104 A MANUAL OF BACTEEIOLOGY 



6. Grind up the dry powder and store it in a clean glass- 

 stoppered bottle. 



7. Prepare 2 per cent solutions of cane sugar, dextrose, levu- 

 lose, maltose, and lactose. Put them into fermentation tubes 

 and add to each as much yeast powder as will lie on the point 

 of a penknife. Examine the tubes for gas at the end of ten to 

 eighteen hours. Compare the results with those obtained with 

 the same sugars in Exercise 143. 



Exercise 146. The Estimation of the Chief Products of the 

 Fermentation of Sugars 



The chief products of the fermentation of an invert sugar are 

 carbon dioxide and alcohol. In addition, small amounts of glycerin 

 and succinic acid are produced, together with smaller quantities 

 of other compounds, such as aldehydes, fusel oil, and furfural. 

 This exercise demonstrates methods of estimating the carbon 

 dioxide and alcohol. 



1. Prepare about a liter of apple- or grape-juice must. Instead 

 of must, other saccharine solutions may be used — for example, 

 4 per cent dextrose, or molasses diluted with two volumes of 

 water. Determine quantitatively the sugar content of the must. 

 To do this, make 20 cc. of Fehling's solution by mixing 10 cc. 

 of each of the two stock solutions (see Appendix) in a casserole, 

 and boil. Above the casserole adjust a burette containing 50 cc. 

 of must and add, drop by drop, until the blue color of the 

 Fehling's solution disappears. 



1 cc. normal Fehling's solution = 4.94 mg. invert sugar. 

 1 cc. normal Fehling's solution = 4.53 mg. dextrose. 



For example, 20 cc. of Fehling's solution required 16.2 cc. of 

 must to reduce the copper. 



20 cc. Fehling's solution = .0988 g. invert sugar. 

 Then 16.2: .0988: : 100 : a;; 



therefore x = .60988 per cent invert sugar in the must. 



2. Select three Erlenmeyer flasks of 500 cc. capacity. Put 

 into each 150 cc. of must and sterilize fractionally in the Arnold 



