Properties of Gelatin
- by Dr. Bernard Cole
- Copyright - 2006 Bernard Cole - This is not Open Source Copyright
- Dr. Bernard Cole's Web Site
- 1 INTRODUCTION
- 2 CHEMISTRY and BIOCHEMISTRY
- 3 GELATIN MANUFACTURE
- 4 GELATIN SAFETY
- 5 GELATIN PROPERTIES AND USES
- 5.1 Solubility in water
- 5.2 Adhesive properties
- 5.3 Gelling properties
- 5.4 Foaming properties
- 5.5 Protective Colloid/Crystal habit modifying properties
- 5.6 Film Forming properties
- 5.7 Emulsifying properties
- 5.8 Stability
- 5.9 Microencapsulation - Mixed film forming properties
- 5.10 Milk - Food stabilising properties
- 5.11 Fruit Juice Clarifying properties
- 5.12 Texturising properties
- 5.13 Nutritional properties
- 5.14 Corrosive properties
- 5.15 Fish skin gelatin
- 6 GELATIN TESTING METHODS
- 7 BIBLIOGRAPHY
Gelatin is a substantially pure protein food ingredient, obtained by the thermal denaturation of collagen (1), which is the structural mainstay and most common protein in the animal kingdom. Today gelatin is usually available in granular powder form, although in Europe, sheet gelatin is still available.
There are two main types of gelatin. Type A, with isoionic point of 7 to 9, is derived from collagen with exclusively acid pretreatment. Type B, with isoionic point of 4.8 to 5.2, is the result of an alkaline pretreatment of the collagen. However, gelatin is sold with a wide range of special properties, like gel strength, to suit particular applications.
Gelatin (2) forms thermally reversible gels with water, and the gel melting temperature (<35°C) is below body temperature, which gives gelatin products unique organoleptic properties and flavour release. The disadvantage of gelatin is that it is derived from animal hide and bone (not from trotters as is a common perception), hence there are problems with regard to kosher and Halal status and vegetarians also have objections to its use. Competitive gelling agents like starch, alginate, pectin, agar, carrageenan etc. are all carbohydrates from vegetable sources, but their gels lack the melt in the mouth, elastic properties of gelatin gels.
CHEMISTRY and BIOCHEMISTRY
Gelatin is an amphoteric protein with isoionic point between 5 and 9 depending on raw material and method of manufacture. Like its parent protein, collagen (3), it is unique in that it contains 14% hydroxyproline, 16 % proline and 26 % glycine. The only other animal product containing hydroxyproline is elastin and then at a very much lower concentration, so hydroxyproline is used to determine the collagen or gelatin content of foods. In brief, the protein is made up of peptide triplets, glycine - X - Y, where X and Y can be any one of the amino acids but proline has a preference for the X position and hydroxyproline the Y position (3). Approximately 1050 amino acids produce an alpha-chain with the left-handed proline helix conformation. Collagen exists in many different forms but gelatin is only derived from sources rich in Type I collagen which contains no cystine, however, hide or skin contains some Type III collagen which can be the source of traces of the traces of cystine found in some gelatins. Although Type I collagen contains no cystine, the alpha procollagen chains excreted by the cell do contain cystine at the C terminal end of the protein which is thought to be the site of assembly of 3 alpha-chains. The three chains then spontaneously (4) coil together, zipper fashion, to form a right-handed helix. After spontaneous helix formation, cross links between chains are formed in the region of the N terminal telopeptides (globular tail portion of the chains) and then the telopeptides (containing the cystine and tyrosine of pro-collagen) are shed leaving the rod-like ca. 3150 amino acid containing triple helix . These collagen rods assemble together with a quarter-stagger to form the collagen fibre and the fibres are stabilised by further cross-links.
Gelatin is the product of denaturation or disintegration of collagen. Initially the alpha-chains of collagen are held together with several different but easily reducible cross-links. As the collagen matures, so the cross-links become stabilised (3). Then as time progresses the eta-amino groups of lysine become linked to arginine by glucose molecules (Maillard reaction) to form the pentosidine type cross-links which are extremely stable (5). Hence when the alkaline processing is used on young animal skin the alkali breaks one of the initial (pyridinoline) cross-links and as a result, on heating, the collagen releases, mainly, denatured alpha-chains into solution (5). Once the pentosidine cross-links of the mature animal have formed in the collagen, the main process of denaturation has to be thermal hydrolysis of peptide bonds resulting in protein fragments of various molecular weights i.e. polydisperse protein fragments. With the "acid process", the collagen denaturation is limited to the thermal hydrolysis of peptide bonds with a small amount of alpha-chain material from acid soluble collagen in evidence (6).
Nutritionally, gelatin is not a complete protein food because the essential amino acid tryptophan is missing and methionine is only present at a low level.
Type A gelatin (dry and ash free) contains 18.5 % nitrogen, but due to the loss of amide groups, Type B gelatin contains only about 18 % nitrogen (7). Gelatin is abnormally stable and a special catalyst has to be used to obtain the correct Kjeldahl nitrogen content.
The amino acid analysis of gelatin (8) is variable, particularly for the minor constituents, depending on raw material and process used, but proximate values by weight are: glycine 21 %, proline 12 %, hydroxyproline 12 %, glutamic acid 10 %, alanine 9 %, arginine 8%, aspartic acid 6 %, lysine 4 %, serine 4 %, leucine 3 %, valine 2 %, phenylalanine 2 %, threonine 2 %, isoleucine 1 %,hydroxylysine 1 %, methionine and histidine <1% with tyrosine < 0.5 %. It should be remembered that the peptide bond has considerable aromatic character, hence gelatin shows an absorption maximum at ca. 230 nm.
Collagen is resistant to most proteases and requires special collagenases for its enzymic hydrolysis. Gelatin, however, is susceptible to most proteases, but they do not break gelatin down into peptides containing much less than 20 amino acids.
The cross-linking of gelatin with aldehydes is being used to extend the uses of gelatin. In particular, treatment of gelatin films with glutaraldehyde is receiving considerable study in order to improve their thermal resistance, decrease their solubility in water as well as to improve their mechanical properties. In Japan and Brazil the cross-linking of gelatin using the enzyme trans-glutaminase and its use in joining gelatin to other proteins, is approved for food use. An occasional phenomenon is the loss of gelatin solubility after storage in a new kitchen cupboard where the residual formaldehyde vapour from the adhesives used, causes cross-linking of the gelatin. This reaction has been used to make gelatin adhesives water-resistant. Furthermore, the "smokes" used in food preservation are rich in aldehydes and thus can have unwanted reactions with gelatin.
There are a large number of unit processes used in the manufacture of gelatin and the raw materials from which it is derived are demineralised bone (called ossein), pigskin, cow hide, fish skin and in China, donkey hide is also used quite extensively. In theory there is no reason for excluding any collagen source from the manufacture of gelatin, but the ones above are the currently commercially available raw materials. Interestingly, in countries where pork is sold with its skin intact, there is no pigskin available for gelatin manufacture.
There are basically two processes by which collagen is processed to gelatin:
The acid process (studied in detail by Reich (9)) is mainly used with pigskin and fish skin and sometimes bone raw materials. It is basically one in which the collagen is acidified to about pH 4 and then heated stepwise from 50°C to boiling to denature and solubilize the collagen. Thereafter the denatured collagen or gelatin solution has to be defatted, filtered to high clarity, concentrated by vacuum evaporation or membrane ultra-filtration treatment, to a reasonably high concentration for gelation and then drying by passing dry air over the gel. The final process is one of grinding and blending to customer requirements and packaging. The resulting gelatin has an isoionic point of 7 to 9 based on the severity and duration of the acid processing of the collagen which causes limited hydrolysis of the asparagine and glutamine amino acid side chains.
The alkali process (studied in detail by Cole and Roberts (10)) is used on bovine hide and collagen sources where the animals are relatively old at slaughter. The process is one in which collagen is submitted to a caustic soda or lengthy liming process prior to extraction. The alkali hydrolyses the asparagine and glutamine side chains to glutamic and aspartic acid relatively quickly (11), with the result that the gelatin has a traditional isoionic point of 4.8 to 5.2, however, with shortened (7 days or less) alkali treatment, isoionic points as high as 6 are produced. After the alkali processing, the collagen is washed free of alkali and treated with acid to the desired extraction pH (which has a marked effect on the gel strength to viscosity ratio of the final product). The collagen is then denatured and converted to gelatin by heating, as with the acid process. Because of the alkali treatment, it is often necessary to demineralise the gelatin solution to remove excessive amounts of salts using ion-exchange or ultrafiltration. Thereafter the process is the same as for the acid process - vacuum evaporation, filtration, gelation, drying, grinding and blending. Although gelatin is often considered a commodity like sugar, the descriptions of the processes and raw materials above, should indicates that gelatin has the potential for being a variable product and it behoves users to ensure that they are using the best product for each particular application. In the past, little emphasis has been placed on the animal age of the raw material, particularly in the case of gelatins from bovines, however it is now known that this factor plays a significant role in the molecular structure of the derived gelatin. The role of liming in the alkali process used to be considered one of progressive alkali hydrolysis of the collagen, which made it possible to denature the collagen at lower temperatures and thus maximise the yield of top quality gelatin. Recently, however, it has been shown that the role of liming is limited to the hydrolysis of one collagen cross-link which fluoresces at 290/380 nm and that liming has less and less effect on "extractability" as the animal gets older. The result is that alkali treatment times have been greatly reduced. One of the less well recognised effects of alkali treatment is the "opening up" of the hide collagen, as it is termed in leather manufacture, or the destruction of the proteoglycans associated with the collagen fibrils and this probably results in a more pure gelatin via the alkali process as is indicated by electrophoresis of the gelatin proteins (12).
At present, enormous developments are being made in the understanding of the structure of collagen and the changes occurring with senescence, and these developments are bound to have an impact on the appreciation of the variables in gelatin, particularly at the molecular level.
Gelatin is regarded as a food ingredient rather than an additive and it is Generally Regarded as Safe (GRAS). In 1993 the FDA reiterated the GRAS status of gelatin and stated that there was no objection to the use of gelatin from any source and any country provided that the hide from animals showing signs of neurological disease were excluded and also Specified Raw Materials were excluded from the manufacturing process. Although, at the beginning of the Bovine Spongiform Encephalopathy (BSE) scare in Europe the popular media brought suspicion on all products of bovine origin as being possible transmitters of the disease to humans as CJD, this was a thoroughly unscientific assessment of the dangers of spreading infection. It is now recognised that BSE is a neurological and brain problem and not associated with the hide of the animal. It is also recognised that the processes of manufacturing gelatin make it virtually impossible for the survival of a defective prion, if it were present in the first place.
Detailed and unbiased information on BSE is available from the Institute of Food Science and Technology Web site. Hence, today, gelatin retains its GRAS status. Furthermore, the Joint Expert Commission on Food Additives (JECFA) placed no limit on the use of gelatin in 1970.
Gelatin is an excellent growth medium for most bacteria, hence considerable care needs to be taken, during manufacture, to avoid contamination. This care is evidenced by the use of documented HACCP programs by manufacturers. In the same way to ensure product reproducibility, most companies are implementing ISO 9000 quality management systems.
GELATIN PROPERTIES AND USES
Solubility in water
Gelatin is only partially soluble in cold water, however dry gelatin swells or hydrates when stirred into water. Such mixtures should generally not exceed 34 % gelatin. On warming to about 40°C gelatin that has been allowed to hydrate for about 30 minutes melts to give a uniform solution. Alternatively, dry gelatin can be dissolved by stirring into hot water, but stirring must be continued until solution is complete. This method is normally only used for dilute solutions of gelatin.
If gelatin solutions are spray dried or drum dried from the sol state, the resulting gelatin is "cold water soluble" and such gelatins gel quickly when stirred into cold water. These gels are generally not clear, so the use of this form of gelatin is limited to milk puddings and other products where solution clarity is not required.
The compatibility of gelatin in aqueous solution with polyhydric alcohols like glycerol, propylene glycol, sorbitol etc. is virtually unlimited and they are used to modify the hardness of gelatin films.
In products with limited moisture availability, as in confectionery, and where there is another polymer, as in glucose syrup, competing for the available water, then gelatin can be precipitated resulting in loss of gelation and cloudiness. In these cases the gelatin solubility is very dependent on the charge on the protein molecule or the pH of the product. Hence, the further the product pH is from the isoionic pH the better will be the solubility and performance of the gelatin.
Possibly the oldest use of gelatin was as animal glue. For adhesion to take place a warm gelatin solution must be used and the gelatin must not have gelled before the surfaces to be glued are brought together. An example of this use of gelatin is in pharmaceutical or confectionery tableting and in liquorice all-sorts where it can be used to join the layers.
The most common use of gelatin is for its thermally reversible gelling properties with water, for example, the production of table jellies. Gelatin is also used in aspic to add flavour to meat products while on gelling it also provides a pleasing shiny appearance to the product. In some cases gelling is known as its "water absorbing property": For example, in canned hams, gelatin can be added to the can before cooking. On cooking the exudate from the meat is absorbed by the gelatin and appears as a gel when the can is opened.
In confectionery, gelatin is used as the gelling binder in gummy products, wine gums etc. In the manufacture of these products gelatin is combined with sugar and glucose syrups. Incompatibility between gelatin and glucose syrup can occur (13) and is a function of the concentration of glucose polymers containing more than 2 glucose units, contained in the syrup. Competition between gelatin and glucose polymers for water in low water content products can result in, at worst, precipitation of the gelatin and at best a marked loss in gelling properties or hardness of the product. It is also known that different gelatins with similar properties in water, can have very different properties in confectionery formulations.
Some raw fruits like pineapple and papaya contain proteolytic enzymes like bromelin which hydrolyse gelatin and destroy its gelling ability. In such cases it is essential that the fruit is cooked to destroy the protease before the fruit is added to gelatin solutions.
In general one can say that the lower the mean molecular weight (MW) of a gelatin the lower the gel strength and viscosity of its solution, however it has been shown that the collagen alpha-chain (MW 100 kD and gel strength = 364 g Bloom) is the main contributor of gel strength (14) and that higher molecular weight components (beta-chain with MW 200 kD, gama-chain with MW 300 kD and "microgel" with MW > 300 kD) make a relatively low contribution to gel strength but a high contribution to viscosity.
Gelatin is a very efficient foam stabiliser and this property is exploited in the manufacture of marshmallows. Different gelatins have different foam stabilising properties and gelatin for this use needs to be carefully selected. However, the foaming properties can be standardised by the use of sodium lauryl sulphate (15), if this is permitted by local food additive regulations. In marshmallows the gelatin's film forming properties are also used to stabilise the foam on cooling, and because the product is normally not acidified, it has to have a much lower moisture content (>85 % solids) than gummy products (76 % solids) to avoid mould growth in storage.
Protective Colloid/Crystal habit modifying properties
If a gelled jelly is frozen, the product will suffer from syneresis and on thawing the clear jelly will disintegrate with much exuded water. However, if water containing 0.5 % gelatin is frozen, the water will freeze as millions of small discrete crystals, instead of forming a single solid block of ice. This effect is most desirable in "ice lollies" and is also used in ice cream manufacture to obtain a smooth product with small ice crystals and also to ensure that any lactose precipitates as fine crystals avoiding the development of graininess with time.
Film Forming properties
Gelatin's film forming properties are used in the manufacture of both hard and soft (pharmaceutical) capsules. Gelatin films shrink with great force on drying, hence such uses usually involve the addition of polyhydric alcohols to modify the adhesion and flexibility of the dry film. Also, for film forming, a gelatin with a high viscosity is preferred to one with a low viscosity, hence for hard capsules and in photography, ossein gelatin is preferred and commands a premium price.
The amphoteric character as well as hydrophobic areas on the peptide chain gives gelatin limited emulsifying and emulsion stabilising properties used in the manufacture of toffees and water in oil emulsions like low fat margarine.
Dry gelatin has an almost infinite shelf life as long as the moisture content is such as to ensure that the product is stored below the glass transition temperature.
The stability of gelatin in solution depends on temperature and pH. Generally, to minimise loss of gel strength and viscosity with time, the pH of the solution should be in the range 5 to 7 and the temperature should be kept as low as possible, consistent with the avoidance of gelation and the suitability of the solution viscosity to the particular application. Often the cause of degradation or hydrolysis of gelatin in solution is microbial proliferation, so gelatin solutions should not be stored for longer than is absolutely necessary, and after addition of the acid to confectionery formulations, the solution should be used and cooled/gelled with minimal delays.
Microencapsulation - Mixed film forming properties
Besides being precipitated by polymers competing for water, gelatin is amphoteric, i.e. it has both positive and negative charges on the molecule (and no net charge at the isoionic point). Hence, at a pH where the basic side chains do not carry a charge, acid groups for example from gum arabic can react with the basic groups of gelatin to form an insoluble gelatin-arabate complex which can be precipitated around emulsified oil droplets, forming micro-ecapsulated oil. The microcapsules are hardened with formaldehyde or glutaraldehyde before harvesting and drying. In this application the pI of the gelatin is critical. This process has been used in the food industry for encapsulating flavours.
Milk - Food stabilising properties
Gelatin is used as a stabilizer particularly in yoghurt, where the addition of 0.3 - 0.5 % acts to prevent syneresis thus allowing the production of stirred and fruit containing products. In this instance the gelatin reacts with the casein in the milk to reduce its tendency to separate water from the curd. Gelatin can also be used in cheese manufacture to improve yield and in the stabilisation of thickened cream.
Fruit Juice Clarifying properties
In "fining" applications, gelatin reacts with polyphenols (tannins) and proteins in fruit juices forming a precipitate which settles leaving a supernatant which is stable to further cloud formation with storage time. In wine, usage levels are about 1 to 3 g/hL and excess usage, which would lead to protein instability, needs to be avoided. Traditionally, low Bloom strength gelatins are used but it has been shown that high Bloom strengths are equally effective (16). However, from the practical point of view, the use of low Bloom Strength gelatin is cheaper and makes it easier to mix the gelatin into the bulk of the cold juice before gelation can occur. In this regard, it has become common practice to treat cold grapes, during the initial crushing process, with gelatin that has been hydrolysed to the extent that it can no longer gel.
Gelatin is used in dried soups to provide the appropriate mouth feel (viscosity) to the final product.
As stated earlier, gelatin is not a complete protein source because it is deficient in tryptophan and low in methionine content, however the digestibility is excellent and it is often used in feeding invalids and the high level of lysine (4 %) is noteworthy. More controversially, studies have shown that the consumption of 7 to 10 g/day can significantly improve nail growth rate and strength (17) and it also promotes hair growth (18). Gelatin has also been shown to benefit arthritis sufferers in a large proportion of cases (19).
Although 304 stainless steel (s/s) can be used with milk, gelatin attacks 304 s/s and tubing can be perforated after a few months of continuous usage. With gelatin, it is essential to use 316 s/s and if heat exchanger plates are involved, the use of 316 s/s with the minimum specified molybdenum content of only 2 % can be unacceptable.
Fish skin gelatin
Fish skin gelatin is available commercially and can be produced for kosher use provided that the appropriate conditions are met (such as the use of fish having scales). Fish gelatin with normal gel strength has a normal hydroxyproline content (21) and is made from fish from warmer waters and not necessarily from fresh water, although this is normally the case. Fish gelatin with low or no gel strength (20), has a low hydroxyproline content (7) and is produced from cold water species which are sourced typically from the sea.
The low gel-strength gelatin has been used to emulsify vitamin A before spray drying to give another type of microencapsulated product using gelatin.
GELATIN TESTING METHODS
The best published sources of gelatin testing methods are British Standard 757 of 1975 (22) or Standard Methods for sampling and testing gelatin, published by the GMIA (23) or the Pharmacopoeias. Many of the methods used in laboratories need to be modified to suit the peculiarities of gelatin.
Gelatin gives the normal positive trichloroacetic acid, biuret, ninhydrin tests for protein. The precipitate with 5 % tannic acid is a particularly sensitive test for very dilute solutions of gelatin. In addition the thermally reversible gelation of a 6 % solution in water between 10 and 60°C is unique for this protein.
The most important attribute of gelatin is its gel strength and when determined by the standard method (22), is called the Bloom Strength. This is the force in grams required to press a 12.5 mm diameter plunger 4 mm into 112 g of a standard 62/3% w/v gelatin gel at 10°C. Several penetrometer type instruments have been adapted to determine Bloom Strength.
A frequent question is how to substitute gelatin of one Bloom Strength for a gelatin of another. As a guide one can say:
C x B½ = k (24)
or C1(B1)½÷(B2)½ = C2
Where C = concentration, B = Bloom strength and k = constant, however, there are other considerations besides gel strength which can invalidate such a substitution calculation. For example, in a gummy formulation, the texture using 250 Bloom gelatin is far shorter than when 180 Bloom gelatin is used.
From the point of view of functionality, the solution viscosity of gelatin is probably the second most important parameter. The standard method calls for the viscosity of a 62/3 % solution at 60°C. Low viscosity (and a high gel strength) is required for poured confectionery, and high viscosity for film forming applications.
In viscosity calculations, usually C logV = k but the model is not as good as is the mathematical model for Bloom calculations.
Colour and Clarity
Solution colour and turbidity or clarity are attributes which may or may not be important depending on the application. Poor clarity markedly affects the ability to measure colour (25) and at this stage there are no internationally accepted methods for determining these attributes, however, if clarity is good, then gelatin colour obeys Beer's Law.
Solution pH (1%) is usually about pH 5 but can vary considerably. At this pH the viscosity of Type B gelatin is minimal and the gel strength is maximal, hence from the manufacturers point of view it is advantageous to manufacture gelatin at this pH. However, due to the strong buffering capacity of gelatin this pH may not be the most advantageous for the customer.
The moisture content of gelatin may be as high as 16 %, however, more normally it is about 10 % to 13 % because at 13.0 % moisture content the glass transition temperature (26) of gelatin is about 64°C which allows particle size reduction to be a simple operation. In addition, at 13 % moisture content and 25°C gelatin is close to equilibrium with ambient air moisture contents of ca. 46 % RH. At 6 % to 8 % moisture content gelatin is very hygroscopic and it becomes difficult to determine the physical attributes with accuracy.
Due to the variable granule size of gelatin, the rate of moisture loss at 105°C can be variable. Hence it is normal to add water to the gelatin powder before placing the sample in the drying oven. This means that the gelatin melts and water is lost from a uniform thin film of protein. It should be noted that metal dishes have to be used because, on drying, the film of gelatin shrinks and breaks containers of glass or ceramic.
Finally, the drying of gelatin to very low moisture contents results in cross-linking and loss of solubility. It is thus difficult to distinguish between free and bound water in gelatin.
The gelatin ash content is determined by pyrolysis at 550°C. Usually ash contents up to 2.5 % can be accepted in food applications. However the nature of the ash can be important. For example, 2 % CaSO4 in gelatin can have excellent clarity in spite of the solubility product of the ash being exceeded (due to the crystal-habit modifying effect of gelatin), however on dilution of the gelatin in a confectionery formulation, the ash can precipitate. Furthermore, ammonia is often used as a pH modifier in gelatin preparation and salts like NH4Cl are not determinable by pyrolysis.
Sulphur dioxide content
Sulphur dioxide is used as a biocide and bleach in gelatin manufacture. The nationally permitted level of residual SO2 in gelatin is variable and the methods for its determination can give a great variation in results. It is known that gelatin promotes oscillating redox reactions (28,29) and the control of this contaminant is not easy. Hydrogen peroxide is often used to control the SO2 content of gelatin and sometimes the permitted level of this contaminant is also specified. It is interesting to note that both H2O2 and SO2 can be shown to coexist in gelatin.
Heavy Metal content
Once again the determination of heavy metals in gelatin can be a problem because of the difficulty of completely degrading gelatin and also because the main component of the ash in gelatin can be of low solubility, like calcium sulphate, hence with a variable ability to absorb traces of heavy metals. It must be recommended that internal standards be used wherever possible.
The isoionic point of gelatin (27) is best determined by passing a 1 % solution of the gelatin at 40°C through a mixed bed column of ionexchange resin (Rohm & Haas MB3) at a flow rate of not more than 10 bed volumes per hour and measuring the pH of the eluate. It should be noted that on cooling, isoionic gelatin has poor clarity and the conductivity should be between 1 and 5 s/cm for Type B gelatin.
Gelatin is an excellent nutrient for most bacteria, hence the manufacturing processes have to carefully avoid contamination. Most countries have microbiological specifications for gelatin, but generally they are not very onerous. Total mesophyllic plate counts of 1000 are generally accepted with various countries limiting the presence of Coliforms, E. Coli, Salmonella, Clostridial spores, Staphylococci, and sometimes even Pseudomonades.
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