35 



Addition of acid to boiled milk makes lime salts soluble again and 

 restores power of rennet coagulation. 



(g) Milk from dilTerent cows of the same herd coagulates with rennet 

 in (|uite varying periofls of time (reference No. 3, p. 67). 



6. Galactase is an enzym existing naturally in milk. It has the 

 power of coagulating and then dissolving milk casein. Galactase 

 resembles pepsin, but appears to be much slower in its action (refer- 

 ences No. 1, p. 161; No. 13, p. 77; and No. 14, p. 157). 



EXPERIMENT AND PRACTICE WORK, SIXTH LECTURE. 



Time required for exj)eriments. — Four hours or more can be advan- 

 tageously given to the following experiments: 



Apparatus required. — (1) Monrad's or Marschall's rennet test; (2] 

 rennet extract; (3) aseptic scale pepsin, 1-3000; (4) dilute sulphuric 

 acid; (5) sodium carbonate; (6) salt; (7) calcium chlorid, and (8) 

 thermometer. 



1. Effect of acids on rennet coagulation of milk. — Take 2 or 3 quarts 

 of milk, test acidit}^ (see Lecture 5, paragraph 2), then determine 

 rapidity of coagulation by the rennet test (Lecture 5, paragraph 3). 

 Record results. To 1 pint of the remaining milk add 10 cubic centi- 

 meters of dilute sulphuric acid made by adding 1 cubic centimeter 

 of sulphuric acid, such as is used in Babcock test, to 100 cubic centi- 

 meters of water. Test acidity of milk again and make rennet test 

 again. Increase amount of acid in the milk by adding 20 cubic 

 centimeters more. Again test acidity and make rennet test. Com- 

 pare results. Then add 20 cubic centimeters more of dilute acid and 

 make acid and rennet tests again. 



2. Effect of alkali on coagulation of milk hy rennet. — ^Add to 1 pint of 

 milk 1 gram of sodium carbonate and add rennet in usual amount. If 

 it coagulates, note the time required. 



3. Effect of letting milk stand on rennet coagulation. — Make acid and 

 rennet tests of a sampje of milk. Put milk in warm place (80° to 

 90° F.) and repeat acid and rennet tests at intervals of a half hour for 

 four or five hours or more. Keep careful record of all tests. 



4. Effect of temperature on rennet coagulation. — Make rennet test of 

 some milk at 84° F. Then make tests at following temperatures and 

 note results: 60° F., 70° F., 75° F., 80° F., 90° F., 100° F., 110° F., 

 120° F., and 140° F. 



5. Effect of heat on coagulating power of rennet. — Heat some rennet 

 extract to 140° F. for fifteen minutes and then test its coagulating 

 power. 



6. Effect of amount of rennet upon coagulation of milk. — Perform the 

 rennet test upon portioiis of the same milk, using (a) 5, (b) 10, (c) 15, 

 (d) 20, and (e) 25 cubic centimeters of rennet extract. Note and 

 compare the results. 



