20 The Literature of Gelatin T H O M A S T.
HILL,
Consultant and Lecturer
Rochester Institute of Technology, P . O . Box 3408, Rochester, Ν. Y . 14608
Gelatin,
the
partial-hydrolysis
product
animal
bone and skin raw materials,
colloid,
used in many industries,
fications.
This discussion
References and use
photographic. specific
include
industries,
those of interest
Included
a
with widely
is an annotated
particularly
of
variety
is a valuable varying
basic
to the papers
speci
bibliography. manufacturing
food, pharmaceutical,
are review
of
natural
with
and extensive
bibliographies.
' T p h i s discussion is not a state-of-the-art report, such as that by Idson and -1· Braswell (see Selected Periodical A r t i c l e s ) , but a survey of the most perti nent literature covering new uses and new ways of applying the old uses for gelatin. The literature of gelatin has its own peculiar pitfalls, partly since most of the literature grew out of its application rather than manufacture, and partly because of the lack of uniformity among gelatin products derived from loosely regulated raw materials and varied processes of manufacture. This has led to numerous subdivisions according to use and quality. Some of these variations result from the many types of skin and bone used i n making gelatin. Commercial gelatin is only a partially hydrolyzed collagen, meaning that the substance is not a single entity but a mixture of fragments of chain molecules having a variety of molecular weights and substituents. F o r mass uses, such as food, physical properties of reasonable uniformity are obtained by blending lots of varied gelatins to meet a set of compromise specifications. Specialized manufacturing processes are used to prepare lots w i t h particular physical a n d / o r chemical properties for unique purposes. Physical properties to be considered by a l l users, usually of a 6 - 2 / 3 % solution, are as follows: (1) Viscosity. Ranging from 10 to 70 millipoises. (2) Stiffness. Also called bloom value, after the inventor of a standardized gelometer to measure the pressure required to fracture the surface of set gelatin. (3) Setting Point. Reported i n terms of time and temperature. (4) M e l t i n g Point. Determined on set, "aged" gelatin. (5) p H of the Solution. Differs from isoionic p H . 381
382
LITERATURE
OF CHEMICAL TECHNOLOGY
(6) Isoionic p H (and Isoelectric p H ) or Point. This is the point on the p H scale at w h i c h the solution of the gelatin has the same number of protons combined on basic groups as it has dissociated from acidic groups; for alkalineprocessed gelatins this value w i l l range from 4.8 to 5.1, and for acid-processed gelatins it w i l l be around 9.0 to 9.4. In the absence of other ions, this is the same as the isoelectric p H , but conditions often result i n a different value since the latter is defined as the p H value at w h i c h the net charge is zero. This is of practical importance because for many solution properties, maxima or minima occur at these points—i.e., an opalescence owing to aggregate formation is highest at these points. (7) Color and Turbidity. Kept to specific limits for various uses. (8) A s h . Expressed as percent, usually below 2 % , although certain uses require lower levels. (9) Sterility of the D r y Gelatin. U S P limit is 10,000 per gram; normally food gelatins test out at about 1 0 % of this value. (10) Stability i n Solutions. A n important property i n pharmaceutical capsules or other preparations i n w h i c h the gelatin must be held for a length of time before setting. Although food gelatin is judged mainly by the 10 physical properties listed above, other properties are important to pharmaceutical and photographic users, who require greater care i n handling and segregating raw materials and more quality control testing. F o r photographic gelatins, for example, the above properties are only the beginning of a characterization because "trace" impurities present i n a few parts per million or billion have critical effects on the performance of photographic emulsions. L i m i t s of a few p.p.m. are usually specified for heavy metals, sulfur compounds, aldehydes, and other such impurities. F o r instance, one photo gel sets the limit for thiosulfate, usually sodium thiosulfate, at 10 to 25 p.p.m. T i n and mercury can both cause heavy fog i n a photographic emulsion at 0.5 p.p.m. Since analytical methods are seldom able to detect such small concentrations, the emulsion must be tested b y actual photography. Because manufacturing methods also affect the gelatin's properties, one must know something about the procedure being used i n order to select a gelatin appropriate to the need. The demineralized bones of animals (called "ossein") and clean, dehaired animal skins are the two basic raw materials i n gelatin. These are soaked i n a dilute acid or alkali to "condition" them for easier hydrolysis. The hydrolysis itself is a short cooking process usually performed i n stages, resulting i n dilute gelatin solutions of varying properties w h i c h are then blended together to form large lots at a mean quality. Because all types of gelatin are usually made i n the same plant, the F o o d and D r u g Administration regulations on sanitation prevent the bacterial damage w h i c h can occur i n a l l grades of the product. There are two main groups of technical grade gelatins—(1) the low quality materials just below good grade, w h i c h are available at lower prices and are used for paper sizing, adhesive, and other non-critical purposes, and (2) the higher quality materials selected for some special property, such as high bloom, w h i c h cost more. Therefore, to match need to the characteristics of the supply, close cooperation between the user and the supplier is necessary. Unfortunately, misunder-
20.
HILL
Gelatin
383
standings about the properties of a given type of gelatin often arise from contradictory lab results caused by minor changes i n procedure. In addition, the tradition of secrecy among producers may b i n d a user to a particular supplier and prevent both the user and other suppliers from obtaining information on new, improved preparations. Because of the difficulty i n obtaining uniformity from batch to batch, much work has been done to develop substitutes, w i t h little success. Because gelatin usually performs more than one function i n a given application, synthetics have often been found helpful i n one aspect and defective i n the others. Therefore, the trend has been toward developing specialized gelatin derivatives rather than turning to a wholly unrelated substitute; i n many products "compatible" synthetics are used w i t h gelatin, particularly to increase its stability to temperature or humidity. T w o broad sources of information on the topic merit special attention. The trade associations, such as the Gelatin Manufacturers' Institute of America and the British Gelatin and Glue Research Association, are excellent starting points for a literature search on gelatin. The patent literature is more specialized, and i n this country at least is aimed primarily at modes of application rather than manufacture. Gelatin itself is included i n Class 260, subclasses 117 and 118 being most important, especially for purification and recovery. Some patents w i l l be found under Class 303 ( C o l l o i d s ) , subclasses 315 and 316. Hardening is covered under Class 95 (Photography), subclasses 7 and 111. Other photographic uses w i l l be found under Class 96, including colloid transfer i n subclass 28 and diffusion transfer reversal processes i n 96-29. Class 95-7 also includes gelatin derivatives and their uses, while Class 96-114 includes synthetic co-polymers used w i t h gelatins. Coacervation techniques such as those found i n the currently active field of microcapsules w i l l be found i n Classes 252-316. In other cases gelatin used as a raw material is classified under the finished product. W h i l e gelatin is derived from the bones and skins of animals, glue is a product of the horns and hooves. This distinction is important i n many uses, particularly i n adhesives where only about 3% of the lowest quality gelatin is used along w i t h glues. However, i n Europe, especially Germany and Great Britain, glue and gelatin are often made i n the same plants and are therefore discussed together i n the trade and technical literature and share the same trade associations. T h e leather industry, as discussed by Shreve and Strieby i n this volume, contains reference of interest to the gelatin user because tanning agents, such as alum and the aldehydes, were the original hardeners for gelatin. Also in this volume the chapters on the food industry and on photographic chemicals contain references concerning gelatin. The biomedical literature also contains studies of collagen from a medical or scientific angle. There are many ways of subdividing the literature of gelatin. In addition to a division into different applications, sources may be listed according to raw materials, process, or physical properties. Hence the bibliography here is a collection of information sources w h i c h should be valuable to any searcher, coded to indicate the application it emphasizes. Should this bibliography fail
384
LITERATURE
OF CHEMICAL TECHNOLOGY
to uncover the desired information, the reader is invited to query the author, who has a more extensive file at hand. As the literature contains more tech nology than science, the reader must expect to encounter frequent statistical inconsistencies stemming from defects i n the methods of testing and the materials tested. The bibliography is coded according to primary uses as follows: A
F o o d Gelatins
A3
General applications. Special types emphasizing specific properties. Special uses requiring special colors.
Al A2
Β
Pharmaceutical Gelatins
Bl B2 B3
H a r d capsules. Soft capsules. Other, such as silver proteinates, etc.
C
Photographic Emulsions
CI
Non-sensitive layers, base-paper coatings. Photographic emulsions, highest purity gel. Graphic arts uses, collotype, e t c , where physical properties are central.
C2 C3 D
Technical Gelatins
DI D2 D3 D4 D5
Uses requiring high quality w i t h definite specifications. Uses tied to certain property. Uses requiring only low quality gelatin w i t h a wide range of properties; scrap. Scientific laboratory uses, such as a source of amino acid and derivatives. Scientific laboratory uses as a "model protein" for structural purposes, etc.
BIBLIOGRAPHY
Books Alexander, J., "Glue and Gelatin," Chemical Catalog Co., New York, 1923 (ABCD). Bogue, R. H., "The Chemistry and Technology of Gelatin and Glue," McGraw-Hill Book Co., New York, 1922 (ABCD). Croome, R. J., Clegg, F. G., "Photographic Gelatin," Focal Press, London and New York, 1965. The most recent and comprehensive study in English with an excellent bibliography (C, esp. C2). Fernbach, R. L., "Glues and Gelatine," D. Van Nostrand Co., Princeton, 1907 (ABCD). Gerngross, O., Goebel, E., "Chemie and Technologie der Leim- und Gelatine-Fabrikation," Steinkopff Verlag, Dresden, 1933 (ABCD). Glafkides, Pierre, "Photographic Chemistry," Fountain Press, London; The Macmillan Co., New York, 1958. Chapter XVII on Gelatin; other chapters on the uses of gelatin in the English translation, Vol. I (C). Gustavson, Κ. H., "The Chemistry and Reactivity of Collagen," Academic Press, New York, 1956. Deals mainly with the theoretical and scientific aspects of gelatin raw materials (ABCD). Kessling, A., "Leim and Gelatine," Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1923 (ABCD). Kirk, R. E., Othmer, D. F., "Encyclopedia of Chemical Technology," 2nd ed., John Wiley & Sons, New York, 1963, gelatin, p. 145; photographic gelatin, p. 153 (ABCD).
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385
Mees, C. Ε. K., "Theory of the Photographic Process," 2nd ed. 1954, 3rd ed. 1966, The Macmillan Co., New York ( C ) . Russell, G., "Chemical Analysis in Photography," Focal Press, London and New York, 1966. Two chapters on analytical methods for gelatin with much data not available elsewhere; references ( C ) . Sheppard, S. E . , "Gelatin in Photography," Vol. I, D . Van Nostrand Co., Princeton, 1923; pioneer monograph covering theoretical studies ( A B C D ) . Smith, P. I., "Glue and Gelatin," Chemical Publishing Co., New York, 1922 ( A B C D ) . Stainsby, G., Saunders, P. R., Ward, A. G., "Proceedings of the 13th International Congress Pure and Applied Chemistry, Uppsala," 1953; concerned mainly with theoretical studies on collagen ( A B C D ) . Stainsby, G., ed., "Recent Advances in Gelatine and Glue Research," Pergamon Press, London and New York, 1958; report of Conference of British Gelatin and Glue Research Association at Cambridge; excellent discussion of the properties and preparation of gelatins and glues; little on applications ( A B C D ) . Thiele, L . A., "Fabrikation von Leim und Gelatine," Janecke, Leipzig, 1922 ( A B C D ) . Veis, A., "Macromolecular Chemistry of Gelatin," Academic Press, New York, 1964; excellent and updated ( D ) . Specifications and Reports "Federal Specification for Dessert Powders and Plain Gelatin," U.S. C-D-22/a, Super intendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402, Sept. 1945. Revised periodically ( A ) . "Gelatin," Gelatin Manufacturers Institute of America (formerly the Gelatin Research Society of America), 55 West 42nd St., New York, Ν. Y., 1962; 42-page booklet summarizing manufacturing, properties, and uses of gelatin ( A B C D ) . "German Gelatin and Glue Industries, with Subsidiaries," BIOS Final Report 1431, PB 86033, Her Majesty's Stationery Office, London, 1946 ( A B C D ) . "German Gelatin and Hide Glue Industry," Final BIOS Report 1212, Items 22 and 31, Her Majesty's Stationery Office, London, 1946 ( A B C D ) . "Testing Methods for Photographic Gelatin," 2nd e d . , Commission on Testing Methods for Photographic Gelatin, Photographic, and Gelatin Industries, Tokyo, 1964 ( C 2 ) . tt
Periodicals Abstracts of the British Gelatin and Glue Research Association. 2A Dalmeny Ave., Holloway, London N7, England, quarterly ( A B C D ) . Adhaesion, Hadert-Lexikon-Verlag, Martin-Luther-Str. 88, Berlin W30, Germany, monthly. Chemical Abstracts, American Chemical Society, 1155 Sixteenth St., N.W., Washing ton, D . C. 20036, weekly: Year 1912 1915 1962 1965 1967
Sections 5. Photography 5. Photography 29. Leather & Glue 11. Radiation Chemistry & Photochemistry 45. Leather & Glue 11. Radiation Chemistry, Photochemistry, & Photographic Processes 45. Synthetic High Polymers 55. Leather & Related Materials 35. Synthetic High Polymers 41. Leather & Related Materials 74. Radiation Chemistry, Photochemistry, & Photographic Processes
Gelatine, Leim, Klebstoffe, discontinued with Vol. 12 (1944). Reappeared in 1957 as Adhaesion, which see. Selected Periodical Articles Ammann-Brass, H . , "Caracterisation des Gelatines Photographiques," Sci. Ind. Phot. XIX, 11, 401 (1948). Benesch, Reinhold, Benesch, Ruth, "Thiolation of Proteins," Proc. Natl. Acad. Sci. U.S. 44, 848 (1958) ( D ) .
386
LITERATURE OF CHEMICAL
TECHNOLOGY
Borginon, H . , "Photographic Properties of the Gelatin Macromolecule," J. Phot. Sci. 15, 207 (1967) ( C ) . One of 25 papers presented at a Conference on Photographic Gelatins at the British Cambridge University, Sept. 1967 and published in this Journal. This paper has an excellent bibliography of 105 entries. Courts, A., "Properties of Gelatin," Nature 171, 1099 (1953) ( A B C D ) . Edelstein, S. D., "Papin and his New Digester, Invention of the Pressure Kier and Pressure Dying," Am. Dyestuff Reptr. 51, 25 (1961) ( A B C D ) . Grettie, D. P., "Isoelectric Points of Collagen and Gelatin," J. Am. Leather Chem. Assoc. 60, 572 (1965); recent experimental studies related to earlier work ( A B C D ) . Highberger, J . H . , " A Half Century of Progress in Collagen Chemistry," J. Am. Leather Assoc. 48, 704 (1953); also as a reprint; an excellent review with references ( A B C D ) . Hilder, W . E . , "Gelatin and its Use in Paper Sizing," Brit. Paper Board Makers Assoc., Proc. Tech. Sect. 35, 121 (1954); review paper with many references. Hillson, P. J . , "Gelatin as a Protective Colloid," Nature 195, 1298 (1962) ( A B C D ) . Idson, B., Braswell, E . , "Gelatin," Advan. Food Res. V I I , 235 (1957); also in reprint, this is a review and bibliography on the state-of-the-art to the date of publication, with about 250 references ( A B C D ) . Janus, J . W., Darlow, R. L . R., "Intrinsic Viscosity of Gelatin," Nature 194, 1075 (1962); a study of anomalies and errors in determining viscosity ( A B C D ) . Kuntzel, A., Le Nanaon, J . C., "Effects of Anionic and Cationic Surfactants on Gelatin," Das Leder 12, 9, 82 (1961) ( A B C D ) . Leach, A . A., Williams, A . P., "Organic Constituents of Gelatins and Glues," J. Appl. Chem. 10, 367; 11, 10, 100 (1960/1961); detailed study with references ( A B C D ) . Rousselot, A., "The Isoelectric Point of Gelatin," Compt. Rend. 219, 62 (1944); in French ( A B C D ) . Sera, T. Y., Gumenyuk, A . G., "The Action of Ultra-Violet Radiation on Gelatin," Nauchn. Ezhegodnik Odessk. Univ. No. 2, 175 (1961); in Russian; abstracted in Zh. Fiz Abstr. (1962) ( A B C D ) . Tomoda, Y., Tsuda, M . , "Effect of Radiation on Gelatin Solution," Bull. Soc. Sci. Phot. Japan, No. 10 (Dec. 1960); special annual edition in English containing the previous year's Japanese articles; discusses exposure of cobalt 60 to dry and dissolved gelatin of various strengths ( A B C D ) . Ward, A. G., "The Chemical and Physical Properties of Gelatin," J. Phot. Sci. 3, 60 (1955); review with many references ( A B C D ) . Ibid., "Gelatin; The Current Position," J. Soc. Leather Trades Chemists 44, 505 (1960) ( A B C D ) . Ibid., "Present Position in Gelatin and Glue Research," J. Phot. Sci. 9, 56 (1961) (ABCD). Ibid., "Recent Advances in Gelatin Research," Chem. Ind. (London) 18, 502 (1954) (ABCD). Ibid., "Utilization of Tannery Waste for Gelatin and Glue Production," Res. Ind. 5, 239 (1960) ( A B C D ) . RECEIVED November 26, 1965. Updated 1967,
