LESSON XIV 



CARBOHYDRATES: ACTION OF MALT UPON STARCH 



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 mixture 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 erytlirodextriri), 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. Take 50 c.c. of a solution of maltose and determine how much of it 

 is necessary to reduce 10 c.c. of Fehling's solution. 



5. Take another 50 c.c. and boil it with 1 c.c. of strong sulphuric acid fo 

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

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

 its increased reducing power with Fehling's solution. If x - c.c. of maltose 



2# 

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



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 correspond to 0'05 gramme of dextrose. 



x 2 



