268 FERMENTATIONS IN MILK 



To do this, measure out with a pipette 25 or 50 c.c. into a 

 porcelain dish. Add to it five drops of the following solution of 

 phenolphthalein : 



Phenolphthalein . . . . . 5 gr. 

 Alcohol, 90 per cent 150 c.c. 



(This solution turns pink when an alkali is added to it and 

 remains colourless when acid : try it.) 



Now run into the milk a decinormal solution of caustic soda 

 from a graduated burette, carefully noting the point at which the 

 caustic soda solution stands in the burette before beginning the 

 experiment. 



At first when the caustic soda solution touches the milk a pink 

 coloration is produced, which disappears on stirring with a glass 

 rod. 



Run in more, drop by drop, until the pink colour is per- 

 manent, stirring vigorously after each addition of the soda; 

 when the pink colour remains for ten to twenty seconds after 

 stirring, all the acid has been neutralized. 



Read off on the scale of the burette how many c.c. of the soda 

 solution has been used. 



Calculate what percentage of acid was present in the milk, 

 assuming that each c.c. of the soda solution corresponds to .009 gr. 

 lactic acid. 



Draw up a table showing the amount of acid in each sample 

 of milk from day to day. 



Lactic acid exists in three forms, all having the 

 same chemical composition, yet differing from each 

 other in molecular constitution and physical properties. 

 A solution of one of these, known as dextro-lactic acid, 

 has the power of affecting a beam of polarized light when 

 passed through it, the plane of the light being rotated 

 towards the right. Another form rotates the light 

 equally to the left, when passed through solutions of the 

 same strength ; it is termed laevo-lactic acid. When 



