136 PHYSIOLOGICAL CHEMISTRY. 



pears, as the activity of the yeast cell is destroyed by alcohol. We have 

 good familiar illustrations of this in the self-preservation of certain 

 " heavy " wines, as ports, sherries and malagas, while " light " wines, which 

 contain 10 to 12 per cent of alcohol usually, must be kept tightly bottled 

 for preservation. 



Ex. TEST FOR ALCOHOL. We have many tests by which the presence or 

 formation of alcohol may be shown. The fermentation of a saccharine 

 liquid is followed by a lowering of the specific gravity as may be easily 

 found by experiment, and a practical quantitative test is based on this 

 fact. A simple chemical test for the presence of alcohol, which in most 

 cases is sufficient, is the following: Add to the clear liquid to be examined 

 a few small crystals of iodine, warm to about 60 C, and then add enough 

 sodium hydroxide or sodium carbonate to produce a colorless solution. 

 An excess of the alkali must not be used. In a short time bright yellow 

 crystals of iodoform precipitate, easily recognized by their color and odor. 

 Certain other liquids give the same test. 



Ordinary yeast contains the soluble ferment called invertase, 

 which has been already referred to. This may be shown by 

 experiment, as follows : 



Ex. Crush some yeast, add water and wash by decantation or on a 

 filter thoroughly. Then rub up the washed yeast with some water in a 

 mortar and add the mixture to a solution of pure cane sugar which has 

 previously been treated with a few drops of a strong alcoholic solution of 

 thymol. 50 cc. of a 5 per cent cane sugar solution will answer. The 

 thymol prevents the action of the yeast cell fermentation, but does not 

 prevent the action of the invertase. The mixture should be kept about 

 24 hours at a temperature of 40 to 50 C. At the end of this time it is 

 filtered and the filtrate tested for invert sugar by means of the Fehling 

 solution. Ether and chloroform are sometimes employed in place of the 

 thymol; the latter must be removed by heating before making the Fehling 

 test. 



Zymase. It has been intimated already that the activity 

 of yeast as an alcoholic ferment is due to the presence of an 

 enzyme. This fact, long suspected and much debated, was 

 finally demonstrated by E. Buchner, as explained above. 

 'Buchner rubbed the yeast with fine, sharp sand and water and 

 then subjected the mixture to great pressure. The liquid 

 pressed out was carefully filtered and was found to be as 

 active as the original yeast. The enzyme in it he called zy- 

 mase. It clings tenaciously to the yeast cell, hence the neces- 

 sity of destroying the structure by grinding with sand, and 

 employing great pressure. 



