Gelatin, method for producing it and its use

A gelatin containing microgel, alpha-gelatin, oligomers of alpha-gelatin, and less than 25% by weight of the gelatin, of fragments of alpha-gelatin having a molecular weight of no more than 9.times.10.sup.4.

BACKGROUND OF THE INVENTION 
The present invention relates to a gelatin containing microgel, oligomers 
of alpha-gelatin, alpha-gelatin and fractions of alpha-gelatin (peptides), 
a method for producing such gelatins as well as the use of gelatins 
according to the invention, particularly as a binder for layers of 
photographic materials. 
Gelatin is known to be a purified protein obtained by partial hydrolysis of 
the scleroprotein collagen. Due to the different types of raw materials 
used in the production of gelatin, namely skin material from beef cattle 
and calves, bacon rind and ossein (demineralized, usually comminuted bone) 
and due to the differences in the manufacturing processes, the resulting 
gelatin varies considerably in its chemical and physical properties. 
Usually, the raw material is subjected to alkali decomposition, for example 
by means of milk of lime or soda lye, (the so-called "liming") and then 
melted out in an essentially neutral solution. Besides the alkali 
decomposition method, there is also the so-called "acid decomposition 
method" in which there is no alkali pretreatment and the melting out takes 
place in an acid medium. The respectively developed gelatin solutions are 
filtered, concentrated and dried. Gelatin manufacturing methods are 
discussed in detail in G. Reich, Kollagen, 1966, pages 242 et seq. 
published by Verlag Theodor Steinkopff, Dresden and in A. G. Ward and A. 
Courts, Science and Technology of Gelatin, 1977, Academic Press. 
The properties of the gelatin thus obtained depend to a great extent on the 
raw material employed, on the decomposition process selected and 
especially on the reaction conditions during decomposition, extraction and 
drying. The methods for producing gelatin and the realization of certain 
desired properties are based to a large extent on empiric experience. 
Although it is possible to obtain a considerable degree of 
reproduceability in this way, it has been found that gelatins intended for 
the manufacture of photographic products must be tested in practical 
experiments, in order to enable the manufacture of photographic materials 
having the required uniformity. 
The main component of the raw materials is the so-called tropocollagen, a 
well defined protein molecule consisting of two identical alpha.sub.1 
chains and one alpha.sub.2 chain which is somewhat different. These chains 
are linked together in the vicinity of their N-terminal amino acid. The 
amino acid sequence of the alpha.sub.1 chain is well known for the case of 
calfskin collagen; the polypeptide consists of a linear chain of 1052 
amino acids. In this connection, see P. I. Rose & S. Gross, Photographic 
Gelatin, (published by R. J. Cox), page 89, Academic Press, 1976. 
Gelatin consists of a mixture of various fractions of tropocollagen 
produced during the acid or alkali decomposition. The following four 
fractions are distinguishable as major components of gelatin and are 
present in different quantity ratios depending on the origin of the 
gelatin: 
1. alpha-gelatin, consisting of complete alpha polypeptide chains; 
molecular weight of 9.5.times.10.sup.4 ; 
2. oligomers of the alpha chain, consisting of 2 to 15 linked alpha chains; 
molecular weight of 10.sup.5 -10.sup.6 ; 
3. "microgel", consisting of polymers of up to 1000 linked alpha chains; 
molecular weight 10.sup.7 -10.sup.8 ; 
4. "peptides"; different sized cleavage products of the alpha chain; 
molecular weight 10.sup.4 to 9.times.10.sup.4. 
A typical composition of conventional types of gelatin obtained by alkali 
pretreatment of the raw material and subsequent extraction with water at 
about 45.degree. to 60.degree. C., is listed in Table 1 below. 
TABLE 1 
______________________________________ 
content 
Component molecular weight 
(weight %) 
______________________________________ 
migrogel 10.sup.7 - 10.sup.8 
0-15 
oligomers of alpha gelatin 
10.sup.5 - 10.sup.6 
10-30 
alpha gelatin 9.5 .times. 10.sup.4 
10-40 
fragment of alpha gelatin chain 
(peptides) 10.sup.4 - 9 .times. 10.sup.4 
30-80 
______________________________________ 
In this connection, see A. Veis, The Macromolecular Chemistry of Gelatin, 
Academic Press 1978, and I. Tomka, Chimia 30, pages 534 et seq. (1976, No. 
12). The separation of the gelatin into various fractions has been 
described in detail by I. Tomka et al in J. Phot. Sci., 23, 97 (1975). 
SUMMARY OF THE INVENTION 
It has now been found that the four major fractions of gelatin determine 
its physical properties and usability in very different ways. 
The most valuable components are the alpha gelatin fraction and its 
oligomers up to a size of about 10-15 alpha units. Due to the special 
configuration of the amino acid sequence, they are substantially 
determinative for the setting properties of the gelatin solutions. Because 
of upward limitations of their molecular weights, solutions of these 
fractions have low to medium viscosity as is often desirable for the 
manufacture of photographic coatings. 
Experience has shown that, due to its small proportion, the fraction called 
"microgel" which has the highest molecular weight does not contribute much 
to the lattice structure of the gelled gelatin and is unable, in 
particular, to significantly influence the setting speed. However, because 
of its sometimes extremely high molecular weight, this fraction is 
determinative to a great extent of the viscosity of the aqueous gelatin 
solutions. A high proportion of microgel is therefore desirable in those 
cases where high viscosity is preferred for reasons of pouring; in such 
cases no viscosity increasing additives need be used. If in other cases 
low viscosity is desired, a gelatin having a low microgel content will be 
preferable. 
The peptide fraction, the cleavage products of the alpha-gelatin, is the 
least valuable component of gelatin, with respect to physical properties. 
It has been found that the peptides do not take part in the formation of 
the lattice but remain substantially in the sol form. They thus weaken the 
lattice structure and retard gel formation. Slowly setting gelatins 
therefore contain a large proportion of peptides. 
A further factor has been found that can interfere with gel formation. 
Native collagen contains amino acids exclusively in the L-configuration, 
but with increasing decomposition times, there may gradually occur 
racemization causing part of the L-amino acids to be rearranged to the 
D-configuration. With greater proportions of D-amino acids in the chains, 
the build-up of a contiguous network may be interfered with for steric 
reasons, and this considerably lengthens the setting time of the 
solutions. It is therefore desirable to obtain gelatins having as low a 
content of D-amino acid radicals as possible. 
In many uses of gelatin in the food, pharmaceutical and photographic 
industries, it is extremely desirable to utilize quick setting gelatins 
because these are particularly easy to handle, for example, and provide 
reproduceable products of uniform quality. The setting times of known 
gelatins (measured at 16.degree. C. and with 2.5 g gelatin in 1 dl water) 
lie far above one minute which brings about considerable difficulties in 
many cases. 
It is an object of the present invention to provide a gelatin having a 
setting time less than that of prior art types of gelatins. 
It is a further object of the present invention to provide a gelatin having 
a setting time of less than one minute. 
To achieve these objects and in accordance with its purpose, the present 
invention provides a gelatin including microgel, alpha-gelatin, oligomers 
of alpha-gelatin, and less than 25% by weight of the gelatin, of fragments 
of alpha-gelatin having a molecular weight of no more than 
9.times.10.sup.4. 
The present invention further relates to methods for producing such 
gelatins and to their use as binders in food products or pharmaceutical 
preparations and preferably in coatings of photographic materials as well 
as to the use of such coatings of photographic materials produced by the 
use of the gelatins of the present invention. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary, but are not restrictive of 
the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to gelatins having an alpha-gelatin fragment 
or peptide content of less than 25%, preferably less than 20%, and most 
preferably less than 10% by weight of the gelatin. These fragments or 
peptides have a molecular weight of no more than 9.times.10.sup.4. 
The preferred gelatin compositions contain, by weight, 5 to 15% microgel, 
30 to 70% oligomers of alpha-gelatin (n=2-15), 20 to 60% alpha-gelatin and 
less than 25% peptides. 
At the same time, in the gelatin fractions, the content of amino acid 
radicals having a D-configuration is preferably no more than 5 weight 
percent. 
The molecular weight of the components of the microgel lies approximately 
between 10.sup.7 and 10.sup.8, and the oligomers of alpha-gelatin have 
molecular weights in the region between about 10.sup.5 to 10.sup.6, while 
the molecular weight of the alpha-gelatin is about 9.5.times.10.sup.4 and 
the fragments of alpha-gelatin (peptides) have a molecular weight of about 
10.sup.4 to 9.times.10.sup.4. 
A gelatin according to the present invention is distinguished by the fact 
that it has a high viscosity, i.e. more than 180 mP (millipoise) in a 10 
percent gelatin solution (10 g gelatin in 100 ml water) at 60.degree. C. 
The preferred viscosity range lies between 200 and 350 mP, most preferably 
between 200 and 300 mP, but the viscosity can also exceed such values. For 
6.67% gelatin solutions and at 40.degree. C., the viscosity is about 8 to 
20 cP (80 to 200 mP). 
It is also possible to achieve particularly short setting times by mixing 
suitable gelatin fractions which are each distinguished by low peptide 
content or high viscosity, respectively. In corresponding tests, setting 
times, for example, of 2 to 20 seconds were obtained at 16.degree. C., 
with 2.5 g gelatin in 1 dl water. 
A preferred method for producing a gelatin of composition according to the 
invention is to melt (extract) the gelatin out of a conventionally alkali 
pretreated raw material in a temperature range (brew temperature) between 
70.degree. and 100.degree. C. for a time (brewing time) from 5 to 120 
minutes at a pH (brew pH) between 5.5 and 7.0. 
In the manufacture of gelatin according to the present invention it is 
preferred to start with a conventionally alkali pretreated (e.g. with 
aqueous calcium hydroxide solution or sodium hydroxide solution) raw 
material, although, in principle, it is also possible to operate in an 
acid medium. To obtain the desired short setting times it is significant 
to perform, not the gentle long time extraction (with longer period of 
dwell of the extraction water or of the steadily enriched gelatin solution 
at low temperature) of the prior art, but a high temperature short term 
reaction. This is understood to mean that the gelatin is extracted or 
melted out (brewed), in an aqueous medium, in a temperature range (brew 
temperature) between about 70.degree. and 100.degree. C. for a time 
(brewing time) of about 5 to 120 minutes. The pH (brew pH) will lie 
between about 5.5 and 7.0. A weakly alkali brew pH up to about 8.5 may 
also be suitable. 
A particularly advantageous range for the brew temperature lies between 
about 70.degree. and 82.degree. C., and a particularly preferred brewing 
time lies between about 20 to 40 minutes. 
The aqueous gelatin solutions obtained in this way are cooled within 1 to 
60 minutes, preferably within 1 to 5 minutes, to temperatures below 
55.degree. C. It is also important to obtain a quick transition to the gel 
phase. This transition is obtained in the manufacturing processes 
according to the present invention within 5 to 45 minutes, preferably 
within 5 to 15 minutes. 
While the extraction takes place in the prior art methods for more than 2 
hours and at relatively low temperatures below 70.degree. C., it is 
necessary for the gelatin according to the present invention to be 
extracted very quickly at higher temperatures while assuring good heat 
transfer conditions. With the use of the electrophoresis method which 
permits a determination of the composition of the gelatin being produced, 
the process according to the invention can be regulated in such a way that 
gelatins having high viscosity and simultaneously low peptide contents are 
obtained at defined times in defined draft sequences. ("Draft" means 
"Extraction"). 
A further method for producing the gelatins according to the invention 
involves the fractionation of commercially available gelatins with 
composition being monitored by means of gel chromotography. 
The gelatins according to the invention not only have particularly 
favorable setting times, they are particularly well suited, due to their 
high viscosity, for use in certain modern casting techniques, e.g., 
curtain casting, because no viscosity increasing substances such as sodium 
cellulose sulfate, need be added, to produce coatings such as photographic 
coatings. In some cases, such additives adversely influence other 
properties of the gelatin, possibly causing discoloration. 
A further decisive advantage of the gelatin according to the invention is 
that in industrial production, the gelatin drying process can be 
shortened. For example, while maintaining the conventional drying line, 
the temperatures of the drying air can be raised since the melting point 
of the gelatin according to the invention lies about 1.degree. to 
3.degree. C. higher than that of conventional types of gelatin. In other 
industrial processes the blowing in of cooling air can be omitted or 
reduced. 
Photographic materials generally consist of a planar substrate onto which 
is applied at least one, usually several thin coatings. At least one of 
these coatings is photosensitive and, in the case of conventional 
photographic material, consists of a fine dispersion of silver halide in a 
hydrophilic colloidal binder. The photosensitive layer and possible 
further nonphotosensitive layers may additionally contain a number of 
other substances, such as dyes, color couplers, sensitizers, stabilizers, 
solvents, wetting agents, hardeners, and additional nonhydrophilic binders 
in dispersed form. 
Since the invention of the dry plate, gelatin has been the preferred 
hydrophilic binder for photographic coatings and, despite the advances in 
the manufacture of polymeric substances, is practically irreplaceable 
today. The reason for its usefulness is the unique properties of gelatin 
which are combined in such a favorable way in no other natural or 
synthetic material. The chemical properties of gelatin and its natural 
accompanying substances permit the production of silver halide layers 
having particularly high sensitivity to light. Physically, gelatins have 
the swellability and permeability properties necessary for aqueous 
processing solutions. Gelatins also have favorable properties as 
protective colloids which permit the production and stabilization of 
finely dispersed emulsions and dispersions, particularly of silver 
halides. Finally, the physical properties of the gelatin solutions enhance 
the application and drying of the gelatin in thin, uniform layers. 
It is inherent in the technology of the manufacture of photographic 
materials that gelatins with widely varying properties must be available. 
This applies mainly to their chemical properties which determine to a 
great extent the sensitometry of the photosensitive layers. The gelatin 
manufacturing industry has been able to meet the changing requirements of 
the photographic art over an increasingly broader range of needs, and 
especially to meet the demands for reproduceability of the material. 
When manufacturing photographic materials, particularly for the precise and 
economical production of thin photographic layers, next to its chemical 
properties the physical properties of the gelatin employed play a decisive 
roll. Two of these physical properties are of special significance: the 
viscosity of the aqueous gelatin solutions and their setting speed. 
Aqueous gelatin solutions are known to be liquid only at higher 
temperatures. Below about 30.degree. C. they harden to an elastic gel 
within a shorter or longer period of time. The only exceptions are very 
highly dilute solutions having a concentration of less than about 1% which 
remain liquid at all temperatures. The capability of gelatins to solidify 
to a nonflowing gel is an important feature which greatly facilitates the 
production and drying of precise, thin layers or even makes it possible at 
all. 
The application of the photographic layers onto the substrate takes place 
with these layers in the liquid state, often a plurality of liquid layers 
being applied simultaneously. The layers must then be dried, which is 
accomplished in the fastest and most appropriate manner by blowing on warm 
air. Since it is impossible to roll up a material containing undried 
layers, the application and drying of the layers must take place 
continuously in one passage. For the drying process it is important that 
the layers which have been applied with great precision are not deformed 
by gravity or the moving warm air. This requirement can be met in a 
particularly easy manner with gelatin-containing layers by having the 
layers set to a gel as quickly as possible before the start of the drying 
process. To accomplish this, the coated substrates are conducted through a 
cooling line. The faster the layers set, the less are the industrial 
expenditures for the cooling line and the higher the advancing speed that 
can be utilized. A general description of the coating and drying 
techniques for photographic materials can be found in B. M. Deryagin et 
al, Film Coating Theory, Focal Press, 1964. 
The setting speed of gelatin solutions is generally dependent on their 
concentration and temperature, and can be improved by increasing the 
gelatin concentration in the coating solution. However, such a mode of 
operation has certain limits set by the coating technique and such limits 
cannot be exceeded. With respect to colloidal chemistry, casting solutions 
which are too concentrated are often unstable and may tend, for example, 
to demix, or individual components of the solutions may flake out. 
These drawbacks in the production of coatings of photographic materials can 
be substantially overcome with the gelatins according to the present 
invention. The present invention thus also relates to the use of the 
gelatins according to the present invention as binders for producing 
coatings of photographic materials and to a method for producing 
photographic materials comprising a substrate and at least one 
photographic coating, wherein the binder for at least one layer is a 
gelatin according to the present invention. The invention further relates 
to photographic materials produced in this way. These materials and 
coatings of the material are distinguished by good mechanical properties. 
The good compatibility of the gelatin according to the invention with 
photographic additives (e.g., dyes and dye couplers) as well as the 
optimum behavior of the gelatin during photographic processing (after 
exposure of the photographic material) have an extremely advantageous 
effect on image reproduction. 
The gelatins according to the invention are of particular advantage not 
only in the photographic industry but also in the pharmaceutical and 
foodstuffs industries. In the pharmaceutical industry, medicinal 
preparations are often packed in hard gelatin capsules which are produced 
by an immersion process in which particularly uniform capsules can be 
obtained in a reproduceable manner if the setting time of the gelatin 
employed is very short. This manufacture of hard capsules by the immersion 
process is a further important field of application for the quick setting 
gelatin according to the invention, the flow properties of this gelatin 
being utilized in an advantageous manner due to its high viscosity. 
The corresponding principles apply for food gelatins where short setting 
times may also be extremely desirable. In the candy industry, for example, 
in the production of marshmallows, bands of foam must solidify in about 30 
seconds to the extent that they can be cut. During this time the band of 
foam travels on cooling bands which have a length of 20 to 30 meters. 
Shortening of the setting times here results in earlier opportunities for 
cutting and considerable savings in the size of the plant. In the 
manufacture of gum candy as well, shortening of the setting times results 
in acceleration of the manufacturing process and savings, for example, in 
powder boxes and storage space. Likewise, in the fish and meat product 
industry, reduction of the setting time of gelatin would result in 
increased output and reduction of plant size since, for example, shorter 
cooling tunnels could be used. In household applications, gelatin having a 
shorter setting time is also advantageous since, for example, it will no 
longer be necessary, when making gelatin deserts or aspics to place the 
bowls filled with the gelatin solution in the refrigerator for several 
hours. 
Additionally, the slow setting of commercially available gelatin in the 
past constituted an insurmountable obstacle to the use of gelatin in cake 
icings. The slow setting caused the still liquid gelatin solution to 
penetrate into the cake itself and make it soggy. Only quick setting 
gelling agents such as pectin, agar-agar, carrageenan and alginate have 
therefore been used for cake icings. Shortening the setting time opens 
this field for gelatin as well and the replacement of the abovementioned 
gelling agents by gelatin is of particular advantage because those gelling 
agents which are often used both in the cake baking industry and in other 
branches of the foodstuffs industry because of their short setting times 
have drawbacks regarding taste and texture.

The following examples are given by way of illustration to further explain 
the principles of the invention. These examples are merely illustrative 
and are not to be understood as limiting the scope and underlying 
principles of the invention in any way. All percentages referred to herein 
are by weight unless otherwise indicated. 
EXAMPLE 1 
Bone granules from freshly slaughtered raw bones are carefully degreased by 
way of water degreasing and are macerated in the conventional manner under 
gentle conditions. Thereafter, liming and neutralizing takes place in the 
customary alkali manner. 
A first draft is made at a brew pH of 6.5 and a temperature of 72.degree. 
C. after 35 minutes and a second draft is made at the same pH at 
78.degree. C. after a further 20 minutes. Then the third draft is 
extracted at a brew pH of 6.5 and 80.degree. C. after 25 minutes in a 
conventional stirring vessel, to correspond to 50 to 65% of the total 
yield. A partially continuous extraction process produces the same 
results. The resulting gelatin solution is cooled to 50.degree. C. within 
3 to 5 minutes and is converted to the gel phase within about 10 minutes. 
The gelatin obtained in the third draft has the characteristics listed in 
Table 2 under Sample No. 4. 
The second draft to the fourth draft (brew pH 6.7, at 84.degree. C. for 20 
minutes), generally the middle drafts, corresponding to about 45 to 75% of 
the total yield may still produce usable, highly viscous gelatins 
according to the invention with a low peptide content. 
EXAMPLE 2 
Healthy beef split is cut as usual, washed and limed under conventional 
conditions with aqueous calcium hydroxide or sodium hydroxide solution. 
Then the brewing procedure continues as explained in Example 1. In the 
first drafts, which correspond to 0 to 20% of the total yield, the brew pH 
is 7.0, brewing temperature is 90.degree. C. and brewing time is 15 
minutes. As in Example 1, the resulting gelatin solutions are cooled to 
50.degree. C. within 3 to 5 minutes and are converted to the gel phase 
within about 10 minutes. Highly viscous gelatins which are low in peptides 
are obtained. The data for a gelatin obtained in this way are listed in 
Table 2 under Sample No. 2. 
EXAMPLE 3 
Fresh or frozen bacon rinds are washed and acidified in the customary 
manner. Then the procedure continues under conditions similar to those in 
Example 1, with the brew pH being at 5.0, brewing temperature at 
72.degree. C., and the brewing time 15 minutes. The characteristic data 
for the resulting gelatin are listed in Table 2 under Sample No. 5. 
TABLE 3 
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Gelatin Sample No. 
1 2 3 4 5 
______________________________________ 
Proportion of alpha- 
gelatin fragments 
4 13 7 7 21 
(wt %) 
Limiting viscosity 
0.85 0.95 0.8 1.0 0.85 
(dl g.sup.-1) 
at 45.degree. C. A.sub.2 = 0 
Gel strength 
(dyn. cm.sup.-2) 
at 16.degree. C., 2.5 g gelatin 
9.times. 10.sup.4 
9.times. 10.sup.4 
9.times. 10.sup.4 
9.times. 10.sup.4 
9.times. 10.sup.4 
in 1 dl 
Setting time (sec) 
at 16.degree. C., 2.5 g gelatin 
48 42 54 27 49 
in 1 dl 
______________________________________ 
For an explanation of the values given for limiting viscosity, gel strength 
and setting time, see N. Veis, The Macromolecular Chemistry of Gelatin, 
1964, Academic Press and J. Brandrup, E. M. Immergut, Polymer Handbook, 
1978, Interscience. 
EXAMPLE 4 
Table 3 below illustrates the significance of the gelatin composition of 
the gelatins according to the invention and their content of the various 
fractions, particularly peptide and microgel. The gelatin samples marked 
Nos. 6 through 10 were produced by fractionation of commercially available 
gelatins. 
The composition of the gelatins was determined by preparative gel 
chromatography. 
TABLE 3 
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Sample No. 6 7 8 9 10 
______________________________________ 
Peptides (alpha cleavage 
fragments) % 18 10 14 1 1 
Microgel (high polymers) % 
8 2 10 3 80 
Setting time (2.5% solution, 
16.degree. C.) sec. 
70 50 50 8 2 
Gel strength (2.5% solution, 
16.degree. C.) dyn. cm.sup.-2 
10.sup.5 
9 .times. 10.sup.4 
10.sup.5 
10.sup.5 
10.sup.5 
Viscosity (6.67% solution 40.degree. C.) 
cP 13 8 14 9 40 
______________________________________ 
Table 3 shows a direct connection between the setting time and the peptide 
content of the gelatin. However, gel strength as well as viscosity of the 
gelatin solutions are substantially independent of the peptide content 
within the range covered by Table 3. 
It is clear however, that a good correlation exists between microgel 
content and viscosity of the solutions. 
Table 4 shows, for comparison, the corresponding values of five different 
commercially available gelatins. 
TABLE 4 
__________________________________________________________________________ 
Sample No. 11 12 13 14 15 
__________________________________________________________________________ 
Raw material bone skin bone bone skin 
Decomposition process 
HCl HCl Ca(OH).sub.2 
Ca(OH).sub.2 
NaOH 
Peptides 
(alpha cleavage fragments) % 
60 30 30 40 30 
Microgel (high polymers) % 
3 2 2 15 3 
Setting time 
(2.5% solution, 16.degree. C.) sec 
400 200 120 90 150 
Gel strength dyn. cm.sup.-2 
(2.5% solution, 16.degree. C.) 
5 .times. 10.sup.4 
7.5 .times. 10.sup.4 
6 .times. 10.sup.4 
5.5 .times. 10.sup.4 
6.5 .times. 10.sup.4 
Viscosity cP 
(6.67% solution, 40.degree. C.) 
6 5 5 18 6 
__________________________________________________________________________ 
As in the gelatins according to the invention listed in Table 3, the 
connection between setting time and peptide content on the one hand and 
between viscosity and microgel content on the other hand can be 
recognized. Since, however, the commercially available gelatins 11 through 
15 all have higher peptide contents, the short setting times attained with 
the gelatins according to the invention, Nos. 6 through 10, are not 
attained with the commercially available gelatins. 
EXAMPLE 5 
Using a gelatin produced according to Example 1 (gelatin Sample No. 4 in 
Table 2) a silver halide emulsion containing a chromogeneous coupler is 
produced in the following manner and processed into a photographic 
coating: 
150 g of a red sensitized silver halide emulsion containing 24.37 g silver 
bromide and 7 g gelatin are mixed with 300 g of a finely dispersed 
emulsion containing 20 g of a two-equivalent cyan coupler of the formula 
##STR1## 
10 g tricresyl phosphate, 2.5 g of an anionic active dispersant and 15 g 
gelatin. 550 g of a 7% aqueous gelatin solution are added to the mixture. 
1000 g of a pourable solution containing 1.4% silver, 2% coupler and 6.05% 
gelatin are produced. The viscosity of the solution at 40.degree. C. is 16 
cP and the setting time, measured at 16.degree. C., is 20 seconds. This is 
precisely the same setting time measured for a 6.05% solution of the same 
gelatin No. 4, without any further additives. 
The ready-to-pour solution is applied onto a glass substrate to a layer 
thickness corresponding to a weight per area of 20 g per m.sup.2. The 
solution is set by a short cooling, then dried by blowing on warm air. 
A photosensitive layer is obtained which, after exposure under a 
transparent sample and after customary processing by color development, 
silver bleaching and fixing, furnishes a negative blue-green image of the 
sample. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.