Fresh or unsalted butter of good quality should present a rich straw-yellow colour. At ordinary temperatures it has a firm uniform consistency, while it is soft enough to cut and spread easily under the knife without breaking or crumbling. It should possess a faint sweet odour, and a bland, soft, delicate flavour, melting in the mouth without any indication of grittiness. Pure butter is a complex chemical compound, consisting in large part of fats or glycerides of the non-volatile acids, palmitic acid, and butyroleic acid, with occasionally stearic acid. With these there occur small proportions of glycerides of the volatile acids, butyric, capronic, caprylic, and caprinic acid, to which the butter owes its distinguishing flavour and characteristics, as it has the non-volatile acids in common with other fats, though in different proportions. Butter when unadulterated and prepared with ordinary care should contain at least 85 per cent, of pure fat, the remainder consisting of casein, water, and salt. The casein is derived from milk, which is never perfectly washed out, but in butter of good quality this ought not to amount to more than from 3 to 5 per cent. Water may be present to the extent of from 5 to 10 per cent, without the butter being subject to a charge of adulteration; and a small proportion of salt is commonly worked into the butter in its preparation, but in what is sold as fresh or sweet this should only be from ⅜ to 2 per cent, of the whole weight.

When butter is exposed to the air for some time, especially in warm weather, or in hot, confined situations, it quickly becomes rancid, acquiring thereby a distinct disagreeable odour and a biting taste, owing to the development of a volatile fatty acid under the influence of a species of fermentation, which is doubtless caused by the nitrogenous substance, casein, it contains. The more completely, therefore, all milky and curdy matter is washed out of butter the less will be the tendency to set up and develop fermentation. The preservation or curing of butter depends for its efficacy on the employment of some agency by which fermentative action may be prevented; but there are also several ways by which its development may be retarded and the material kept sweet for a considerable period. Rancidity may be corrected to some extent by melting the affected butter and pouring it into ice-cold water. As a means also of retarding rancidity, butter is in some parts of France and the East melted up and heated till the water it may contain is evaporated, when the casein which rises as a scum to the surface is carefully skimmed off; but butter cannot be so melted without injuriously affecting its delicate flavour. By keeping fresh butter in a very cool place covered with pure water renewed daily, it will remain sweet for a considerable time. A still better method, recommended by Μ. Payen (Substances Alimentaires) is to use water acidulated with either tartaric acid or vinegar. It is also said that sugar in the form of a syrup poured over the butter is an excellent medium for retarding rancid fermentation. Butter, however, which is to be kept for a considerable length of time is “cured,” or preserved by incorporating with it some substance or substances which act upon the nitrogenous material it contains, and thus prevent fermentation; and for this purpose common salt is the agent chiefly relied on. The salt used should be pure, dry, and finely powdered. About 5 per cent, of salt is sufficient for the purpose of curing; and when the quantity exceeds 8 per cent, it ought to be regarded as an adulteration. Butter very lightly salted for keeping only a short time is said to be powdered. A mixture much used for curing butter in Continental dairies is thus prepared :— One part each of sugar and nitre are mixed up with two parts of common salt and reduced to a very fine powder. This mixture is thoroughly kneaded into the butter in the proportion of about 1 oz. to every lb. After standing over for a fortnight butter so prepared will be ready for use and have a soft, agreeable taste, which it will retain a long time. In the preservation of all butter, the exclusion of air, as much as possible, is of the utmost consequence. Jt is, therefore, packed for sale in oaken kegs or glazed earthenware jars, filled quite full, and covered with a clean linen cloth on which salt is sprinkled. When in use the kegs should also be closely covered over, and the surface of the butter kept under brine.

Butter of good quality is a most digestible form of fat, while its flavour is so delicate and little pronounced that it is always acceptable to the palate. It is used most extensively by all classes, not only in the direct form with bread at nearly every meal, but also as entering very largely into the preparation of pastry, puddings, sauces, fancy cakes, and biscuits. Taking into account the daily consumption of this article, it is evident that the amount used in a year by a population such as that of Great Britain must be very great, an inference borne out by the fact that in 1875 the imports were 1,619,808 cwts., valued at £9,050,025, and, though no means exist of accurately estimating the home produce, it may safely be regarded as equal to the whole imports. The countries whence butter is imported into Great Britain are chiefly Germany, Holland, Denmark, and especially France. A large proportion of the French butter comes from the department of Calvados—Isigny being the centre of the best butter-making district. The value of the total produce of France in 1867 was estimated at 250 millions of francs.

Butter is a substance which affords great scope for adulteration, and its composition makes accurate detection of certain foreign matters a matter of considerable labour and difficulty. Other animal fats, such as lard, beef and mutton dripping, and tallow, with certain vegetable fats, are the chief adulterants. Such adulterations may be suspected by their characteristic smell, and detected by their different melting points, by microscopical examination, and by their ethereal solutions. Messrs Angell and Hehner have proposed a convenient method of estimating the fusing points of fat by placing a given weight of definite size on the fat, and observing the temperature at which it sinks Into the substance. They find that the sinking-point for genuine butter is remarkably constant, and that it is affected in proportion to the amount and sinking point of any adulterant fats used. Their analysis of butter depends upon the much smaller proportion of non-volatile, insoluble fatty acids it contains, as compared with fats used as adulterants.

Under the name of butterine an artificial substitute for butter has been introduced in America, and imported into England from New York. It is the same as the artificial butter or “margarine-mouries,” which has been for some years manufactured in Paris according to a method made public by the eminent chemist Μ. Mège-Mouries. Having surmised that the formation of butter contained in milk was due to the absorption of fat contained in the animal tissues, Μ. Mouries was led to experiment on the splitting up of animal fat. The process he ultimately adopted consisted in heating finely-minced beef suet with water, carbonate of potash, and fresh sheep’s stomachs cut up into small fragments. The mixture he raised to a temperature of 45° 0. (113° Fahr.) The influence of the pepsine of the sheep’s stomach with the heat separated the fat from the cellular tissue; he removed the fatty matter, and submitted it when cool to powerful hydraulic pressure, separating it into stearin and oleomargarin, which last alone he used for butter-making. Of this fat about the proportions of 10 lb with 4 pints of milk and 3 pints of water were placed in a churn, to which a small quantity of arnotto was added for colouring, and the whole churned together. The compound so obtained when well washed was in general appearance, taste, and consistency like ordinary butter, and when well freed from water it was found to keep a longer time. According to French official reports artificial butter goes much further as food than the genuine article, and forms a perfectly wholesome dietetic material. The Parisian octroi officials have recognized the efficiency of the substitute by imposing on it the same duties which are chargeable on ordinary butter. The company established for the manufacture in France had in 1874 seven manufactories, in which four hundred men were employed. There can be no doubt that a pure, sweet fat, such as is manufactured by the process of Μ. Mège-Mouries is a safer and more wholesome article than the unsavoury rancid butter which is sold so freely among the poorer classes. See Dairy.