That curious epicurean food, the edible birds’-nests, which has been a subject of much controversy concerning its composition, is commonly described as a delicate kind of gelatin. This does not appear to be quite correct. It is certainly gelatinous in its mechanical properties, but it more nearly resembles the material of the slime and organic tissue of snails, a substance to which the name of mucin has been given. Thus the birds’-nest soup of the East and the snail soup of the West are nearly allied, and that made from callipash and callipee supplies an intermediate reptilian link.
The birds’-nests, when cleaned for cooking, are entirely composed of the dried saliva of swallows, or rather swiftlets (collocalia), and this saliva probably contains some amount of digestive ferment or pepsin, which may render it more digestible than the vulgar product from shin of beef, and consequently more acceptable to feeble epicures. Those who have sufficient vital energy to supply their own saliva will probably prefer the vulgar concoction to the costly secretion. The bird saliva sells for its own weight in silver, when freed from adhering impurities.[6]
Those who are disposed to bow too implicitly to mere authority in scientific matters will do well to study the history and the treatment which gelatin has received from some of the highest of these authorities. Our grandmothers believed it to be highly nutritious, prepared it in the form of jellies for invalids, and estimated the nutritive value of their soups by the consistency of the jelly which they formed on cooling, which thickness is due to the gelatin they contain. Isinglass, which is simply the swim-bladder of the sturgeon and similar fishes cut into shreds, was especially esteemed, and sold at high prices. This is the purest natural form of gelatin.
Everybody believed that the callipash and callipee of the alderman’s turtle soup contributed largely to his proverbial girth, and those who could not afford to pay for the gelatin of the reptile, made mock turtle from the gelatinous tissues of calves’-heads and pigs’-feet.
About fifty or sixty years ago, the French Academy of Sciences appointed a bone-soup commission, consisting of some of the most eminent savants of the period. They worked for above ten years upon the problem submitted to them, that of determining whether or not the soup made by boiling bones until only their mineral matter remained solid, is, or is not, a nutritious food for the inmates of hospitals, &c. In the voluminous report which they ultimately submitted to the Academy, they decided in the negative.
Baron Liebig became the popular exponent of their conclusions, and vigorously denounced gelatin, as not merely a worthless article of food, but as loading the system with material that demands wasteful effort for its removal.
The Academicians fed dogs on gelatin alone, found that they speedily lost flesh, and ultimately died of starvation. A multitude of similar experiments showed that gelatin alone will not support animal life, and hence the conclusion that pure gelatin is worthless as an article of food, and that ordinary soups containing gelatin owed their nutritive value to their other constituents. According to the above-named report, and the statements of Liebig, the following, which I find on a wrapper of Liebig’s ‘Extract of Meat,’ is justifiable: ‘This Extract of Meat differs essentially from the gelatinous product obtained from tendons and muscular fibre, inasmuch as it contains 80 per cent. of nutritive matter, while the other contains 4 or 5 per cent.’ Here the 4 or 5 per cent. allowed to exist in the ‘gelatinous product’ (i.e. ordinary kitchen stock or glaze), is attributed to the constituents it contains over and above the pure gelatin.
The following, from a text-book largely used by medical students,[7] shows the estimation in which gelatin was held at that date: ‘But there is another azotised compound, Gelatin, that is furnished by animals, to which nothing analogous exists in Plants; and this is commonly reputed to possess highly nutritious properties. It may be confidently affirmed, however, as a result of experiments made upon a large scale, that Gelatin is incapable of being converted into Albumen in the animal body, so that it cannot be applied to the nutrition of the albuminous tissues. And, although it might à priori be thought not unlikely that Gelatin, taken in as food, should be applied to the nutrition of the gelatinous tissues, yet neither observation nor experiment bears out such a probability.’ Further on, Dr. Carpenter says: ‘The use of gelatin as food would seem to be limited to its power of furnishing a certain amount of combustive material that may assist in maintaining the heat of the body.’
Subsequent experiments, however, have refuted these conclusions. I must not be tempted to describe them in detail, but only to state the general results, which are, that while animals fed on gelatin soup, formed into a soft paste with bread, lost flesh and strength rapidly, they recovered their original weight when to this same food only a very small quantity of the sapid and odorous principles of meat were added. Thus, in the experiments of MM. Edwards and Balzac, a young dog that had ceased growing, and had lost one-fifth of its original weight when fed on bread and gelatin for thirty days, was next supplied with the same food, but to which was added, twice a day, only two tablespoonfuls of soup made from horseflesh. There was an increase of weight on the first day, and, ‘in twenty-three days the dog had gained considerably more than its original weight, and was in the enjoyment of vigorous health and strength.’
All this difference was due to the savoury constituents of the four tablespoonfuls of meat soup, which soup contained the juices of the flesh, to which, as already stated, its flavour is due.