CHEESE.

§ 188. Cheese consists essentially of the coagulated albuminous matters of milk, especially of casein, with a variable quantity of fat, common salt, and alkaline and earthy phosphates. Cheese may be made from the milk of any animal, but the great majority of cheeses in commerce are made from that of the cow. The principle of the manufacture of cheese from fresh milk is very simple: the casein is precipitated by rennet, and carries down with it most of the milk-fat, as well as some portion of the milk-sugar. The thin whey is allowed to run off, and the precipitated "curds" submitted to pressure, which has the effect of not only getting rid of the whey, but also of giving to the mass shape and consistency. Cheese may be

made from sour milk without the addition of rennet, the lactic acid precipitating the casein; but most of the cheeses in commerce are made from fresh milk. Cheeses may be divided into two varieties--the soft and the hard; the former are manufactured by precipitating with rennet at a low temperature, and using but little pressure; they have mostly an alkaline reaction. The hard cheeses are subjected to a higher temperature and stronger pressure, and have, when first made, an acid reaction.

§ 189. Soft Cheeses. - Examples of soft cheeses are cream cheese. Neufchatel (a Swiss cream cheese), fromage de Brie, and Camembert.

Hard Cheeses. Examples of hard cheeses are American, Cheddar, Stilton, Dunlop, Gloucester, and others.

The general composition of the chief cheeses of commerce may be gathered from the following table:

* An analysis of curds by M. Rubner is as follows:

$ 190. Parmesan Cheese is a peculiar cheese, never made in this country, but imported from Parma and elsewhere. The essential points in the manufacture are, that the rennet is heated to about 46°6 (116° Fahr.), and an hour afterwards the milk set over a slow fire until heated to about 65°5 (150° Fahr.) These operations cause the curd to separate in hard lumps. It is usually coloured with saffron. The outer crust of the cheese at the end of fourteen days is cut off, the new surface varnished with linseed oil, and one side coloured red. It is a very dry cheese, with a large amount of casein, and only a moderate percentage of fat.

§ 191. The Ripening of Cheese.-The transformation that cheese undergoes, and by which it usually acquires a more agreeable taste and flavour, is without doubt a fermentation of a slow character, induced by the agency of minute mycoderms; possibly, as F. Cohn suggests, the very active thread-bacteria, which rennet always contains, have something to do with the process. Fresh cheese has an acid reaction, but the development of ammonia ultimately renders the reaction alkaline. There is with age a continuous loss of water, and a slow development of carbon dioxide the fat decomposes, and the fatty acids * unite in part with the lime and in part with the ammonia. The casein also gradually passes into a soluble condition, so that a cheese, which originally gave very little to water, ultimately becomes almost wholly soluble.

Blondeau and others considered that, in the ripening of cheese, the albuminoids slowly changed into fat; thus a Roquefort, a cheese made from ewe's milk, analysed by Blondeau,† had when fresh

The same cheese, in the condition most highly prized (after it had been kept two months in a cold cellar), had the following composition :

* Valerianic acid has been detected in a Roquefort cheese by M. Balard, and by Messrs. J. Genko and Laskowski in a Limbourg cheese.

+ Annales de Chimie et de Physique, (4), t. i., 1864.

The cheese was again analysed at the end of a year, and its composition found to be as follows:

Alexander Müller has, however, shown that the albuminoid and fatty constituents of old cheese bear the same relation to each other as in new cheese, which he proves by calculating the determinations according to an equal quantity of water,

N. Siber* has also contested the change of albuminoids into fat, and gives the following analyses of a Roquefort :—

Now, although there is in the above analyses an apparent increase in the fat, yet, if reckoned on 100 parts of the dry substance, there is no very decided change. Thus

Brassier, some years ago, made several careful analyses, which may be of assistance in following the changes that cheese undergoes through age. Five pieces of the same cheese in the salted and unsalted condition were analysed at the end of two, four, and seven months, the results of which are tabulated in Table XXI. The development of ammonia, the production of nitro* Journ. f. Prak. Chemie, xxi. 213.

+ Ann. de Chem. Phys., v.

genous products soluble in alcohol, and the wasting of the fat and total nitrogen, are well shown in these analyses.

The researches on the "ripening" of cheese hitherto made by no means exhaust the subject, and there appears room for much interesting work in this direction.

§ 192. The Analysis of Cheese.-The chief difficulty in the analysis of cheese is in the extraction of the fat; one method, recommended and used by the American chemists, is to take a weighed quantity of cheese, mix it intimately with anhydrous copper sulphate, and then exhaust it in a Soxhlet with petroleum, ether, or other suitable fat solvent. A second method is to rub up 25 to 50 grms. of the cheese with sand or gypsum, and similarly exhaust with ether or petroleum. The fat in cheese may also be estimated by "whirling." Chattaway, Pearmain, and Moor operate as follows, using a Leffmann and Beam apparatus:-2 grms. of cheese are reduced to as fine a division as possible, transferred to a small dish, and heated in the waterbath with 30 cc. of concentrated hydrochloric acid until dissolved. The mixture is poured into a Leffmann-Beam bottle, the dish rinsed with the hydrochloric acid and fusel-oil mixture into the bottle, and, finally, enough strong hot acid to fill the