PROPERTIES OF THE SUGAR OF MILK. 543, 



§ 5. Of the sugar of milk, and of the acid of milk or lactic acid. 



Before I can hope to make you understand the nature of the changes 

 which take place during the souring, the churning, and the curdling of 

 milk, it will be necessary to make you acquainted with the sugar of 

 milk, and with lactic acid or the acid of milk. 



1°. Sugar of milk. — When the curd is separated from milk, the raw 

 whey afterwards boiled — with or without the addition of new and butter 

 milk — and the floating churd skimmed off or separated by straining 

 through a cloth, the whey is obtained nearly free from butter and cheese. 

 By mixing it while hot with well beat white of egg, the remainder of the 

 curd is coagulated, and may be removed by again straining through 

 cloth. If the clear whey, thus obtained, be boiled down in a pan to one 

 fourth of its bulk, then poured into an earthen dish, and set aside for a 

 few days in a cool place, minute hard white crystals gradually de- 

 posit themselves upon the sides and bottom of the vessel. These crystals 

 are sugar of milk. A second portion may be obtained by evaporating 

 the remaining whey still further, and again setting aside. If the whey 

 be at once evaporated to dryness a white mass of impure sugar is pre- 

 pared, which in many places is used as an article of food. Of the purer 

 variety large quantities are extracted from milk by the Swiss shepherds, 

 and in their country it forms an important article of commerce. 



The sugar of milk is less sweet than that of the grape, or of the sugar 

 cane. It is harder also, and much less soluble in water, and is gritty 

 between the teeth. This sugar undergoes no change when exposed to 

 the air, either in the dry state or when dissolved in water. But if a little 

 of the curd of milk (casein) be introduced into the solution it gradually be- 

 comes sour, lactic acid is formed, and the lifjuid begins to ferment. Car- 

 bonic acid is given oti^— as is the case during the fermentation of other 

 liquids — and alcohol is produced. In milk the two substances are na- 

 turally intermixed, and it is the presence of the cheesy matter, as we 

 shall hereafter see, which at favourable temperatures always causes milk 

 of every kind first to become sour and then to ferment. 



The gluten of wheat and animal membranes of various kinds produce 

 a similar effect upon solutions of sugar of milk. A piece of bladder, or 

 of the gut or stomach of an animal, immersed into a solution of the sugar, 

 changes it by degrees into lactic acid, and upon this influence depends 

 the effect of the calf's stomach, in the form of rennet, in the curdling of 

 milk. The effect of such membranes is more speedy after they have 

 been some time taken from the body of the animal, a fact which also ac- 

 cords with the long experience of the dairy districts in the preparation of 

 rennet. 



When a little sulphuric or muriatic acid is added to a solution of milk 

 sugar, it is slowly converted into grape sugar. This change is hastened 

 very much by boiling it with the acid. It is supposed that previous to 

 the fermentation of milk the sugar it contains undergoes a similar change 

 into the sugar of grapes. 



Milk sugar has not hitherto been formed by art. It exists in the milk 

 of all mammiferous animals, and from this source alone have we hith- 

 erto been able to obtain it. 



2°. The acid of milk — lactic acid. — When milk is exposed to the air 

 for a length of time it acquires a sour taste, which gradually increases in 



