

In the churning of cream the motion should be slow at first until the cream is thoroughly broken up. In churning milk the agitation should neither be violent nor irregular; about 40 or 50 motions of the plunger or board per minute will be sufficient. In steam-worked churns the motion is often excessively rapid, and the separation of the b.u.t.ter is effected in a few minutes; but the article obtained in this hasty way very quickly becomes rancid, and must be disposed of at once. An hour's churning of sour cream appears in general to produce good b.u.t.ter.

Sweet cream and whole milk require a longer period--the latter about 3 hours--but in any case prolonged churning is certain, by incorporating cheesy matter with the b.u.t.ter, to produce an inferior article.

Sweet milk becomes sour, evolves a considerable quant.i.ty of gas during churning, and its temperature ascends four or five degrees. Oxygen is unquestionably absorbed, and it is probable that a portion of the sugar of milk is converted into acid products.

I have already stated that even the most carefully prepared b.u.t.ter contains a small proportion of casein and sugar of milk. This casein is the good genius of the cheese-maker, but the evil genius of the b.u.t.ter manufacturer. How? In this way:--When b.u.t.ter containing a notable proportion of casein and sugar of milk is exposed to the air, the following changes take place: the casein pa.s.ses into a state of fermentation, and acting upon the sugar of milk, converts it, firstly into the bad-flavored lactic acid, and secondly into the bad odorous butyric, capric, and caproic acids. The first of these compounds in a state of purity emits an odor resembling a mixture of vinegar and rancid b.u.t.ter; the second possesses an odor resembling that of a goat--hence the name _capric_; the third has an odor like that of perspiration. In addition to these acids, there is another simultaneously generated--the caprylic, but it does not unpleasantly affect the olfactory nerve.

The casein also injuriously affects the fatty const.i.tuents of the b.u.t.ter; under its influence they absorb oxygen from the air, and become converted into strong-smelling compounds. The was.h.i.+ng of b.u.t.ter is intended to free it from the casein and unaltered cream, and the more perfectly it is freed from those impurities the better will be its flavor, and the longer it will remain without becoming rancid. Some people believe that too much water injures the quality and lessens the quant.i.ty of b.u.t.ter. It cannot do the former, because the essential const.i.tuents of b.u.t.ter are totally insoluble in water; it may do the latter, but, if it do, so much the better, because the loss of weight represents the amount of impurities--milk, sugar of milk, &c.--removed.

I have already remarked that b.u.t.ter is so susceptible of taint that even a perspiring hand is sufficient to spoil it; naturally cool hands should alone be allowed to come in contact with this delicate commodity, and the hands should be made thoroughly clean by repeated was.h.i.+ngs with warm water and oatmeal--the use of soap in the lavatory of the dairymaid being highly objectionable. Wooden spades are now being commonly made use of in manipulating the b.u.t.ter, and there is no good reason why they should not come into universal use.

The yield of b.u.t.ter per cow is subject to great variation. Some breeds of the animal are remarkable as milkers; such, for instance, as the Alderneys and Kerrys--indeed, I may say all the small varieties of the bovine race. There are instances of cows yielding upwards of twenty pounds of b.u.t.ter per week, but these are extraordinary cases. In Holland a good cow will produce, during the summer months, more than 180 lbs.

of b.u.t.ter. In these countries I think the average annual yield of a cow is not more than 170 lbs. It sometimes happens that cows yield a large quant.i.ty of milk and a small amount of b.u.t.ter, but it far more frequently occurs that the cow which gives most milk also yields most b.u.t.ter.

An estimate of the amount of b.u.t.ter contained in milk may be made by determining the amount of cream. This may be effected by means of an instrument termed a _lactometer_, which is simply a gla.s.s tube about five inches long, and graduated into a hundred parts. The specimen to be examined is poured into this tube up to zero or 0, and allowed to stand for twelve hours in summer and sixteen or eighteen in winter. At the end of that time the cream will have risen to the top, and its per-centage may be easily seen. In good milk the cream will generally extend 11 to 15 degrees down from 0. This instrument, although very useful, is not reliable in every case, especially in detecting the adulteration of milk.

I have already stated that the complete separation of the b.u.t.ter from the other const.i.tuents of the milk is never accomplished in the dairy.

Now although the proportion of curd in the b.u.t.ter is very small--rarely more than two per cent. and often not a fourth of one per cent.--yet it is more than sufficient, under a certain condition, to cause the b.u.t.ter to become speedily rancid. That condition is simply contact with the air. If the curd, before it becomes dry and firm, is subjected to the influence of the air, it rapidly pa.s.ses into a state of fermentation, which is very soon communicated to the fatty and saccharine const.i.tuents of the b.u.t.ter (substances not spontaneously liable to sudden changes in composition) and those peculiar compounds--such, for example, as butyric and capric acids, are generated, which confer upon rancid b.u.t.ter its characteristic and very disagreeable odor and flavor. The fermentation of the curd is prevented by incorporating common salt with the b.u.t.ter, and by preventing, so far as possible, the access of air to the vessels in which the article is placed. If fresh b.u.t.ter be placed in water--which apparently protects it from the influence of the air--it will soon become rancid. The reason of this is, that water always contains air, which differs in composition, though derived, from the atmosphere, by being very rich in oxygen. Now, it is precisely this oxygen which effects those undesirable changes in the casein, or curd, to which I have so repeatedly referred; hence its presence in a concentrated state in water causes that fluid to produce an injurious effect on the b.u.t.ter placed in it. A saturated solution of salt contains very little air, and, so long as the curd is immersed therein, it undergoes no change. The salt, too, acts as a decided preservative; for although it was long considered to be capable of preserving animal matters, merely by virtue of its property of absorbing water from them (the presence of water being a condition in the decomposition of organic matter), it has lately been shown to possess very antiseptic properties.

The mixing of the salt with the b.u.t.ter is effected in the following manner:--The b.u.t.ter, after being well washed, in order to free it from the b.u.t.ter-milk, is spread out in a tub, and the salt shaken over it; the b.u.t.ter is then turned over on the salt by the lower part of the palm of the hand, and rubbed down until a uniform mixture is attained. A good plan in salting is to mix in only one half of the quant.i.ty of salt, make up the b.u.t.ter in lumps, and set them aside until the following day; a quant.i.ty of milk is certain to exude, which is to be poured off, and then the rest of the salt may be incorporated with the b.u.t.ter.

According to b.u.t.ter-makers, the quality of the article is greatly dependent on the quality of the salt used in preserving it. I think there is a good deal of truth in this belief, and I therefore recommend that only the very best and _driest_ salt should be used in the dairy.

Common salt is essentially composed of the substance termed by chemists chloride of sodium, but it often contains other saline matters (chloride of magnesium, &c.), some of which have a tendency to absorb moisture from the air, and to dissolve in the water so obtained. These salts are termed _deliquescent_, from the Latin _deliquere_, to melt down. When, therefore, common salt becomes damp by mere exposure to the air, it is to be inferred that it contains impurities which, as they possess a very bitter taste, would, if mixed with b.u.t.ter, confer a bad flavor upon it.

The impurities of salt may be almost completely removed by placing about a stone weight of it in any convenient vessel, pouring over it a quart of boiling water, and mixing thoroughly the fluid and solid. In an hour or two the whole is to be thrown upon a filter made of calico, when the water will pa.s.s through the filter, carrying with it all the impurities, and the purified salt, in fine crystals, will remain upon the filter.

The solution need not be thrown away: boiled down to dryness it may be given as salt to cattle; or, if added in solution to the dung-heap, it will augment the fertilising power of that manure.

The proportion of salt used in preserving b.u.t.ter varies greatly. When the b.u.t.ter is intended for immediate use, I believe a quarter of an ounce of salt to the pound is quite sufficient; but when designed for the market, about half an ounce of salt to the pound of b.u.t.ter will be sufficient. Irish b.u.t.ter at one time commanded the highest price in the home and foreign markets, but latterly it has fallen greatly in public estimation; indeed, at the present moment the price of Irish b.u.t.ter at London is nearly twenty s.h.i.+llings per cwt. under that of the Dutch article. It is really painful to be obliged to admit that the Irish farmer is solely to blame for this remarkable depreciation in the value of one of our best agricultural staples. In a word, by the stupid (and _recent_) practice of putting into b.u.t.ter four times the quant.i.ty of salt necessary to its preservation, the Irish dairy farmers--or at least the great majority of them--have completely ruined the reputation of Irish b.u.t.ter in those very markets in which, at one time, the Cork brand on a firkin was sufficient to dispose of its contents at the very highest price. It is a great mistake to think that the greater the quant.i.ty of salt which can be incorporated with the b.u.t.ter, the greater will be the profit to the producer. No doubt, every pound of salt sold as a const.i.tuent of b.u.t.ter realises a profit of two thousand per cent.; but then the addition of every pound of that substance, after a certain quant.i.ty, to the cwt. of b.u.t.ter depreciates the value of the latter to such an extent as to far more than neutralise the gain on the sale of salt at the price of b.u.t.ter. In the county of Carlow, less salt is used in preserving b.u.t.ter than is the case in the county of Cork and the adjacent counties; the price, therefore, which the Carlow commodity commands in the London market is higher than that of the Cork b.u.t.ter: but in every part of Ireland the proportion of salt added to the b.u.t.ter is excessive.

The results of the a.n.a.lyses of b.u.t.ter supplied to the London market, made by the _Lancet_ a.n.a.lytical Commission, showed that the proportion of salt varied from 030 to 824 per cent. The largest proportion of salt found in fresh b.u.t.ter was 221 and the least 030. In salt b.u.t.ter the highest proportion of salt was 824 and the lowest 153. The b.u.t.ter which contained most salt was also generally largely adulterated with water. Indeed, in several samples the amount of this const.i.tuent reached so high as nearly 30 per cent. Nothing is easier than the incorporation of water with salt b.u.t.ter. The b.u.t.ter is melted, and whilst cooling the salt and water are added, and the mixture kept constantly stirred until quite cold. In this way nearly 50 per cent. of water may be added to b.u.t.ter; but of course the quality of the article will be of the very worst kind.

A correspondent of the _Lancet_ states that, on awakening about three o'clock in the morning at the house in which he was lodging, he perceived a light below the door of his room; and apprehending a fire, he hurried down stairs, and was not a little surprised to discover the whole family engaged in manipulating b.u.t.ter. He was informed in a jocose way that they were making Epping b.u.t.ter! For this purpose they used inferior Irish b.u.t.ter, which, by repeated was.h.i.+ngs, was freed from its excessive amount of salt; after which it was frequently bathed in sweet milk, the addition of a little sugar being the concluding stroke in the process. This "sweet fresh b.u.t.ter from Epping" was sold at a profit of 100 per cent. Our dairy farmers might take a hint from this anecdote.

Does it not prove that the mere removal of the salt added to Irish b.u.t.ter doubles the value of the article?

It is as necessary to pay attention to the packing of b.u.t.ter as it is to its salting. If old firkins be employed, great care should be taken in cleaning them, and if the staves be loose, the firkins should be steeped in hot water, in order to cause the wood to swell, and thereby to bring the edges of the staves into close contact. New firkins often communicate a disagreeable odour to the b.u.t.ter. In order to guard against this, it is the practice in many parts to fill the firkins with very moist garden mould, which, after the lapse of a few days, is thrown out, and the firkin thoroughly scrubbed with hot water, rinsed with the same fluid in a cold state, and finally rubbed with salt, just before being used.

In packing the b.u.t.ter, the chief object to be kept in view is the exclusion of air. In order to accomplish this, the lumps of b.u.t.ter should be pressed firmly together, and also against the bottom and sides of the vessel. When the products of several churnings are placed in the same firkin, the surface of each churning should be furrowed, so that the next layer may be mixed with it. A firkin should never be filled in a single operation. About six inches of b.u.t.ter of each churning will be quite sufficient, and in a large dairy two or more firkins can be gradually but simultaneously filled. I strongly recommend the removal of the pickle jar from the dairy. When the layers of b.u.t.ter have been carried up to within an inch or so of the top of the firkin, the s.p.a.ce between the surface of the b.u.t.ter and the edge of the vessel should be filled with fine dry salt, instead of pickle. A common mistake made is the holding over for too long a time of the b.u.t.ter: the sooner this article can be disposed of the better, for _it never improves by age_.

[Footnote 23: From two Greek words, signifying odour and soup.]

[Footnote 24: "A New Inquiry, fully ill.u.s.trated by coloured engravings of the heart, lungs, &c., of the Diseased Prize Cattle lately exhibited at the Smithfield Cattle Club, 1857." By Frederick James Gant, M.R.C.S.

London, 1858.]

[Footnote 25: Professor John Wilson's Report of the Agricultural Exhibition, Aarhuus, 1867.]

PART V.

ON THE COMPOSITION AND NUTRITIVE VALUE OF VEGETABLE FOODS.

SECTION I.

THE MONEY VALUE OF FOOD SUBSTANCES.

The flesh-forming principles of food are, as I have already stated, almost identical with the princ.i.p.al nitrogenous const.i.tuents of animals.

Unlike the non-plastic substances, they are convertible into each other with little, if any, loss either of matter or of force. Not many years since it was the fas.h.i.+on to estimate the nutritive value of a food-substance by its proportion of nitrogen; but this method--not yet quite abandoned--was based on erroneous views, and yielded results very far from the truth. No doubt all the more concentrated and valuable kinds of food are rich in nitrogenous principles; but there are other varieties, the nutritive value of which is very low, and yet their proportion of nitrogen is very high. This point requires explanation.

Both the plastic and the non-plastic materials of food exist in two distinct states--in one of which they are easily digestible, and in the other either altogether una.s.similable or so nearly so as to be almost useless. Thus, for example, the cellular tissue of plants, when newly formed, is to a great extent digestible, whilst the old woody fibre is nearly, if not quite, incapable of a.s.similation. Gelatine, which in raw bones is easily digested in the stomachs of the carnivora, loses a large proportion of its nutritive value on being subjected to the action of steam. Again, a portion of the nitrogen of young succulent plants is in a form not sufficiently organic to admit of its being a.s.similated to the animal body. But, independently of these strong objections to the method of estimating the nutritive value of food by its per-centage of flesh-formers, there are many other reasons which as clearly prove the fallacy of this rule. If we were, for instance, to estimate the value of alb.u.men according to the tables of food equivalents which were constructed some years ago by Boussingault and other chemists, we would find one pound weight of it to be equivalent to four pounds weight of oil-cake, or to twelve pounds weight of hay; yet, it is a fact that a horse would speedily die if confined to a purely alb.u.minous diet, whereas hay is capable of supporting the animal's life for an indefinite period.