PRACTICAL EXERCISES 693 



of bag, and knead it with the fingers in a capsule of water. The 

 starch grains come through the cheese-cloth. Pour the water into a 

 beaker. It is opaque, and on standing the starch grains sink to the 

 bottom, (a) Test for starch with the iodine test, and also examine 

 microscopically. The grains are round, with a central hilum, and are 

 smaller than those of potato starch (p. n). (b) Test for sugar by 

 Trommer's test (p. 10). None is present unless the flour has been made 

 from inferior grain in which some germination has taken place. 



(2) Go on kneading the dough till no more starch comes through. 

 The sticky mass which remains in the bag is a protein called gluten, 

 which is formed from certain globulins and other proteins in the flour 

 on addition of water. Oatmeal, ground rice, and other grains poor in 

 gluten-forming globulins do not form dough when mixed with water. 

 Suspend some of the gluten in water in a test-tube, and apply to it the 

 general protein colour tests (p. 8). 



7. Bread. (i) Rub up a small piece of the crumb of a stale loaf in a 

 mortar with water. Strain through cheese-cloth. The fluid which 

 passes through contains starch grains, (a) Filter it, and test a portion 

 of the filtrate for dextrose by Trommer's test. A positive result 

 is obtained. Test another portion with iodine for erythrodextrin. 

 (b) Test a portion of the residue of the bread which has not passed 

 through the cheese-cloth for protein by the general protein tests e.g., 

 the xanthroproteic or Millon's tests. 



(2) Repeat (i) using the crust of the bread. Both dextrose and 

 erythrodextrin are present in the cold-water extract, but the dextrose 

 is less plentiful than in the crumb, having been converted into caramel 

 in the baking. The sugar and dextrin are formed from the starch of the 

 flomr by the ferments of the yeast employed to make the bread rise. 



8. Variations in the Total Nitrogen (p. 514) and in the Quantity of 

 Urea excreted, with Variations in the Amount of Proteins in the Food. 

 The student should put himself, or somebody else if he can, for two days 

 on a diet poor in proteins, then (after an interval of forty-eight hours 

 on his ordinary food) for two days on a diet rich in proteins. A suitable 

 table of diets will be supplied. The urine should be collected on the 

 six days of the period of experiment, on the day before it begins, and 

 on the day after it ends. Small samples of the mixed urine of the 

 twenty-four hours for each of these eight days should be brought to the 

 laboratory, and the quantity of urea determined by the hypobromite 

 method. The volume of the urine passed in each interval of twenty- 

 four hours being known, the total excretion of urea for the twenty-four 

 hours can be calculated, and a curve plotted to show how it varies 

 during the period of experiment.* If sufficient time is available, the 

 experiment will be made still more instructive by determining the 

 total nitrogen in each sample in addition to the urea. A curve showing 

 the variation in the total nitrogen can then be plotted on the same paper 

 as the urea curve, and a table calculated giving the percentage of the 

 total nitrogen contained in the urea for each day of the experiment. 



9. Action of Epinephrin (Adrenalin). Several experiments to illus- 

 trate this are given in the Practical Exercises following other chapters, 

 but may equally well be performed here. (See Experiment 8, p. 66; 

 Experiment 3, jp. 447.) 



* In 17 healthy students the average amount of urea excreted in twenty 

 four hours on the ordinary diet was 29*51 grammes (minimum 19*35 grammes 

 maximum 46*01 grammes) ; on a diet poor in protein, average 20*75 grammes 

 (minimum 9*52 grammes, maximum 32-86 grammes) ; on a diet rich in protein, 

 average 38*83 grammes (minimum 23*26 grammes, maximum 67*82 grammes). 



