THE INDIA RUBBER WORLD 



[Dkkmbfr 1. 1918. 



What the Rubber Chemists Are Doing. 



THEORY OF LATEX COAGULATION. 



IN a paper on the theory of latc.x coagulation, by G. Stafford 

 Whitby, published in the "Agricultural Bulletin of the Federal 

 Malay States," June, 1918, the author states that he has con- 

 firmed the views of Barrowcliff expressed in a recent article 

 (The Indi.\ Rubber World, October 1, 1918), namely, that the 

 agency responsible for the natural coagulation of Hevca latex is 

 enzymic ; not only natural coagulation, but coagulation resulting 

 from the addition of acetic acid appears to be due to enzymic 

 activity. The function of the acid in acetic acid coagulation 

 appears to be the activation of a coagulating enzyme. Barrow- 

 cliff found that the latex which had been sterilized by running it 

 into an equal volume of boiling water was no longer coagulated 

 by acetic acid, but became so if treated with a few drops of fresh 

 latex. The author remarks that the conclusion to which this 

 observation points — that the function of acetic acid in the ordinary 

 process of latex coagulation is to activate an enzyme — resolves 

 certain anomalies involved in the view that the separation of rub- 

 ber from its colloidal emulsion by acid is a physico-chemical phe- 

 nomenon in the ordinary sense. 



The author states that the object of his paper is primarily to 

 compare, in certain aspects, the coagulation of latex with the 

 coagulation of cow's milk and to record a number of experi- 

 mental results. Whitby makes closer examination of certain 

 aspects of the analogy indicated by Barrowcliff between latex 

 coagulation and the coagulation of milk by rennin. Somewhat 

 condensed, the author's studies are given as follows: 



1. Coagulation of Sterilized Milk. Boiled milk can be 

 coagulated by added acid in the same way as unboiled milk. 

 Sterilized cow's milk in contradistinction to latex sterilized by 

 Barrowcliff's procedure, was coagulable with acetic acid. 



2. Critical Concentr.'vtion of Acid for Milk Coagulation. 

 In the coagulation of milk by added acetic acid or added lactic 

 acid there is what may be called a critical concentration of acid. 

 Below this concentration no coagulation occurs. Even when the 

 acid added is only ten per cent less than this quantity no coagula- 

 tion occurs, but when the critical amount or more is added imme- 

 diate clotting takes place. 



This behavior of milk stands in very marked contrast with 

 the behavior of Herea late.x towards increasing amounts of added 

 acid. In the case of latex there is no critical concentration at 

 which immediate clotting occurs and below which no clotting is 

 obtained. There is a wide range over which acid concentration 

 can be varied and great differences in the rapidity with which 

 coagulation can be brought about. 



3. .'\nti-Coagulating Effect of a Deficit of Acetic .-Vcid in 

 Milk. A more striking contrast with the effect of added acetic 

 acid on Hever latex is that an amount of acetic acid below the 

 critical amount not only fails to produce coagulation in milk, but 

 has a pronounced anti-coagulating effect due presumably to an 

 unfavorable effect on the growth of the lactic acid bacterium. In 

 the case of Hezva late.x, acetic acid in amounts less than that 

 necessary for rapid coagulation has no anti-coagulating effect. 

 Diminishing the amount of acid added simply renders the coagula- 

 tion slower. The fact that acetic acid has no anti-coagulating 

 effect on Hevea latex tells against the bacterial hypothesis of 

 latex coagulation. 



4. Time Factor in the Formation of a Clot. The clotting 

 of milk by added acid is a practically instantaneous process, 

 whereas the time occupied by the formation of a clot in latex 

 may, by adjustment of the amount of acid added, be varied over 

 a period of from three minutes to eight hours. There is thus, 

 negatively, a failure of analogy between latex coagulation and the 

 acid coagulation of milk in regard to the time factor, but there 



is, positively, a close analogy between the effect of acids on latex 

 coagulation and their effect on the coagulation of milk by rennin, 

 as shown by the results of Gerber. 



In the absence of all other considerations, the slowness with 

 which coagulation takes place in ordinary plantation practice 

 woul'd seem to afford a strong presumption that the coagulation 

 of latex under the influence of added acid is not a direct physical 

 phenomenon but involves the activity of an enzyme. 



5. Acid Concentration for Natural CoAGUL.^TION. In the 

 case of the coagulation of milk by the addition of acid or by 

 natural souring, the occurrence of coagulation appears to depend 

 substantially upon the presence of a certain concentration of acid. 

 When, as a result of natural souring, the acidity has reached a 

 certain point, clotting occurs. Under ordinary conditions of 

 souring at 20 degrees C. milk will coagulate when the lactic acid 

 reaches 0.6 to 0.7 per cent. 



In the case of latex the development of acidity has not been 

 studied as closely as in the case of milk, but it seems clear that 

 the natural coagulation of latex is not a simple function of its 

 acidity. It has been demonstrated experimentally by the author 

 that the amount of acidity engendered naturally in latex may be 

 very greatly in excess of that which it would be necessary to 

 add to fresh latex in order to produce coagulation which reveals 

 a further failure of analogy between latex coagulation and that 

 of milk. 



6. Nature of the Clot. In outward character, particularly as 

 regards coherency, the clot produced in milk by enzymic action 

 is more analogous to the ordinary rubber coagulum than is the 

 clot produced in milk by natural or added acid. The ordinary 

 rubber coagulum resembles the coherent clot produced by rennin 

 rather than the floccular clot which is first produced in milk by 

 souring or by the addition of acids. 



7. Influence of Caloum Salts. That calcium chloride is 

 capable of leading to the coagulation of latex was recorded some 

 years ago. It has been suggested by Eaton that the influence 

 of soluble calcium salts in promoting natural coagulation is due 

 to the presence of calcium being favorable to certain anaerobic 

 organisms. There does not, however, seem to be any analogy for 

 this suggestion, either in the coagulation of blood or in the acid 

 or enzymic coagulation of milk. An analogy between the action 

 of calcium chloride on latex and its action on the enzymic coagu- 

 lation of milk is indicated in its activation of rennin. 



8. Bactericides and Enzyme Paralyzers. Natural coagula- 

 tion is not prevented or retarded by toluol, thymol or chloroform 

 water but is prevented by such an agent as hydrocyanic acid. 

 Hydrogen sulphide, although acidic, retards or prevents coagula- 

 tion. 



9. Influence of Oxygen. In apparent contrast to the case of 

 milk the natural coagulation of Hevea latex is not dependent 

 upon the surface exposed. 



10. Possible Significance of the "Gap." Barrowcliff points 

 out that, on the view that an enzyme is concerned in latex coagu- 

 lation, the fact that hydrochloric acid in certain comparatively 

 high concentrations fails to coagulate latex may be due to the 

 destruction or paralysis of the enzyme by this concentration of 

 acid. In such case, at still higher concentrations, the acid would 

 appear to have a direct clotting effect. It has been shown that 

 on adding increasing quantities of hydrochloric acid to samples 

 of latex a point is reached comparatively soon at which no 

 coagulation occurs, but that on continuing to increase the amount 

 of acid added, a point is reached at w-hich coagulation again be- 

 gins to take place. There is a gap over which no coagulation 

 occurs. The position of this gap was found to be approximately 

 from a concentration of hydrochloric acid corresponding to three 



