143 



LESSON XIV 

 CABBOHYDBATES : ACTION OF MALT UPON STABCH 



1. Prepare a 0"5-per-cent. solution of starch. 



2. Prepare some malt extract by digesting 10 grammes of powdered malt 

 with 50 c.c. of water at 50° C. for three hours, and subsequently straining. 

 This extract contains the diastatic or malting ferment. 



Solutions 1 and 2 may be conveniently prepared beforehand by the 

 demonstrator. 



3. To the starch solution add one-tenth of its volume of malt extract, and 

 place the mixtmre in a water-bath at 40° C. From time to time test portions 

 of the liquid by mixing a drop with a drop of iodine solution on a testing 

 slab. The blue colour at first seen is soon replaced by a violet (mixture of 

 blue and red), and then by a red reaction (due to erytTirodextrin) which 

 gradually vanishes. Alcohol added to the liquid when all starch and erythro- 

 dextrin have gone still causes a precipitate of a dextrin, which, as it gives no 

 colour with iodine, is called achroo-dextrin. The liquid also contains a 

 reducing sugar, maltose. 



4. Treat the solution of starch as before with one-tenth of its voltmie of 

 malt extract, and keep it in the warm chamber (50° C.) for three hours. 

 This may be conveniently done beforehand by the demonstrator. 



5. Take 50 c.c. of the product (which is a solution of maltose and isomal- 

 tose, the reducing action of which is the same as that of maltose), and 

 detei-mine how much of it is necessary to reduce 10 c.c. of Fehling's solution. 

 The manner of carrying out Fehling's quantitative method is given in 

 Lesson XII. 



6. Take another 50 c.c. and boil it with 1 c.c. of strong sulphuric acid for 



half an hour in a flask. This converts it into dextrose. After cooling bring 



the Uquid to its original volume (50 c.c.) by adding water, and again determine 



its increased reducing power with Fehling's solution. If a; = c.c. of maltose 



• . . 2x 



solution necessary to reduce 10 c.c. of Fehling's solution, then— - = c.c. of 



o 



dextrose solution necessary for the same purpose. The strength of the 



maltose solution can be calculated from the fact that 10 c.c. of Fehling's 



solution corresponds to 0K)5 gramme of dextrose. 



