The Chemistry of Food and Nutrition - A. W. Duncan (latest books to read .TXT) 📗

Some of the "Liebig's Extract of Meat" so called, contains yeast extract; some even, is almost entirely, if not altogether made from yeast. The latter can be manufactured at a very low cost from brewers' and distillers' waste products, and there is a strong incentive for unscrupulous dealers to substitute it secretly. Artificial meat extracts prepared from yeast have the appearance and taste of meat extracts, but some, at least, have a considerably sharper flavour. In one method of manufacture common salt is added, and this renders it unfit for use in more than very small quantities as a flavouring. J. Graff has made analyses of ten yeast extracts, and contrasted them with meat extracts (see Analyst 1904, page 194), and says, "It will be seen that the chemical composition of yeast extract does not greatly differ from that of meat extract." Yeast extracts contain purin bodies, and are probably equally as injurious as meat extracts. Such strong and rank flavours (the odour is suggestive to us of putrefaction) should be discouraged by those who would cultivate a refined taste in food.

Flesh Bases and Waste Products.—As the result of destructive metamorphosis or the wearing out of the body, there remain certain waste products which have to be expelled as soon as is possible. Their retention and accumulation would soon produce death. A part is expelled by the lungs as carbon-dioxide, or as it is generally though less correctly termed, carbonic acid. Upon the breaking down of the complex proteid and other nitrogenous matter, the nitrogen is left in comparatively simple combinations. These effete nitrogen compounds are commonly termed flesh bases or nitrogenous extractives. They exist in small quantity in flesh meat, but are concentrated and conserved in the making of beef-tea or beef-extract. The spleen, lymphatic and other glands, and especially the liver, break these down into still simpler compounds, so that the kidneys may readily separate them from the blood, that they may pass out of the body. By far the largest part of this waste nitrogen is expelled from the bodies of men and many other mammals in the form of urea. Pure urea is an odourless transparent crystalline substance, of cooling saline taste like nitre. It is soluble in an equal volume of water, and is expelled from the body with great ease. In the herbivora the nitrogenous waste takes the form of another body called hippuric acid. The nearly solid light-coloured urinary excretion of birds and serpents consists of urates; this is uric acid in combination with alkalies. In man, in addition to the urea excreted, there is also a little hippuric and uric acid or compounds of these. Uric acid is a transparent colourless crystalline body almost insoluble in water but soluble as urates in the presence of alkalies. As deposited from urine it is of a dull red sand-like appearance, as it has a great affinity for any colouring matter that is present.

It is only possible to make a brief reference to the chief organic bases. The xanthine bases are closely related to uric acid. Some of these occur in small quantity in the urine and animal tissues, others, such as caffeine, occur in plants. Creatine is a constant constituent of muscle substance. In fowl's flesh there is said to be 0.32 per cent., in cod-fish 0.17 per cent., and in beef 0.07 per cent. Creatinine is produced from creatine with great facility; it exists in urine. Both creatine and creatinine are readily soluble in water. A series of bases, closely allied to creatinine have been isolated from the flesh of large animals by A. Gautier; they are known as Gautier's flesh bases. When administered to animals, these act more or less powerfully on the nerve centres, inducing sleep and in some cases causing vomiting and purging in a manner similar to the alkaloids of snake venom, but less powerfully than the ptomaines. These bases are formed during life as a result of normal vital processes and are termed leucomaines.

Another class of bases of an alkaloidal nature, are termed ptomaines; these differ from the leucomaines, being produced by putrefactive or bacterial agency from dead flesh. The poisoning which has occasionally resulted from the eating of sausages, pork-pies, tinned meats, etc., is due to their having contained ptomaines.

Such quantities of waste products as are produced in the healthy body are excreted with ease, but it is otherwise in certain diseases. Either specially noxious substances are produced, or the usual substances are in excessive quantity and not eliminated with sufficient rapidity; in consequence the body is poisoned. Those who eat largely of flesh, introduce into their system the excretory matter contained therein, which super-added to the excretory matter resulting from the vital processes of the body puts an unusual and unnatural strain upon the liver and kidneys. It has been observed, that the eating of the flesh of some trapped animals has produced severe symptoms of poisoning. The pain and horror of having a limb bleeding and mangled in a most cruel steel trap, the struggles which only add to the misery, slowly being done to death during hours or even days of torture, has produced in their bodies virulent poisons. Leucomaine poisons have also been produced by the violent and prolonged exertions of an animal, fleeing from its pursuers, until its strength was completely spent. Cases are also known, where a mother nursing her infant, has given way to violent anger or other emotion, and the child at the breast has been made violently ill. We must not expect the flesh of any hunted or terrified animals to be wholesome. Animals brought in cattle ships across the Atlantic, suffer acutely. After rough weather they will often arrive in a maimed condition, some being dead. To this is added the terror and cruelty to which they are subjected whilst driven by callous drovers, often through a crowded city, to the slaughter house to which they have an instinctive dread. It is only to be expected that the dead flesh from such animals, should contain an unusually large quantity of the more poisonous flesh bases.

Purin Bodies.—The term purin has been applied to all bodies containing the nucleus C5N4. It comprises the xanthine group and the uric acid group of bodies. The principal purins are hypoxanthin, xanthin, uric acid, guanin, adenin, caffeine and theobromine. Purins in the body may either result from the wear and tear of certain cell contents, when they are called endogenous purins; or they are introduced in the food, when they are distinguished as exogenous purins. These purins are waste products and are readily converted into uric acid. The production of some uric acid by tissue change is, of course, unavoidable; but that resulting from the purins in food is under control.

An excess of uric acid is commonly associated with gout and similar diseases. The morbid phenomena of gout are chiefly manifested in the joints and surrounding tissues. The articular cartilages become swollen, with ensuing great pain. There is an accumulation of mortar like matter about the joints. This is calcium urate (not sodium urate as is generally stated). These nodular concretions are called tophi or chalkstones.

Very many are the hypotheses which have been propounded on the cause of gout and the part played by uric acid; many have had to be discarded or greatly modified. Though much light has recently been thrown on the subject, there remains much that is obscure. The subject is one which is surrounded with great difficulties, and would not be suitable for discussion here, were it not for the following reason: Certain views on uric acid as the cause of gout and several other diseases, are at the present time being pushed to the extreme in some health journals and pamphlets. Unfortunately many of the writers have very little knowledge, either of chemistry or physiology,