544 THK ACID OF MILK, OR LACTIC ACID. 



intensity till at length the whole heghis to ferment. This sour taste is 

 owing to tlie production of a jieculiar acid, to which the name of acid 

 of milk or lactic acid has been given. The same acid is formed during 

 the fermentation of the juices of the beet, and of the turnip, in sour cab- 

 bage {sauer kraut), and sour malt, in brewers' grains which have become 

 sour, in the sour vegetable mixtures with which cattle are often fed, in 

 the waste liquor of the tanners, in the fermented extract of rice, and in 

 large quantity during the fermentation of the gluten in the manufacture 

 of starch from wlieaten flour, or of a mixture of oat-meal or bean- 

 meal with water, which is allowed to stand and become sour. 



The acid, therefore, differs from the sugar of milk in so far that it can 

 readily be formed, and in any quantity, by anificial means. As it is 

 not employed for any economical purposes, I shall not trouble you with 

 the methods by which this acid is obtained in a state of purity. 



It is rarely found in milk when first drawn from the cow, but it very 

 soon begins to be formed in it. It is produced from the sugar, through 

 the influence of the cheesy matter of the milk. The pure acid may be 

 mixed with cold milk without causing it to curdle, but if the mixture be 

 heated, the curd forms and speedily separates. It is for the sarne reason 

 that milk may be distinctly sour to the taste, and yet may not coagulate. 

 But if such milk be heated it will curdle immediately. So cream when 

 sour may not appear so, till it is poured into hot tea, when it will break 

 and leave its cheesy matter floating on the surface. 



§ 6. Of the mutual relations which exist between lactic acid and the cane^ 

 grape, and milk sugars. 



It is important, and I think it will prove interesting to you, to under- 

 stand the beautifully simple relation which exists between the sugar of 

 milk and this lactic acid, which plays so important a part in nearly all 

 your daily operations- 

 Cane sugar, grape sugar, milk sugar, and lactic acid, as they exist in 

 solution in water or in milk, may all be represented as compounds of car- 

 bon with water — or of carbon with hydrogen and oxygen in the propor- 

 tions in which they exist in water. Thus they consist respectively of — 



12 Carbon + 12 Water 

 12H + 120 or 12C + 12HO* 



12 Carbon + 14 Water 



14H 4- 140 or 12C + 14HO 



24 Carbon -|- 24 Water 

 24H 4- 240 or 24C -f 24HO 



6 Carbon + 6 Water 

 6H + 60 or 6C + 6HO 



4 Carbon + 3 Water 

 3H + 30 or 4C -|- 3HO 



I have added acetic acid to this list, to show you that the lactic acid 

 bears a similar relation to the sugars as this acid does. You will recol- 

 lect that starch, gum, and woody fibre, have also a similar relation to 

 the sugars — and tliat by certain apparently simple transformations these 



• C, H, and O, as in our former lectures, representing respectively carbon, hydrogen, and 

 ^Jtygen, and HO watpr— a compound of hydrogen with oxygen. 



