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Vol. 21, No. 3
Photometric and Electrometric Measurements of Gelatin Behavior’ M. Briefer AT~ANTIC GELATINSCOMPANY, WOEURN,MASS.
It is shown that the physical characteristics of gelatin T T H E meeting of the or physical state. However, vary with the chemical treatment of the raw material Leather and Gelatin both isoelectric positions are and not with its biological history. Division of the AMERIsubstantially correct, as will The prevailing theory that gelatin is at its isoelectric CAN CHEMICAL SOCIETYheld be shown. In any event, it point specifically at pH = 4.7 is shown to have been a t Detroit in September, 1927, seems quite necessary that originally postulated on a half-truth and no longer a committee was appointed this question be studied and, tenable. to develop specifications for a if possible, more accurately Experimental data are given in proof that gelatin has standard gelatin. This comstated before any standard no specific isoelectric point but that there exist two m i t t e e w a s composed of specifications are proposed. types of gelatin, one carrying positively charged and the ClarkeE. Davis, of the NaTo this end a study is made other negatively charged ions, depending on the origitional Biscuit Company; S. E. of gelatins of different origins nal chemical treatment of the raw material. Sheppard, of the E a s t m a n and of different chemical It is experimentally shown that the ionic state of K o d a k C o m p a n y ; and M. treatments. animal skins, resulting from originally acid or alkaline Briefer, of the Atlantic GelaMethod treatment, is not changed with respect to the isoelectine Company. tric point by subsequent treatment with electrolytes, What such a standard gelaFor the determination of and that the gelatins extracted from these materials tin should be or what purpose the physical p r o p e r t i e s of have corresponding physical and chemical characterit shall serve has been left gelatin, the writer has found istics. The specific reactions of animal hide to chemiundefined. No restrictions of the jelly-consistency apparacal treatment, and the persistence of the initial effect, the committee’s a c t i v i t i e s tus of Briefer and Cohen5 should be an important factor in the chemistry of have been proposed. As a resufficiently well adapted to leather. t h e p r e s e n t study. Their sult of a few informal conferSome additional details are given with respect to isoprocedure and method of plotences and considerable interelectric turbidity. ting the curves have been committee correspondence, a retained. Likewise. for the ~ l a for n conductinn an investigation into the properties and behavior of gelatin has been measurements of turbidity, the photometric method deprovisionally suggested, the idea being to collect a symposium scribed in the same paper5 was used except that a Wratten embodying the more recent investigations of gelatin and A filter No. 25 was substituted for B No. 58. This is a red to add such new information as may develop from the work monochromal providing greater accuracy in matching the of the committee and such other investigators as it may be fields. able to impress into the service. This study is the writer’s JELLY CONBISTENCY-T~~ jelly-consistency apparatus meascontribution as a member of the committee. ures the resistance of an approximately 1 per cent chilled gelatin jelly to the fall of an 11-mg. round lead shot from a Scope of Work height of 50 cm. The fall of the shot is arrested a t different One of the fundamental conditions of gelatin, that of its levels according to the strength of the jelly. The measureisoelectric point, has not been fully investigated. LoebZ ments were made over a range from about pH 4 to 10, thus places it at pH = 4.7 for practically ash-free gelatin. Bogue3 passing through the isoelectric point in each experiment. and other investigators of gelatin behavior either concur Within the above pH range there are always two maxima, in or confirm this pH position from independent investi- one on each side of the isoelectric point wherever it may be gations. The correctness of this conclusion has been ques- found. It is necessary that the titration be sufficient to intioned at various times because of discrepancies found in clude these two maxima. Between these peaks there is a attempts to verify it for different specimens of gelatin. I n depression varying in depth according to conditions to be 1924 the National Association of Glue Manufacturers‘ pub- noted later. The isoelectric point will be found i n t h e region lished some standard methods for determining viscosity and of this depression. However, specimens of gelatin have been jelly strength of glue. In this work the following statement found in which the depression is very slight or almost entirely appears: absent and the two maxima are nearly merged into one broad Recent work by J. A. Wilson and his co-workers indicates that peak. These exceptions to the average characteristic curve the isoelectric point of gelatin may be at pH = 7.7 instead of of gelatin behavior preclude the use of the minimum physical at 4.7 or that 4.7 may be the isoelectric point of the sol form values of gelatin-water mixtures for the estimation of the and 7.7 of the gel form. isoelectric point. I n view of the foregoing, it may be well It is difficult to see how gelatin can change from an acid to to anticipate here that the turbidity curve is an almost una basic condition merely through a change of temperature failing index of the approximate position of the isoelectric 1 Presented before the Division of Leather and Gelatin Chemistry at point and with microtitrations the position may be fixed the 76th Meeting of the American Chemical Society, Swampscott, Mass., with precision. September 10 to 14, 1928. TuRBIDIm-considerable information is to be obtained 2 Loeb, “Proteins and the Theory of Colloidal Behavior,” p. 44, from a study of the isoelectric turbidity of gelatin solutions. McGraw-Hill Book Co., New York, 1924. 8 Bogue, “Colloidal Behavior,” Vol. I, p. 25, McGraw-Hill Book Co., Such factors as the dispersity, intensity, extent, and location New York, 1924. on the pH scale-each has a message with referencepoethe 4 National Association of Glue Manufacturers, IND. END.CHEM.,16,
A
3, 310 (1924).
5
Briefer and Cohen, IND. END. CHRM.,20, 408 (1928).
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INDUSTRIAL A N D ENGINEERING
r
267
I
100 = a constant
=
so that
./ quality and condition of the gelatin. I n view of the importance attached to this property, the method of measurement should be such that the results may be expressed mathematically in terms of some standard unit. For the purpose of this investigation it has been convenient to use in part the photometry of Hurter and Driffield.6 The photometer described in an earlier paper5 was used for the density measurements. For convenience, the equation for converting the angle of rotation to density for this instrument is repeated here: loglo tanaA1 - log,, tanZA = D (density) Hurter and DrifEeld6 expressed the relations between incident light, I, transmitted light, It, transparency, T , opacity, 0, and density, D, with the following equations: I It D = loglo 0 = logloz and T = (1) I The density measurements reported in this work were made through a layer of approximately 1 per cent gelatin jelly 15mm. deep a t a temperature of 10" C. In order properly to match the fields and make accurate readings possible, the effect of the orange-red transmitted by a turbid gelatin jelly must be masked. For this purpose a Wratten A filter No. 25 was placed between the light source and the cylinder containing the material to be examined, the light being first atered through white opal glass. With this arrangement the fields may be matched accurately for all measurable turbidity intensities. When we attempt to visualize quantitatively the difference between two degrees of turbidities from their density values, the result is not very impressive. The same is true of pH values and all other logarithmic increments. For this reason it may be desirable to express turbidity in terms of the quantity of light transmitted. The percentage transmission may be computed from the density as follows: From Equation 1 we may write log I - log It = log 0 = D (2) Now since the light incident is always 100 per cent whatever its intensity, we have 1
CHEMIXTRY
Hurter and Dri5eld, J . SOC.Chcm. I n d . , 9, 455 (1890).
100 -log D = 0 and = It = per cent 0 light transmitted Applying these relations and substituting values, we have, for example, when D = 0.301, 100 -log 0.301 = 0 = 2 and 2 It = 50 per cent light transmitted Similarly, when D = 3.0, -log D = 10' and 101 1 0 3 = 102 -= 0.1 per cent light transmitted From the above considerations it is seen that the translation of D in terms of light transmitted may be reduced to -log (log 100 - D) = per cent It (3) -For example, when D = 2.6 -- log (2 - 2.6) 9.4 - 10 = 0.25 per cent It approximately -With the p e r c e n t a g e of l i g h t ___ transmitted known, and applying corrections for diffused and scattered - light, we may apply the term ( l / m ) n for multiple layers, where the parenthesis encloses the fractions of incident light passed by one unit layer of the turbid media but raised exponentially to a power, n, the number of layers employed. Conversely, the per cent transmission for any fraction of a layer employed may be
found from the term &X An example will make this clear. The maximum turbidity of the specimen of ash-free gelatin (Figure 3) is D 2.56 for a layer 15 mm. deep. This density transmits 0.276 per cent incident light (Equation 3); the transmission for a depth of 1 mm. would then be equal to
-
1 5 - lo = 67.6 per cent approximately d0.00276 = 7'441 15
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Vol. 21, No. 3
INDUSTRIAL A N D ENGINEERING CHEMISTRY
268
different from gelatin of calfskin or other cattle material. All gelatin raw stock, whether bone, pigskin, calfskin, sinews, and so on, when origin a l l y conditioned with lime, produces gelatin with maximum turbidity and minimum physical properties on the acid side of the pH scale. It does not matter if the lime is washed out of the material or .,' removed with acid, nor at what ,* pH the extraction is made, the resultant gelatin will still have the same characteristics and the isoelectric point will remain in the region of pH = 5. On t h e o t h e r h a n d , the same Io materials, when originally conditioned with an acid, will yield gelatins having their isoelectric points in the region of pH = 7 to 8 with c o r r e s p o n d i n g physical relations. Even dea s h i n g does not change the main characteristics of either type of gelatin. It should be added that when animal skins are incompletely limed or acid-plumped the resultant gelatin will have its isoelectric point shifted according to the degree of conversion. Skins only partly limed and subsequently treated with an acid nill produce gelatin having its isoelectricpoint corresponding to an equivalent mixture of these two types of gelatin. This problem is treated in detail under another heading. 60
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Isoelectric Point
The "isoelectric point" is defined as the condition of no migration to anode O r cathode in an electric field. This condition is a consequence of the neutralization Of the positive and negative ions in solution. Theoretically it is the point Of minimum physical values of a colloid and the Point of absolute precipitation Of solid substances in solution or collodial suspension.' For the present work liberty is taken with the scientific definition of the isoelectric point and it is approximately determined by the precipitation or maximum turbidity factor. Warrant is given for such treatment by Pauli18L ~ e b Sheppard,lo ,~ and otherslL1who have used the condition of minimum physical values as an index of the isoelectric point of colloids. Briefer and Cohen5 have shown that acid-dumped Pig skins produced gelatin having its isoelectric point and maximum turbidity in the region of pH = 8 while limed calf skins and other parts of limed cattle stock yielded gelatins with isoelectric points and maximum turbidities in the region of pH = 5. Dah1berg,'2 by an independent method, finds the isoelectric point of gelatin to vary with the type of raw material from which the extraction was made and considers that different kinds of gelatin molecules are produced by the respective raw materials. $0 far as the miter is aware, the published works on gelatin behavior, and of the effect of the previous history of the raw material on the properties of fail to account for the different physical characteristics of pigskin gelatin and gelatins extracted from other animal stocks. It will be shown that all gelatins, irrespective of their origin, have similar physical properties and response to electrolytes when the initial chemical treatment of the raw material is of the same order. It is not correct, then, to postulate that gelatin extracted from pig skins is fundamentally
'
Hardy, Proc. Roy. Soc. (London), 66, 110 (1900). Pauli, s~Colloid of the Proteins,,, Blakistons, 1922; l o i d - Z . , 18, 222 (1913). 9 Loeb, 09. c i f . , p. 9. 10 Sheppard, Trans. Soc. Motion Picture En&, 11, NO. 32, 707 (1927). 11 Colloid Symposium Monograph, Vol. 11, pp. 240-243, 352, Chermcal Catalog Co., New York. 1925. 12 Carpenter, Dahlberg, and Hening, IND. ENG.c B B ~ . .20, 397 (1928).
n*7:0,
0.
"!'ilill
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Gelatin Samples Used A-Pigskin gelatin from limed stock extracted at p~ = 4.0. B-As above, but extracted at pH = 5.8. C-Calfskin gelatins from limed skins treated with different acids. D-Pigskin gelatin originally plumped with sulfuric acid. E-Pigskin gelatin originally acid-treated and de-ashed by the method of Loeb. Original ash, 0.72 per cent; after deashing, 0.04 per cent. Final PH = 4.7. by s* E. Sheppard*
g$:;!F$l$$
:z:Ed
~
acid-plumped. ~ i ~
-
~ -Ifskin i ~
Nofe-G and H were extracted from the same material. A quantity of virgm calf skins was divided into two parts and each treated appropriately. The results are shown in Figure 8.
with the work represent more than The one hundred determinations, each determination representing a group of eighteen individual solutions* These groups were each adjusted Over an appropriate pH chilled for a definite time in a carefully controlled bath, and PH, and The then measured for jelly work covered a period of about nine months. Nature of Material Causing Turbidity
Some of the characteristics of the substance responsible for the turbid effect produced in gelatin developed in this study are given here. The optical intensity increases with decreasing temperature and is reversible. With sufficient time the turbid material coagulates and settles out of solution. Turbidity varies with the concentration of gelatin reaching a maximum and again decreasing. Two different types of the turbid substance exist-one carrying positive and the other, negative ions.
March, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I n straight-run gelatin maximum turbidity is coincident with minimum physical properties. Any number of gelatins extracted from limed stock may be mixed and the maximum turbidity and minimum physical properties of their solution will be in the region of pH = 5 . Similarly, any number of gelatins extracted from originally acid-treated stock may be mixed and the maximum turbidity and minimum physical properties of their solutions will be in the region of pH = 8. When the two different types of gelatin are mixed, turbidity will be a t a maximum betxeen pH 4.7 and 8, the precise location bearing some simple ratio to the proportions of the mixture, but the minimum physical properties will not always be found a t the isoelectric point as in the case of straight-run gelatins. The turbidity product varies in quantity in different specimens of gelatin. It seems obvious that the intensity of turbidity is related to the degree of hydration resulting from manufacturing procedure. The ash content of g e l a t i n appears to have no specific rel a t i o n to the turbidity product, ind i c a t i n g t h a t the turbidity product is of purely o r g a n i c origin. Experimental
269
an acid in order to swell (plump) the stock preparatory to extraction. This method was used in the preparation of the specimens for this group. Originally acid-conditioned stock of whatever origin produces gelatin very different in character from that of the same classes conditioned in limewater. Curve A in Figure 4 is from a normal commercial pigskin gelatin (ash = 0.76 per cent); curve B is the same gelatin de-ashed (ash = 0.04 per cent). In Figure 8 a curve of originally acid-plumped calfskin gelatin is shown against the same material originally treated with lime. All curves from Originally acid-conditioned stock are similar. The maximum turbidities are all at about pH = 8. The distinctive feature of this class of gelatin is that one jelly-consistency maximum is on the H + side and the other on the (OH)- side of the pH scale. This must inevitably follow since the two jelly-consistency maxima are always one to the right and one to the left of the isoelectric point. MIXTURESOF GELATINFROM ORIGINALLY ACID-CONDITIONED AKD LIMEDSTocKs-When gelatin from originally acid-conditioned stock is mixed with gelatin extracted from originally limed stock, the isoelectric point and maximum turbidity are both shifted toward the neutral point on the pH scale, the amount of shift varying with the proportions of the mixtures. It will be seen, however, that, although as much as 80 per cent of gelatin from acid-conditioned stock is present in the mixture, the isoelectric point and maximum turbidity still remain on the H+ side of the pH scale. When 90 per cent gelatin from acid-conditioned stock is present in the mixture, the ions are neutralized and the isoelectric point is a t pH = 7. On the other hand, the second jelly-consistency maximum persists on the (OH)- side throughout the entire range of percentage composition of the two classes of gelatin. I n all mixtures of this nature the basic gelatinate characteristics largely predominate with reference to the
G E L A T I NF R O M LIMEDSTOCK-Gelatin extracted from stock that has been conditioned in limewater has a charact e r i s t i c c u r v e for both jelly consistency and turbidity. Removing the lime with an acid or using excess acid during extraction does not change the nature of this curve. The curves are similar for pigskin, calfskin, or any animal material suitable for gelatin extraction. Figure 1 shows jelly consistency and turbidity curves of gelatin extracted from limed cattle skins, treated with different acids and thoroughly washed. All curves are similar. Figure 2 represents similar properties for gelatins extracted from limed pig skins and subsequently treated in a similar manner. Extractions were made at pH = 4 and 5.8, the average cooking conditions of originally acid-plumped pig skins and limed calf skins, respectively. Figure 3 shows a similar curve for ash-free gelatin supplied by S. E. Sheppard. The curve characteristics indicate that this specimen is undoubtedly of limed stock origin. The restricted range or cramping of the curve is probably due to hydrolysis during the de-ashing process. These specimens, different as to TURBID~V CURVE5 origin but alike as to initial chemical treatment (liming), show similar curve characteristics. The turbidity maxima and minimum physical properties are all a t approximately pH = 5. Both jelly consistency maxima are on the Hf side of the pH scale. It should be noted that the form and isoelectric point, while the acid gelatinate completely prelocation of this double curve is distinctive for all gelatins ex- dominates with reference to the second jelly-consistency tracted from originally limed material. maximum. The curves in Figures 5 and 6 show these conGELATINFROM ORIGINALLY ACID-CONDITIONED (PLUMPED) ditions. STOCK-The usual manufacturing procedure in the prepaASH-FREEOR CHEMICAL GELATIN-It has been assumed ration of this class of gelatin is to treat the virgin skins with by Loeb and others that gelatin combines stoichiometrically
i
f
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 21, No. 3
with anions and cations, respectively, to the left and right gelatin solutions are chilled to about 10" C., the turbidity of pH = 4.7. The assumption carries with it the idea that is intensified but diminishes again with rise in temperature. gelatin may be purified or de-ashed only a t pH = 4.7, when Thus the solubility of the precipitate is a function of the temit is not in combination with either ion. To test this widely perature. accepted theory, specimen E, extracted from originally acidAt relatively high concentrations of isoelectric gelatin, the conditioned stock, having its isoelectric point at approxi- turbid effect is greatly diminished even in the cold and again mately pH = 8, was de-ashed by the method of Loeb. The when the concentration is too low, so that the turbidity effect ash was reduced from an original content of 0.76 to 0.04 per is also a function of the concentration. cent, the whole operation being conducted a t pH not greater A number of measurements were made (Figure 7) and it than 4.5 a t any time. This position corresponds to the first was found that maximum turbidity is in the region of a 1 jelly-consistency maximum of this class of gelatin and pre- per cent solution of isoelectric gelatin in water (with very sumably in the condition of its greatest combining power weak or low-grade gelatin the concentration necessary may with anions. After drying, the pH changed to 4.7, some of reach 1.5 per cent). The most logical explanation for these the acetic acid being evolved in the process. This de-ashed effects appears to be that a t concentrations greater than 1 gelatin shows a second maximum a t pH = 9. From the per cent the gelatin acts as a protective colloid, inhibiting turbidity maxima the isoelectric point remains a t approxi- precipitation a t the isoelectric point, while a t concentrations mately pH = 8. The original characteristics are unchanged less than 1 per cent the solubility in water diminishes the and obviously gelatin may be de-ashed without reference to relative volume of the precipitates. Accordingly, at approxiits isoelectric point. mately 1 per cent concentration water a t 10' C. may be conThe suggestion has been made that gelatin may not be in sidered as saturated with respect to isoelectric gelatin. The chemical combination with electrolytes and the above experi- measurements were made optically, through a depth of 15 ment appears to lend support to the adsorption theory.13 mm. as in the other turbidity experiments. TURBIDITY CHARACTERISTICS-Anumber of determinations were made to study the behavior of the substance reAcknowledgment sponsible for the turbid effect. In solution a t normal temperature this phenomenon is known as the Tyndall effect Grateful acknowledgment is made to S. E. Sheppard for and is due to selective absorption of blue light observable a very fine specimen of ash-free gelatin, to A. W. Thomas when a beam of white light is passed through a weak gelatin for checking some of the pH measurements, and to the Atsolution and viewed a t right angles to incidence. When the lantic Gelatine Company for choice specimens of raw materials and the use of its laboratory equipment. 11 09. c k , pp. 232 and 327.
Cellulose from Cornstalks'" H. A. Webber DEPARIMBNT OF CHEMICAL ENGINEBRING, IOWASIAIB COLLBGB, AXES, IOWA
HE idea of utilizing cornstalks as a source of cellulose in the United States is almost one hundred years old. On August 13, 1838, Homer Holland was granted United States Patent 878 for a process for "Preparing the Lignia or Fibrous Portion of Corn Husks, etc., by digestion in a solution of alkali carbonate causticized with lime." Since that time and up through the year 1927 more than two dozen other United States patents have been granted for making pulp from cornstalks.
T
Quantity of Cornstalks Produced
It is small wonder that the idea of making pulp from stalks occurred to so many, when one considers the vast amount of cornstalks available each year in this country. It is estimated that 175 million tons of cornstalks are produced each year in the United States. The methods used in arriving at this figure may be of interest to those not acquainted with the problem. It may be said that there is no general agreement as to the yield of cornstalks per acre. The data obtained from various sources show considerable variation in 1 Presented by 0. R. Sweeney before the Division of Cellulose Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928. 2 The results which this paper so briefly summarizes have been obtained by the joint codperation during the last few years of the United States Bureau of Standards, the Engineering Experiment Station, and the Agricultural Engineering and Chemical Engineering Departments of Iowa State College. A bulletin giving in detail the work done thus far is being issued by the Engineering Experiment Station, Iowa State College, and copies will soon be avnilable to those interested.
amount, as they have been determined for several varieties of corn grown under widely different conditions of soil and climate. L. K. Arnold, of the Engineering Experiment Station, Iowa State College, has made an extensive study of the yield data extant in the literature. Some reported results were discarded, as it was not always possible to determine whether they represented silage corn, corn fodder, or stover grown for grain. "Stover" is the agricultural term for that part of the corn plant which remains after the corn is husked, and includes the stalk proper, the leaves, tassels, and husks, but does not include the roots. The data considered, therefore, represent only the amounts of mature cornstalks (stover) available after the husking season. The data gathered for this study were tabulated in three groupings. The first group comprised data from nine corngrowing states showing actual ratios of the stover to the grain yields. The second group contained data from twelve state universities and agricultural experiment stations and four private investigators, showing stover yields in tons per acre. The third group included data from six state colleges and agricultural experiment stations, giving the average weight of mature stalks, and data obtained from four investigations, representing nearly two hundred fields of corn, giving the number of stalks per acre. An average of 9000 stalks to the acre with a bone-dry weight of 262 grams (0.576 pound) per stalk was obtained in the third grouping. A summary of the average yields of cornstalks (stover) obtained by these three methods is given in Table I.
