XIII.] MILK, FLOUR, AND BREAD. 97 



the tube can be inverted without the curd falling out. By-and-by 

 the curd shrinks, and squeezes out a clear slightly-yellowish fluid, 

 the whey. Filter. 



(6".) Using commercial rennet extract, repeat (b.), but boil the 

 rennet first ; it no longer effects the change described above. The 

 rennet ferment is destroyed by heat. 



(d,) Boil the milk and allow it to cool, then add rennet ; in all 

 probability no coagulation will take place. Boiled milk is far more 

 difficult to coagulate with rennet than unboiled milk. 



(e.) Take some of the curd of 6 (a.). Dissolve one part in caustic 

 soda and the other in lime-water. Add rennet to both, warm to 

 40° C. The lime solution coagulates, the soda solution does not. 

 (The ferment of rennet has been called rennin.) 



7. The Salts. 



(a.) Using the filtrate of 6 (a.), add magnesia mixture — Lesson 

 XVII. 7, (^.), i.e., ammonio-sulphate of magnesia, which gives a 

 precipitate of phosphates. Calcium phosphate is the most abundant 

 salt. There is a little magnesium phosphate. 



(b.) Silver nitrate gives a precipitate insoluble in nitric acid, 

 indicating chlorides (chiefly potassium and sodium). 



8. Boil milk in a porcelain capsule for a time to cause evapora- 

 tion. It is not coagulated, but a pellicle forms on the surface. 

 Remove it and boil again ; another pellicle is formed. 



9. Coagulation of Milk. — Calcium salts seem to play an important part in 

 this process. 



(i.) Halliburton's Method. — Prepare pure caseinogen by saturating milk 

 with powdered MgSOj. Allow it to stand for a few hours and filter. Keep 

 the filtrate (A). The filter residue consists of caseinogen + fat; wash this with 

 saturated solution of MgS04 until the washings contain no albumin. On 

 adding water to the precipitate, it dissolves, the fat remaining in the filter. 



Precipitate the solution of caseinogen in weak MgS04 by acetic acid. Collect 

 the precipitate on a filter and wash the acid away with distilled water. 

 Dissolve the precipitate in lime water, rubbing it up in a mortar, filter = 

 opalescent solution of caseinogen. 



Place some of this opalescent solution of caseinogen in two tubes A and B. 



To A add rennet and keep at 40"" C. = no coagulation. 



To B add rennet and a few drops of phosphoric acid (.5 percent.). Heat to 

 40° C. = coagulation, i.e., casein is formed from caseinogen in the presence of 

 calcic phosphate. 



(ii.) Ringer's Method to show the conversion of caseinogen into casein. — 

 Precipitate caseinogen ( -f- fat) with acetic acid. Collect and wash the pre- 

 cipitate, and grind it up in a mortar with calcium carbonate. Throw the 

 mixture into excess of distilled water. The fat floats, the excess of calcium 

 carbonate falls to the bottom, while the very opalescent solution contains the 

 caseinogen. Divide the fluid into three tubes A, B, C. Koep all at 40° C. 



To A add rennet = no clot of casein. 



a 



