Cheese consists essentially of the caseine and albumen of milk, together with water, fat, lactic acid, and mineral salts. It is prepared by the coagulation of milk by means of rennet, and is usually obtained from cow’s milk (either fresh, skimmed, or sour), although the milk of the goat, ewe, and other animals is occasionally used. Its colour is very often due to the addition of annato. The following table exhibits the composition of the best-known varieties of cheese, according to the analysis of various chemists:—

Dr. Muter has published the following analyses of cheese:—[47]

According to this chemist, one gramme of genuine cheese should require not less than 220 milligrammes K(OH) for saponification, as executed in Koettstorfer’s process (see p. 71).

The following results were obtained by Griffiths[48] from the analysis of American cheese, and by Gerber[49] from the analysis of artificial American cheese:—

The constituents of cheese are very similar to those of milk; the relations between the soluble and insoluble fatty acids is much the same as in butter. In cheese, however, the milk-sugar is largely decomposed into lactic acid, alcohol, and carbonic acid, during the process of ripening or curing employed in its manufacture.

Another essential change effected by the curing of cheese is the partial decomposition of the caseine into ammonia, which combines with the unaltered caseine, forming soluble ammonium caseates. Other products of the ripening process, also due to the decomposition of the caseine, are tyrosine and leucine (C6H13NO2). The butter-fats are likewise transformed into the corresponding fatty acids, which give rise to the formation of either the ammonia salts, acid albuminates, or amines, such as butylamine or amylamine.

The characteristic odour of many varieties of cheese is chiefly owing to the genesis of these latter compounds.

As with butter, the most important adulteration of cheese consists in the addition of foreign fats. Doubtless, the most frequent sophistication is the admixture of lard. Lard cheese (which is usually sold as “Neufchatel”) is made by first preparing an emulsion of lard and skimmed milk (in the proportion of one part of the former to two parts of the latter). This is subsequently incorporated with skimmed milk and butter-milk, the coagulation of the fat being then effected in the usual manner. In regard to the production of this species of cheese, it is stated that in the 23 factories in the State of New York, the product of six months’ working (ending November, 1881), was about 800,000 pounds, of which the greater proportion was exported. The recent (1885) adoption of a New York State brand for “pure cream cheese” has had a very good effect, and accomplished much in the restriction of the manufacture and sale of the spurious article. Another variety of imitation cheese, know as “anti-huff cheese,” is prepared from skimmed milk without the addition of foreign fat, but with the aid of various chemical preparations, such as caustic or carbonated soda, saltpetre, and borax. The rind of cheese is occasionally contaminated with poisonous metallic salts, including those of lead, mercury, antimony, arsenic, copper and zinc, which are added either for colouring purposes or to prevent the attacks of flies and other insects. This form of adulteration is doubtless of rare occurrence. The methods used in cheese analysis are much the same as those employed in the examination of butter. The fat is determined by exhaustion with ether (or preferably, petroleum naphtha), and evaporation, the remaining solids not fat being likewise dried and weighed. The difference between the combined weight of the fat and the solids not fat, and the amount of the sample taken, represents the proportion of water present. Lactic acid, while insoluble in petroleum naphtha, is also dissolved by ether, and can be estimated by digesting another portion of the sample with water, and titrating the filtered liquid with decinormal soda solution. Its weight is then to be deducted from the amount of fat previously obtained, in case ether was employed in this determination. The relative proportions of the soluble and insoluble fatty acids contained in cheese possess the same significance in indicating the presence of oleomargarine and other foreign fats as with butter; and they are determined by the same methods.

The examination of the colouring matter of cheese can be made by first neutralising the free lactic acid, separating the fat by agitation with water, filtering and drying; the fat is then tested with carbon disulphide and potassium hydroxide (see p. 77).