June, 1921
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
515
The Relations of Hydrogen-Ion Concentration to the Heat Coagulation of Proteins in Swiss Cheese Whey1 By Yuzuru Okuda* and Harper F. Zoller RESEARCH LABORATORIES, DAIRYDIVISION,U. s. DEPARTMENT OF AORICULTURE, WASHINGTON, D. C.
Some time ago, during a n emergent examination of Swiss cheese whey, t o gage roughly t h e zone of reaction t h a t would furnish the best working curd for Ricotta cheese manufacture, one of us (Zoller) observed t h a t t h e indicator methyl red showed this zone t o lie between p H , 5.2 and 5.4. It was a t t h a t time believed t h a t this zone was not t h e true pH zone of the interior of t h e solution, because the quantity of acid used t o reach this zone was great enough t o produce a much lower pH, according t o electrometric titration curves of skim milk. It was desirable, therefore, t o study this zone carefully with the hydrogen electrode, since t h e coagulation of whey proteins is of such tremendous importance t o many of the dairy industries. Whey produced in the case .of cheese making is chiefly a waste product, or has been in t h e past, b u t i t contains appreciable quantities of salts, proteins, fats, lactose, and vitamines. Therefore, its utilization is a n interesting problem from the economic and nutritive point of view. The determination of t h e optimum reaction3 for the heat coagulation of the proteins in t h e whey links itself vitally with practically any method proposed for its utilization. At present i t has its greatest application i n the manufacture of lactose (to be discussed later in t h e text). It is also pertinent t o t h e manufacture of whey cheese, t o t h e determination and isolation of lactalbumin, t o the preparation of “protein-free milk,” and t o t h e milk condensing industry. I n this paper Swiss cheese whey only is reported on, but t h e same principle should hold good in the case of whey from other types of cheese or from casein.
paper is used, but those who use this as a n end-point usually make up for the misgaged reaction by processing their product further. Care was exercised in determining these curves, in view of their ultimate application in,the factory. T o definite portions of whey definite quantities of t h e various acids were added, and t h e influence of dilution
TITRATION C U R V E S
As f a r as t h e authors know, no titration curves of Swiss cheese whey have been published. These are important, inasmuch as direct reference t o them will permit one t o adjust any quantity of Swiss cheese whey t o any definite H+-ion concentration. This is because t h e whey resulting from Swiss cheese manufacture, the country over, is very uniform in reaction. The maximum variation observed by us in the wheys examined was *0.10 p H . This is not sufficient t o cause an appreciable overstepping of the optimum reaction if the identical data furnished in these curves be used in the adjustment of the reaction. Of the methods in use a t present for determining the proper reaction point, the titration t o phenolphthalein is most widely used. It is needless t o say t h a t this is both inconvenient and inaccurate for the average factory man. I n some milk-sugar factories litmus
The concentrations in this and the following tables were: hydrochloric acid, 1.02 M; acetic, 1.02 M; lactic, 0.472 M ; calcium chloride solution, 0.9 M ; and sodium hydroxide, 1.002 M.
Received January 31, 1921. N o t a regular member of the staff of the Department of Agriculture. The Dairy Division granted Dr. Okuda the privilege of working, while a visitor, on the above problem in Mr. Zoller’s laboratories a The optimum reaction for the heat coagulation of proteins in whey l the largest quantity of protein is defined as t h a t reaction which w ~ l remove nitrogen from the whey in a workable form by heating to 98’ C.
The H+-ion concentration was determined electrometrically with Clark rocking electrodes and the saturated calomel electrode recommended by Michaelis, using a Leeds and Northrup type K potentiometer with type R galvanometer and a Weston standard cell.
1 2
was taken into account. I n factory practice i t is unnecessary t o consider the dilution factor. Complete data are furnished in Table I for t h e hydrochloric acid curve. The data for the other acids are given, in t h e form of their curves only, in Fig. 1. TABLEI Whey cc. , 100 100 100
100 100
100
100
HCll cc.
...
1.0 2.5 5.0 7.5 12.5 25.0
HzO Cc. 25 24 22.5 20 17.5 12.5 0.0
E. M. F. 0.6267 0.5579 0,4809 0.3931 0.3493 0.3254 0.2935
pH 6.42 5.26 3.98 2.01 1.77 1.38 0.84
Color PH 6.5 5.5 5.1 4.9 3.3 1.7
...
1
T E E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
516
The resulting potentials were expressed in the pH scale and were calculated according t o the formula: E.M.F. (obs.) - E.M. F. (calomel) = PH 0.0001983 ' I '
Sincemcalcium chloride is sometimes added in the processing of whey in lactose manufacture, a titration curve of whey with a mixture of the chloride and hydrochloric acid was made in order t o determine its effect upon the optimum reaction for removing proteins, etc. Data for this determination are presented in Table 11. Whey cc
100 100 100 100 100 100
... 1.0
2.5 5.0 7.5 12.5
\
T h e quantity of hydrochloric acid added t o each portion of whey t o reach the desired pH was determined by reference t o the curve in Fig. 1. It is apparent t h a t any one of the three methods of removing the curd would be suitable, since the nitrogen curves are parallel and of the same order of magnitude. The cotton filters, however, are more advantageous. The data presented in Tables I11 and I V are summarized in Fig. 2.
TABLE I1
Hz0 c c. 14.5 11.5 10.0 7.5 5.0
HC11 cc.
.
Vol. 13, No. 6
CaCh Cc.
E. M. F.
PH
2.0 2.0 2.0 2.0 2.0
0.6319 0.5365 0.4810 0.3960 0.3483 0.3153
6.48 4.88 3.98 2.56 1.77 1.22
...
...
The relative strengths of the three acids are rather strikingly brought out in Fig. 1. One curve of a n alkali titration upon t h e whey is included in this figure. O P T I M U M REACTION
FOR
COAGULATION OF W H E Y
PROTEINS
Some preliminary tests were conducted t o select a suitable method for removing the coagulated curd from the whey after heat treatment. Whichever method is adopted, i t should be comparable t o factory results. At present the filter-press, centrifugal, and drain-cloth methods are used in factory practice, with slightly different objectives in view, but with the ultimate desire t o remove as much of the protein material as possible. Accordingly, a centrifugal machine, medium filter paper, and a tuft of absorbent cotton placed in a funnel were examined. ' E X P T . 1-One thousand cc. of fresh whey were mixed with the correct quantities of M HC1 and water. k One-tenth of each sample was directly applied in the determination of total nitrogen and electrometric p H , and the remainder was heated in a steam oven for 45 min. a t about 98' C. It was then cooled rapidly and put into a centrifugal machine running a t about 2000 r . p . m. Nitrogen and pH determinations were again made upon the resulting curd-free liquid. Nitrogen was determined according t o Gunning's modification of Kjeldahl's method, taking the results of blank analyses into account, and using methyl red as indicator in the titrations. The results are tabulated a s follows: TABLE111-SEPARATION
MEANSO F CENTRIFUGAL MACHINE ---After Heating----Before HeatingN in N in HCI N Filtrate Curd Cc. E . M . F. pH Grams E. M . F. pH Grams Gram
Whey Cc.
...
1000
1000 1000 1000
3 0
7.0 12.5
0.6266 0.6047 0.5720 0.5353
BY
6 48
6 09 5 54 4.93
1.492 1.492 1.492 1.492
0.6075 0.5975 0.5789 0.5405
6.15 5.97 5 66 5.02
1.049 0.776 0.749 0.725
0.443 0.716 0 743 0.767
2-The above experiment was repeated with the cotton filter and the filter paper, except t h a t the heated whey was poured hot through the filters. Cooling caused the whey t o filter slowly. EXPT.
TABLE IV-SEPARATION B Y MEANSO F COTTON OR FILTERPAPER -Before Heating----After Heating--N N in Filtrate N in Curd Whey HC1 Cotc c . E. M. F. pH Grams E. M. F. pH Cotcc. ton Paper ton Paper
...
1000 1000 3 1000 7 1000 12.5
0.6274 6.50 1.547 0.6026 6.08 1.328 0.895 0.219 0.652 0.6077 6.15 1.547 0.5913 5.90 0.814 0.755 0.733 0.792 0.5707 5.52 1.547 0.5815 5.71 0.7e2 0.739 0.785 0.808 0.5345 4.93 1.547 0.5460 5.08 0.701 7.13 0.796 0.834
FIQ.2 E X P T . 3-With absorbent cotton as filter, a somewhat wider range of H+-ion concentration was studied, using M HC1 and M NaOH. The results are shown in Table V and in Fig. 3. TABLEV Whey
cc. 1000 1000 1000 1000 1000
1000
-Before Heating-After HC1 NaOH E. M. F. N cc. c c . pH Grams E. M. F.
. .. .. ..
12.5 19.0 25.0
.. ....
.. . 8
12
~
0.6278 0.5390 0.5093 0.4879 0.6763 0.7104
6.50 5.00 4.50 4.14 7.35 7.92
1.694 1.694 1.694 1.694 1.694 1.694
0.6058 0.5472 0.5135 0.4927 0.6455 0.6644
pH
Heating-N in N in Filtrate Curd
6.14 5.09 4.5s 4.23
1.451 0.791 0.709 0.770 6.80 1.528 7.12 1.571
0.243 0.903 0.985 0.924 0.166 0.123
From these experiments we can assume the optimum Hf-ion concentration for t h e heat coagulation of proteins in the whey t o be about pH 4.5, a t which point the filtrate was clearest in appearance and contained the least amount of nitrogen, and the curd was firmest for handling. The titration curves, obtained before and after heating of the whey, intersected each other a t a point near t o pH 5 . 7 ; a t t h a t point there was no change in the Hi-ion concentration of the whey as a result of
t
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
June, 1921
beating. This fact has a n important bearing upon t h e precipitation of phosphates from t h e milk serum by 1.6
1.2
1.0
E
-0
r
Y
E z
TABLE VI1
....................... 1000 1000 1000 1000 1000 ........................... 19 ... ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 20 ... . . . . . . . . . . . . . . . . . . . . . 40 ... *.. .................... ... ... 40 H~O, C C . . ....................... 40 ii' 1 E . M . F . ........ 0.6256 0.5078 0.4972 0 . iii4 0.6075 Qefore heating pH.. ............ 6.43 4.43 4.26 4.48 4.43 N total. ......... 1.615 1.615 1.615 1,615 1.615 E . M . F . ........ 0.6050 0.5124 0.5021 0.5139 0.5097 4.51 4.41 After heating 1pH .............. 6.09 4.49 4.32 N in filtrate ..... 1.510 0.658 0.719 0.645 0.656 ( N in curd ....... 0.105 0.957 0.896 0.970 0.960 Whey, cc HCl. cc CaClz cc Acetic' acid, cc... Lactic acid, cc..
i
1.4
--
51 7
0. 08
Ob
04
with about t h e same degree of safety if t h e titration curve is carefully followed. I n this particular experiment the quantity of calcium chloride solution added lowered t h e pH of the mixture somewhat below t h e optimum reaction for t h e coagulation, and hence caused t h e re-solution of a portion of t h e coagulated proteins upon t h e acid side of t h e optimum. It is evident even here t h a t i t exerts no favorable effect upon t h e removal of coagulable proteins, and its use could be discontinued in the factories. U S E GF I K D I C A T O R S T O D E T E R M I N E O P T I M U M R E A C T I O N
cc. ~ / HCI i I N 1000 cc. wnw
CC M,/
NmOH I N 1000 C.C WHEY
Fro. 3
heat. The first four samples of Table V gave their usual whey color reaction after heating (the opalescence changing t o a clear green color), whereas t h e last two showed intense caramelization. This is in keeping with t h e well-known fact t h a t alkalinity favors caramelization and oxidation of lactose. There is no apparent coagulation or increase in t h e turbidity of t h e whey upon t h e addition of acid in t h e cold. This proves t h a t there is no casein as such in the whey, and t h a t t h e majority of the proteins remaining are of t h e heat-coagulable type. E X P T . 4-As a further check upon t h e optimum reaction with hydrochldric acid, another series was run, using a different sample of whey from another day's cheese make, with the results given in Table VI. TABLEV I -Before Whey HC1 E. M. F. Cc. Cc.
Heating-N D H Grams
E. M . F.
pH
1000 1000 1000 1000
6.49 4.76 4.48 4.22
0.6067 0.6312 .0.5142 0.4994
0.01 4.84 4.54 4.30
.. 15
19 23
0.6294 0.5262 0.5098 0.4941
1.538 1.538 1.638 1.538
-----After
Heating---N in Filtrate
1.430 0,702 0.658 0.694
N in Curd
0.108 0.836 0.880 0.844
The results were in keeping with all others bearing upon the optimum reaction. We are safe i n considering t h a t this reaction in t h e whey lies near t o 4.5, which is also very close t o the isoelectric point of casein. INFLUENCE
OF
DIFFERENT
ACIDS
UPON
OPTIMUM
RE-
ACTION
T o ascertain .whether different acids would exert their anion activity t o deflect this optimum as determined. for hydrochloric acid, a separate set of experiments was planned, using the data i n Fig. 1 and Table 11. The results appear in Table VII. Thus, nearly t h e same amount of protein in each whey has been coagulated in nearly equal pH, reached by t h e different acids. I n other words, t h e acids studied have practically t h e same effect upon t h e coagulation of t h e proteins i n t h e whey. While organic acids would be preferable from t h e standpoint of ease in reaching the optimum point, because of their stronger buffer action, hydrochloric acid can be used
I n another section of this paper attention is called t o t h e probable conduct of methyl red in whey, and t o possible misinterpretations accompanying its use. The conduct of methyl red in'skim milk has been strikingly revealed a1ready.l If we now refer t o Table I we observe t h a t t h e colorimetric pH of whey-HC1 is 5 . 5 a t pH 5.26 (electrometric), and 5.1 a t pH 3.98. Upon heating t h e whey as usual, however, t h e methyl red indication of pH changes in t h e opposite direction from t h e pH as indicated by t h e hydrogen electrode. This is brought out in t h e following example: Before Heating
Electrometric p H of whey.. Methyl red p H of whey.
...
......
4.43 5.3
After Heating
4.49 4.8
It is apparent t h a t methyl red cannot be relied upon t o give a n indication of t h e proper reaction in this problem, and it is extremely doubtful if any indicator which covers this region of pH could be employed t o this end, because of the great protein error. One of us (Zoller) has had t h e opportunity t o t r y methyl red in the treatment of whey in milk-sugar factories, I t was there observed t h a t when a great bulk of whey (10,000 to 25,000 lbs.) had been adjusted with hydrochloric acid and lime t o pH 5.4 by methyl red, using a block comparator, and t h e whey heated with live steam t o 95" t o 100" C., the resulting clear green liquor showed a pH of 4.9 t o 5.0 with the same indicator. Hydrogen-electrode measurements on t h e same clear liquor showed a true reaction of about 4.5. It is clear, therefore, t h a t if we make use of methyl red in t h e adjustment of whey t o reactions within its range, we must do so with a realization t h a t i t is only approximate and t h a t we are following color reactions which have a value in themselves other t h a n the expression of pH. SIMPLE ANALYSIS O F CURD
It is imperative t h a t the composition of t h e coagulum or curd be known. This knowledge gives us a n idea of t h e nutritional value of t h e curd, and, in case of t h e 1
W. M. Clark, H. F. Zoller, A. 0. Dahlberg and A. C. Weimar, THIS 12 (1920),1163.
JOURNAL,
518
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
lactose industry, i t informs us of the quantity of substances removed from the whey by simple heating previous t o its concentration for lactose production. A sample of whey having a “-ion concentration of about pH 6.5 was divided into two portions, one of which was acidified with hydrochloric acid t o a pH of about 4.5. These solutions were heated in a steam chamber for 50 min., and the curds separated from them were strained through a piece of muslin, then suspended in alcohol, and strained again. Such samples of curd were dried t o constant weight a t 100” C., and then extracted with carbon tetrachloride t o remove fats. The loss in weight was 60.2 per cent in the acidified curd A and 59.2 per cent in the unacidified curd B.I The results of analysis of the fat and moisture-free curd are given in Table VIII. TABLEVI11 Curd A, pH 4.8 Nitrogen., . . . . . . . . . . . . . . . . . 12.00 Protei
Curd B, pH 6.5
10.79 69.0 6.67 93.33 2.28 2.57 0.07
The above analysis reveals that a t pH 4.5 there is more protein in the whey curd and less calcium phosphate. The increased precipitation of calcium phosphate a t the normal reaction of Swiss cheese whey (higher pH) is in perfect harmony with the facts of general chemistry. S O M E INDUSTRIAL APPLICATIONS
.
LACTOSE PRODUCTION-In the production Of lactose it is imperative, a t some time during the process of its isolation, t o free i t from proteins other than casein. The bulk of these other proteins are coagulable by heat, as has been shown by a number of investigations. No matter whether the whey has resulted from casein manufacture or cheese production, these coagulable proteins can best be removed more thoroughly by one heating if the whey is adjusted t o the reaction of pH 4.5. It is not essential, as the presented data show, t o have excess calcium present. If the whey results from casein manufacture under the grain-curd method,2 then we know t h a t this whey is too acid, i. e., it has a reaction of about pH 4.1 to4.2, as shown by the hydrogen electrode. Therefore, it will be necessary, when treating this whey, t o add some alkali t o bring the reaction back t o pH 4.5. I n case methyl red is used t o indicate this optimum i t will be necessary t o add alkali (lime or soda) until methyl red shows pH 5.4. It would be easier t o make this adjustment only once t o the exact pH and then, having determined the exact quantities of alkali t o be added t o a given weight of whey t o produce this reaction, i t would be practicable t o use these quantities as constant factors as long as the whey resulted from t h e same source, and was treated while fresh. If the whey results from cheese manufacture, then in case of fresh Swiss cheese whey i t is necessary only t o refer t o the titration curve t o find out how much 1 These figures correspond to fat contents in the ordinary sense, but may be somewhat larger than real fat contents because such samples as analyzed contain some water even after drying t o constant weight. 2 Clark, et al., LOC.cit
Vol. 13, No. 6
acid is necessary t o reach a given pH with 100 cc. of whey. From this the quantity can be readily calculated t h a t will be necessary t o adjust a given bulk of whey (say, 10,000 lbs.) t o the correct optimum reaction. From Table VI11 it will be seen t h a t we cannot expect t o remove very much of the salts a t this reaction, but in the lactose processing these will be readily removed in the vacuum pan through concentration and subsequent filtration. WHEY CHEESE I N D U S T R Y - T h e maximum amount of nutritive product can be removed from the whey for cheese making provided i t is first adjusted t o pH 4.5 before coagulating the protein. Thus from 1000 cc. of whey i t is practicable t o remove 0.9 g. in coagulable form (57 per cent), or about 0 . 5 lb. from 100 lbs. of whey. One-half Ib. of protein corresponds t o the quantity of protein in about 2 lbs. of cheese, assuming the average content of protein in various cheeses t o be 26 t o 27 per cent;1 or, upon a nitrogen basis alone, we would be able t o get more than 2 lbs. of cheese from 100 lbs. of whey.2 DISTRIBUTION O F NITROGEN I N WHEY
The following data indicate the distribution of nitrogen in 1000 cc. of whey: Per cent of Total N in Whev
........ ...... ............ ........
Total N in whey.. 1.577 N in curd at p H 4.51 0.902 N in filtrate.. 0.675 Albuminoid N in filtrateg. .. 0.109 Nonalbuminoid N 0.566 1 HCI was used. 8 Stutzer’s method. XOTE
ON PREPARATION
OF
Per cent of Total N in Filtrate
100
...
...
57 43
100
36
84
7
PROTEIN-FREE
16
MILK
AND
ISOLATION O F LACTALBUMIN
Osborne and Mendel,3 as well as Mitchell and Nelson,4 have prepared “protein-free” milk from skim milk or milk powder, by removing lactalbumin from the caseinfree filtrate. Van Slyke and Bosworth6 and Palmer and Scott‘ have conducted some investigations on the coagulation of lactalbumin in milk, but i t seems t h a t they did not actually seek the optimum pH for the heat coagulation of the crude protein, termed “lactalbumin” by so many. I n such cases as the preparation of “protein-free” milk and the determination and isolation of “lactalbumin” from its solutions, i t is advantageous or necessary t o determine the optimum reaction for the heat coagulation of these proteins, because this protein is soluble in water a t any pH. It is difficult t o know whether the addition of acid is short; overstepped, or just enough for complete coagulation by heating, un1 Average protein content of various cheeses is about 26 per cent, calculating from,Sherman’s “Food Products,” 1919, 108; and that of fifteen American Swiss cheeses is 27 per cent, according to U.S. Department of Agriculture, Bulletin 608. 2 According t o T. R. Pirtle of the Dairy Division the total production of whey in the United States in 1919 was about 3,780,000,000 lbs. If we assume the average nitrogen content of the whey is similar t o the whey examined above (perhaps slightly less), the quantity of available pro:eins in the whey corresponds to 76,600,000 lbs. of cheese. 8 “Feeding Experiments with Isolated Food Substances,” Carnegie Institution, Washington, D . C.,Publication 166 (1911), Part. 2, 80. 4 J. B i d . Chenz., 2s (1915), 459. 6. Ibij., 2 0 (1915), 135. 6 Ibrd., 37 (1919),271.
June, 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
less the solution is heated, filtered, and examined very carefully after each addition of acid. It is believed t h a t the above determination of t h e optimum reaction will prove beneficial in the preparation of these so-called “protein-free milks.” If we compare the nitrogen-distribution data with t h e brief analytical data t o be found on the subject of proteinfree milk one cannot help but feel t h a t the method of their preparation can be improved upon. RELATION O F OPTIXUM p H F O R HEAT COAGULATION TO ISOELECTRIC POINT O P WHEY PROTEINS
The optimum reaction for the heat coagulation of proteins (dehydration or denaturation) is not necessarily synonymous with their isoelectric condition. We would not say, therefore, t h a t the reaction pH 4.5 is the isoelectric point of the mixture of those proteins. If we could consider, in view of the fact t h a t a t pH 3.8 the heat-coagulated curd redissolves and thereby becomes positively charged with respect t o the acid with which i t has combined, t h a t the isoelectric zone had been overstepped during the addition of acid, then we could assume t h a t a t the point of maximum curd formation b y heat we had the minimum overstepping in either direction. This being the case, it would be natural
519
t o believe t h a t p H 4.5 is near t o the isoelectric zone for the mixture of heat-coagulable proteins in the whey. The isoelectric point of lactalbumin would then‘ be within this zone. Lactalbumin is only the major constituent of the heat-coagulable proteins of whey. SUMMARY
1-Using methyl red as indicator, titration curves of whey were determined for hydrochloric, acetic, and lactic acids. D a t a are also presented for composing a similar curve for a mixture of hydrochloric acid and c a1ci u m chloride. 2-The optimum reaction for the heat coagulation of the proteins in whey is about p H 4.5 (electrometric). 3-The different acids seem t o have the same effect upon the zone of optimum coagulation. &The inaccuracy of methyl red in the determination of the correct reaction of whey is discussed. 5-The composition of the curd and the distribution of nitrogen in the whey were briefly examined. 6-The utility of this optimum reaction is emphasized in ( a ) the determination of “lactalbumin,” ( b ) production of lactose, (c) manufacture of whey cheese, and ( d ) preparation of “protein-free” milk.
The Variability of Crude Rubber1 By John B. Tuttle
,
68 BANKSTREET.NEW Y O R KN. ~ Y.
When plantation rubber first came on the market in appreciable quantities, the rubber manufacturers found t h a t there was considerable variation between any two lots, a n d for some time this fact created quite a prejudice against the use of plantation rubber. At first, i t was thought t h a t the trouble was entirely due t o the way in which the rubber was coagulated and dried; and, by exposing t h e wet coagulum t o smoke during the drying, attempts were made t o duplicate, as far as possible, the method of coagulation used in preparing the best grade of wild rubber, viz., Fine Para. T h e special efforts t o produce a smoked sheet of high quality were quite successful, and for some time such rubber commanded a premium over the rest of the plantation rubber. These smoked sheets were quite uniform in quality, but from our present knowledge, i t is quite safe t o say t h a t this superiority was not caused by the smoking, but rather by the unusual care which was taken in coagulating, drying, and smoking, and by the fact t h a t only the best quality latex was used in their preparation. Notwithstanding the improvement in smoked sheets, i t soon developed t h a t the problem of variability in crude rubber had not been solved, and a t the Rubber Exposition in London in 1914 the subject received wide attention. By this time, the volume of plantation rubber being marketed had increased enormously, and the importance of this problem of variability grew correspondingly. With the increase in the number of factories using plantation rubber, especially where i t 1 Presented before the Rubber Division a t the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 to 6 , 1919.
included those, factories without adequate control, the losses became more serious t h a n ever. Although the conference held in connection with the 1914 exhibition discussed this subject a t some length, no conclusions were reached as t o the correct explanation of the trouble. Since t h a t time, the results of considerable work have been published, largely from the laboratories of the Department of Agriculture, Federated Malay States. These investigators have advanced many explanations as to the cause of variability, and they have adopted a method of measuring the variability in terms of “the rate of cure” and the tensile properties. The investigations on this subject took the form of vulcanization experiments on compounds containing various proportions of plantation rubber and sulfur. Eaton and his co-workers a t Kuala Lumpur, F. M. S., worked entirely with a compound of 90 per cent rubber and 10 per cent sulfur. Stevens used the same formula, while Schidrowitz used 92.5 per cent rubber and 7 . 5 per cent sulfur. Others used one or the other of these two formulas, but the point t o be noted in this connection is t h a t rubber and sulfur were the only constituents of the mixture. The rubber used was prepared in a variety of ways, and the rate of cure and tensile properties were supposed t o show the effect of changes in the methods of preparation. S U M M A R Y OF EATON’S W O R K
Eaton’s work has been summarized in Bulletin 27 of the Department of Agriculture, F. M. S., which gives in detail the methods of preparing the various
