1 1I 



CHEMISTRY. 



Milk. 



Properties 

 of cow'i 

 milk. 



Cream. 



Butter. 



SECT. X. Of Milk. 



Milk is a fluid secreted by the female of all those ani- 

 mals denominated mammalia, and intended evidently for 

 the nourishment of her offspring. 



The milk of every animal has certain peculiarities 

 which distinguish it from every other milk. But the 

 animal whose milk is most made use of by man as an ar- 

 ticle of food, and with which, consequently, we are best 

 acquainted, is the cow. Chemists, therefore, have made 

 choice of cow's milk for their experiment). 



Milk is an opaque fluid, of a white colour, a slight 

 peculiar smell, and a pleasant sweetish taste. When 

 newly drawn from the cow, it has a taste very different 

 from that which it acquires, after it has been kept for 

 some hours. It reddens vegetable blues. 



When milk is allowed to remain for some time at rest, 

 there collects on its surface a thick unctuous yellowish- 

 coloured substance, known by the name of cream. 



After the cream is separated, the milk which remains 

 is much thinner than before, and it has a bluish white co- 

 lour. If it be heated to the temperature of 100, and a 

 little rennet, which is water digested with the inner coat 

 of a calf's stomach, and preserved with salt, be poured 

 into it, coagulation ensues ; and if the coagulum be bro- 

 ken, the milk very soon separates into two substances ; 

 a folid white part known by the name of curd, and a 

 fluid part called tvhey. Thus we gee that milk may be 

 easily separated into three parts ; namely, cream, curd, 

 and icficy. 



1. Cream is of a yellow colour, and its consistence in- 

 creases gradually by exposure to the atmosphere. In 

 three or four days it becomes so thick, that the vessel 

 which contains it may be inverted without risking any 

 loss. In eight or ten days more, its surface is covered 

 over with mucors and byssi, and it has no longer the fla- 

 vour of cream, but of very fat cheese. 



Cream possesses many of the properties of an oil. It 

 is specifically lighter than water; it has an unctuous feel, 

 stains cloths precisely in the manner of oil ; and if it be 

 kept fluid, it contracts at last a taste which is very ana- 

 logous to the rancidity of oils. These properties are 

 sufficient to show us that it contains a quantity of oil ; 

 but this oil is combined with a part of the curd, and mix- 

 ed with some serum. Cream, then, is composed of a pe- 

 culiar oil, curd, and serum. The oil may be easily ob- 

 tained separate, by agitating the cream for a considerable 

 time. This process, known to every body, is called 

 churning. After a certain time, the cream separates in- 

 to two portions : one fluid, and resembling creamed milk ; 

 the other solid, and called butter. 



Butter is of a yellow colour, possesses the properties 

 of an oil, and mixes readily with other oily bodies. When 

 heated to the temperature of 96 it melts, and becomes 

 transparent ; if it be kept for some time melted, some 

 curd and water or whey separate from it, and it assumes 

 exactly the appearance of oil. But this process deprives 

 it in a great measure of its peculiar flavour. 



Butter may be obtained, by agitating cream newly 

 taken from milk, or even by agitating milk newly drawn 

 from the cow. But it is usual to allow cream to remain 

 for some time before it is churned. Now cream, by 

 standing, acquires a sour taste ; butter, therefore, is com- 

 monly made from sour cream. When very sour cream is 

 churned, evi-ry vne must have perceived, that the butter 

 milk, after the churning, is not nearly so eour as the cream 

 had been. The butter, in all cases, is perfectly sweet ; 

 consequently the acid which had been evolved, hat in a 



great measure disappeared during the progreu of chum- Chemical 

 ing. It has been ascertained that cream may be churn 

 ed, and butter obtained, though the contact of atmo- 

 spheric air be excluded. On the other hand, it has been 

 affirmed, that when cream is churned in contact with air, 

 it absorbs a considerable quantity of it. 



In many cases, there is a considerable extrication of 

 gas during the churning of water. From the phenome- 

 na, it can scarcely he doubted that this acid is carbonic 

 acid. See BI'TTKR. 



2. Curd, which may be separated from creamed milk Curd, 

 by rennet, has many of the properties of coagulated al- 

 bumen. It is white and solid ; and when all the mois- 

 ture is squeezed out, it has a good deal of brittleness. It 

 is insoluble in water ; but pure alkalies and lime dissolve 

 it readily, especially when assisted by heat ; and when 

 fixed alkali is used, a great quantity of ammonia is emit- 

 ted during the solution. The solution of curd in soda is 

 of a red colour, at least if heat be employed, owing pro- 

 bably to the separation of charcoal from the curd by the 

 action of the alkali. The matter dissolved by the alkali 

 may be separated from it by means of an acid ; but it 

 has lost all the properties of curd. It is of a black co- 

 lour, melts like tallow by the application of heat, leaves 

 oily stains on paper, and never acquires the consistence 

 of curd. Its constituents, according to the experiments 

 of Thenard and Gay-Lussac, are as follows : 



Carbon 59.781 



Oxygen 11.409 



Hydrogen 7.429 



Azote 21.381 



100 



Curd, as is well known, is used in making cheese, and Cheeie. 

 the cheeseis the better the more it contains of cream, or of 

 that oily matter which constitutes cream. It is well 

 known to cheesemakers, that the goodness of it depends 

 in a great measure on the manner of separating the whey 

 from the curd. If the milk be much heated, the coagu- 

 lum broken in pieces, and the whey forcibly separated, 

 as is the practice in many parts of Scotland, the cheese 

 is scarce good for any thing ; but the whey is delicious, 

 especially the last squeezed out whey, and butter may 

 be obtained from it in considerable quantity : a full 

 proof that nearly the whole creamy part of the milk has 

 been separated with the whey. Whereas if the milk be 

 not too much heated, (about 100 is sufficient,) if the 

 coagulum be allowed to remain unbroken, and the whey 

 be separated by very slow and gentle pressure, the cheese 

 is excellent ; but the whey is almost transparent, and 

 nearly colourless. 



Good cheese melts at a moderate heat j but bad cheese, 

 when heated, dries, curls, and exhibits all the phenomena 

 of burning horn. From this it is evident, that good 

 cheese contains a quantity of the peculiar oil which con- 

 stitutes the distinguishing characteristic of cream ; hence 

 its flavour and smell. Proust has found in cheese an acid 

 which he calls the caseic acid, to which he ascribes se- 

 veral of the peculiar properties of cheese. See DAIRY. 



3. Whey, after being filtered, to separate a quantity Whey, 

 of curd which, still continues to float through it, is a thin 

 pellucid fluid, of a yellowish green colour, and pleasant 

 sweetish taste, in which the flavour of milk may be dis- 

 tinguished. It always contains some curd ; but nearly 

 the whole may be separated by keeping the whey for 

 some time boiling ; a thick white scum gathers on the 

 surface, which in Scotland is known by the name oljloal 

 i. 'icy. When this scum, which consists of the curdy 

 part, is carefully separated, the whey, after being allow- 



