The object of the cheese-maker is to obtain in a solid form as large a proportion as possible of the casein and butter contained in the milk dealt with. The poverty in these constituents of the whey or liquid matter separated in the process of making cheese is therefore, to some extent, a measure of the success of the operation. The average composition of the whey drained off may be thus stated :— [table]

Milk, as is well known, if allowed to stand for some time, becomes thick, and is then separable into two portions— a solid white curd, and a greenish liquid whey. Such a coagulation and separation is essential in the making of cheese; but only to a small extent, in Holland and some other localities, is the natural acid coagulation taken advantage of. It has been assumed that the solid constituents of milk are held in solution by an alkaline substance, and that coagulation is the result of the neutralization of the alkali by the development within the fluid of lactic acid, as in the case of sour milk, or by the addition of an acid substance as is sometimes the practice; but this theoιy does not satisfactorily account for all the phenomena of coagulation. Acid substances, however, do readily curdle cheese, and hydrochloric acid, tartaric acid, vinegar, and cream of tartar have all been employed to produce coagulation for cheese-making. The curding is also, in practice, produced by the action of such substances as the juice of figs, and decoctions of thistle tops, artichoke flowers, the butter-wort, and other plants. But the substance used uniformly in Great Britain, and in all great cheese-producing districts, is rennet, a preparation of the fourth or digesting stomach of the suckling calf. Rennet is prepared by cutting up the membrane in strips, salting, smoking, and sometimes treating it with spices and aromatics. The influence of rennet is due to the fact of its exciting a kind of fermentative action;· but that it thereby changes the sugar of milk (lactin) into lactic acid, and so coagulates the casein, has been denied by Dr Voelcker, who holds its action to be “sui generis, and as yet only known by its effects.”

In the practice of cheese-making it is found necessary, in order to hasten the coagulating action of rennet, and to produce a curd of sufficient hardness, to heat the milk to a temperature which varies from 72° to 85° or 90° Fahr. The lower temperature, it is found, yields a soft cheese, retaining much whey aud ripening soon; while the increased heat produces a firm curd and a solid slow-ripening cheese. After due coagulation the curd is broken, and by a variety of manipulative processes as much as possible of the whey is drained away, and the curd is reduced to comparatively dry crumbly fragments. At this stage, sometimes, but not frequently, the curd is salted; it is then collected into a clean cheese-cloth, placed in a cheese vat of the form and size of the cheese to be made, and submitted to pressure in the cheese press. While in the press it is frequently turned, a good deal of whey meantime continuing to exude; and it is found that the amount of pressure has much influence on the solidity and rate of ripening of the cheese. As soon as a sufficient skin has formed on the cheese to preserve its shape, it is removed from the press and salted by repeated rubbings of salt over its surface. The cheese is then put aside in a clean, cool, airy situation for ripening, a process which takes a variable period according to the quality of the cheese, its method of preparation, and other circumstances.

The ripening of cheese is the result of a slow process of decay caused by a spontaneous fermentative action. In hard, solid, poor cheeses it acts very slowly, while in those which contain butter in large proportions its action is very energetic, and they cannot be preserved for any considerable period. Cheese when newly made has an acid reaction, but by degrees from without inwards the acid reaction becomes less apparent, and the cheese ripens. A portion of the casein suffers decomposition, evolving ammonia and ammoniacal bases which neutralize the acid of the cheese. In a similar way the fat is partly decomposed, and the resulting fatty acids also combine with the ammonia evolved by the casein. When this action is allowed to proceed too far the cheese becomes alkaline, putrefactive decay ensues, free ammonia is evolved, an offensive odour is produced, and sometimes even poisonous compounds are formed. A satisfactory indication of ripening found in Stilton and other rich cheeses is the appearance of a green mould, streaked throughout the mass, produced by the fungus Aspergillus glaucus. A red mould also develops from Sporendonema Casei, and when the ripening becomes advanced the cheese-mite, Acarus domesticus, is produced with great rapidity in inconceivable numbers.

In England the milk of cows only is used in the manufacture of cheese. Excluding the so-called cream cheeses— a preparation of a soft buttery consistence made from cream gently pressed, which must be used new and fresh, —British cheese may be divided into three classes:—1st, that made with whole milk plus cream; 2d, that made with whole milk; and 3d, that made with milk minus cream, or skim-milk. Stilton and double Gloucester belong to the first class, being made of morning milk to which the cream of the previous evening’s milking is added. Whole milk cheeses are represented by single Gloucester, Cheshire, Cheddar, and Dunlop; and the ordinary country cheese used by the labouring classes represents the third class. It must, however, be understood that all gradations in richness are met with in cheese, as seen by the table below, and that quality depends on other circumstances besides proportion of butter. Of foreign cheese imported into Great Britain, the most important in point of quantity and value is American; and since the introduction of the factory system of cheesemaking in the United States, this has greatly improved in quality and become an important and extensive article of commerce. A large quantity of cheese, both fat and poor, is also made and exported from Holland. Parmesan cheese and Gruyère cheese, which are highly relished on account of their flavour, are skim-milk products, the former being coloured and artificially flavoured with saffron. The celebrated French Roquefort cheese is made from ewe’s milk, and matured in the caves of Roquefort, where a uniform low temperature is found throughout the year. The accompanying table, compiled from various sources, gives the average composition of the principal kinds of cheese at present known in commerce: — [table]

As an article of food cheese is used in a double capacity. Rich cheese in an advanced stage of ripeness is eaten in small quantities partly on account of its piquancy, and partly also as a digestive stimulant. Skim-milk cheese, and all the varieties poor in fat, again, are valuable articles of food on account of their high percentage of nitrogenous matter, and the cheaper qualities are, on this account, extensively consumed among the classes by whom other animal food is not usually obtainable. As an article of ordinary diet, cheese labours under the disadvantage of being hard of digestion; and especially when it is toasted, as is frequently the practice, it really is, as has been observed, ‘ about as digestible as leather.” According to Dr Frankland’s experiments, the maximum amount of force produced by the complete <Jxidation of I lb of Cheshire cheese within the human body is 2704 foot-tons.

The imports of cheese into Great Britain during the year 1875 amounted to 1,626,413 cwt., of an estimated value of £4,705,229; of this quantity about one-half was imported from the United States and one-fourth came from Holland. Of course the imports only represent a small proportion of the total quantity consumed.