Cheese. — The substance known by this name consists chiefly of casein; which hes been precipitated from the milk in company with a variable quantity of its buttery con stituent. Its dietetic value is of course very high. But its digestive properties vary greatly; according to the proportion of fatty matter and salts which it contains, the mechanical aggregation of its mass, and the degree of decomposition which it may have experienced.
Thus as regards its admixture of butter, we inay distinguish three varieties of cheesi:— one which is made from cream, or from milk with the addition of cream ; one from pure milk ; and one froni milk which has been skimmed or deprived of its cream.
In respect to its salts, the chief distinction hitherto established appears referrible to the way in which the casein has been precipitated from its solution in the milk. Where the process has been effected by the addition of rennet, the caseous deposit contains a large proportion — about 5 or 6 per cent.— of phosphate of lime. But where the precipita tion has been produced by the lactic acid which is gradually developetl in milk as the result of its own spontaneous decomposition, the deposit contains scarcely one per cent. of this salt. In such a case, however, the smaller amount of phosphates appears to be partially compensated by the presence of some free phosphoric acid.
The changes which cheese undergoes by keeping are chiefly manifested in the forma tion of various volatile fatty acids, that o'ene rally communicate their characteristic :dour to the whole mass. Such alterations are usnally most marked in those varieties of cheese, in which but a small proportion of rennet has been used, and much fatty matter is present. Bence they seem at least par tially attributable to a metamorphosis—pro bably an oxidation — of the buttery con stituents themselves. In addition to this change, however, the casein also undergoes a somewhat similar fermentation; which is ac companied by the production of oxides of casein, and volatile fatty acids. Occasionally the process is 'carried so far as to constitute a kind of putrefaction, in which the nitrogen originally present is given off in the form of ammonia. The highly poisonous properties which decayed cheese sometimes possesses, and the repulsive odour which it often gives off, may illustrate these statements.
The value of cheese as an article of food rnay be to some extent inferred from the large amount of its proteinous constituent, which often forms more than 70 per cent. of its whole weight. This quantity of casein would correspond to about I per cent. of
nitrogen : a quantity far beyond that contained in an/ other ordinary variety of azotized food. But just as this unexampled chemical com position may suffice to indicate how largely such a proportion of the "histogenetic" prin ciples would require to be diluted with the "respiratory" or " combustible" substances, in order to constitute a food in the true accep tation of the term, — so it partially explains the fact, that cheese is anything but easy of digestion. With many persons even milk is only digested with difficulty; so that much of its casein may be traced through the bowels, but little changed by the action of the gastric juice. And the mechanical aggregation of many kinds of cheese--their extreme hard ness, dryness, and clensity,--often enable them almost to defy digestion. But minute division, cooking, or careful mastication,will obviate one of these objections; and the other is easily met by a proper admixture of vegetable food. With such precautions, cheese becomes a most valuable article of food. So that we need be little surprised to find the extreme value and importance assigned to this variety of azotized aliment amongst rural populations where meat is scarce and expensive. Indeed, the diet on which tradition states old Parr to have attained his remarkable age can hardly have been very unwholesome. And the natives of a country which, like ours, still boasts of large cheese-fairs in some of its country- towns, can find little to wound their national pride in the quaint fancy of Mueller that cheese and freedom flourish together.
Vegetable food.— The general characters of vegetable food have already been alluded to. They are, however, modified by all cir cumstances which materially affect the ar rangement or composition of the vegetable tissues. Thus yottng plants, or the younger shoots of plants, are much more easily digested than the harder and less soluble textures of the older organism. While the approach of fruits towards their maturity determines a series of physical and chennical alterations, which have the result of rendering them much more nutritious. The mode of culture, and the peculiarities of the soil, also exercise im portant influences on the resulting vegetable produce. Thus a rich soil, a warm climate, or a highly azotized manure, have all been noticed to increase the per-centage of protein contained in the corn grown under their in fluence. And the influence of such circum stances will, of course, be in some degree extended to the persons and animals, whose staple food is thus partially dependent upon them.