1-1 Types of gelatin
According to the purpose, gelatin can be divided into three types: edible gelatin, photographic gelatin and industrial gelatin.
According to the viscosity, transparency, melting point and other properties of gelatin, edible gelatin, photographic gelatin and industrial gelatin can be divided into several grades.
1-2 Uses of gelatin
Gelatin is commonly known as an industrial all-purpose and is widely used.
一、Edible
1. Daily consumption In daily life, gelatin is used to make cakes, soups, frozen meats, salads, puddings, egg yolk juice, and icing.
2. Special edible gelatin is an animal protein with high nutritional value. It is especially suitable for infants, malnourished children, patients with lung disease and diabetes, and patients with high fever.
3. Used in the food industry In the food industry, gelatin is used as a binder for the production of fruit paste, jelly pastries, aspic, sausages, canned meat, frozen beef, candied fruit, candy fillings and other candies (such as soft candy, gummy candy, cream cake, nougat, chocolate, flavored candy, chewing gum, butter candy), Western pastries, and stabilizers and emulsifiers for the production of ice cream, cream jelly, French pastries, mayonnaise, puree cake, gravy, frozen milk, jam, gravy powder, jam powder and other foods. Adding some gelatin to beer and soda can also increase the richness.
2. Medical use
1. Therapeutic application In medical treatment, gelatin is a hemostatic agent and an antidote for iodine and alum poisoning. It can treat hemophilia, purpura and hemorrhagic purpura, infantile black stool, arteriovenous disease, epistaxis, hemoptysis, gastric bleeding, intestinal bleeding, hematuria, infantile dysentery, chronic enteritis, arterial calcification and other diseases.
2. Prescription In drug manufacturing, gelatin is the material of capsules, aldehyde gelatin, and glycerol gelatin. Among them, capsules are the outer shells of medicines (such as various mycin pills) and tonics (such as cod liver oil pills); aldehyde gelatin is a surgical dressing, preservative, and colloidal wound medicine; glycerol gelatin is an ointment base, ointment, plaster, external bougie and suppository, coating, microscope slide coating, tablet component, and pill.
3. Lighting
High-grade gelatin (photographic glue) is the main material for silver salt emulsion in the photosensitive layer of dry plates, photographic films, photographic prints, movie films, X-ray films, aviation films and photographic papers.
4. Process
Gelatin is used as an adhesive in industries such as grinding wheels, sandpaper, sandpaper, artificial boards, paper boxes, wood products, pencils, matches, and glued belts. It is used as glazing and sizing in industries such as cotton, silk, printing and dyeing, leather, papermaking, and straw hat weaving. Gelatin is indispensable in products or process processing such as rainproof slurry, printing rollers, plaster soft molds, printing copper plate development, colloid tests, bacterial culture, rubber products, emulsion retention, anti-oxidation pigments, electroplating solutions, and plastics. Gelatin is also used as an ingredient in various other products. In industry, a mixture of gelatin and white pigments (i.e., barium sulfate, etc.) is used as an adhesive material in the white bottom liquid on photographic paper. Industrial gelatin can also be used as a filler for high-grade paper and banknote paper. In the publishing industry, gelatin is used as a component of dyeing liquid, as well as in books, magazines and reproductions of art decoration.
二、Physical properties of gelatin
Gelatin is a colloid, which is a protein extracted from raw gelatin (also known as collagen) of bones, raw hides, tendons, membranes and other animal connective tissues.
2-1 General properties of gelatin
Dry gelatin is almost colorless or slightly yellowish, tasteless, odorless, non-volatile, transparent and hard amorphous substance. Dry gelatin does not melt when heated. It is heated to 105°C for a short time and then dissolved in water, and its properties remain unchanged. However, if the temperature is above 60°C and heated for a long time, the gelatin will become soft and swollen, and its physical and chemical properties such as viscosity will gradually decrease. Finally, it will turn black and carbonize, and emit a smell like burning feathers or hair. When gelatin burns, it produces smoky flames, and the unburned parts form porous and difficult-to-burn carbon. The white residue after burning is 1~1.5% of the original weight of gelatin. Commercial gelatin contains a certain amount of water. Generally, gelatin with a water content of <16% is called dry gelatin. The specific gravity varies slightly depending on the amount of water in the gelatin, generally 1.37.
2-2 Gelatin Solvent
Warm water is the most common solvent for gelatin. At room temperature, gelatin can be dissolved in urea, thiourea, thiocyanate, and solutions of potassium bromide or potassium iodide with high concentrations. It can also be dissolved in organic acids such as acetic acid, benzoyl acetic acid, salicylic acid and phthalic acid. At room temperature, 3N benzenesulfonic acid can dissolve gelatin without causing any chemical changes in gelatin. When alcohol, ammonium sulfate or other salts are added to the aqueous solution of gelatin, gelatin will precipitate as a white precipitate. When the aqueous solution of gelatin is cooled to a temperature below +15°C, it can form a tight jelly (gel), and when the jelly concentration is 10%, its jelly load reaches 2 kg/cm2.
2-3 Isoelectric point of gelatin
Protein is both acidic and alkaline, and is an amphoteric substance. It can form protein salts regardless of whether it reacts with acid or alkali. The behavior of protein depends on the pH value. For example, albumin has acidic properties at high pH values, but shows Zan properties at low pH values. When these two forms of protein are in equilibrium, the pH value is called the isoelectric point of the protein. The gelatin micelle is charged. Under the action of the electric field, it will move to one of the two poles. The direction of movement depends on the pH value of the medium. In an acidic solution, the gelatin micelle moves to the cathode; in an alkaline solution, it moves to the anode. If there is no free charge in the solution, that is, the electric field has no effect on the micelle, then the pH value of the solution is the isoelectric point of the colloid. The isoelectric point of gelatin made by the alkaline method is between 4.7 and 5, while the isoelectric point of gelatin made by the acid method is between 7 and 9.
2-4 Viscosity of gelatin aqueous solution
The aqueous solution of gelatin is viscous. The molecular spatial structure of gelatin in the solution is composed of many simple, flexible, irregularly shaped snake-like chains, so its chemical structure is relatively complex and changeable. Stirring causes some chains to detach from others, and the viscosity of the solution decreases; standing still causes the rigidity of the internal structure to gradually increase, that is, more chains are tied together, and the viscosity of the solution will increase. The longer the standing time, the higher the viscosity of the bath will be (not infinite). Temperature is an important factor affecting viscosity. Except when the temperature rises to 35°C, the viscosity changes very little. Generally speaking, the lower the temperature, the greater the viscosity increase.
The increase in viscosity is also related to the pH value. Experiments have shown that the viscosity of gelatin solution is lowest at the isoelectric point, especially for newly prepared solutions. If the gelatin solution is left at 52°C for 24 hours, its viscosity will have a very significant maximum value at pH=7.6. Stainsly once pointed out that the viscosity of a dilute solution changes with the pH value of the solution, which is due to the influence of the molecular charge that causes the molecules to deform. When the pH value is very high, the added ions overcome the total electrons on the molecules, so the molecules are more tightly packed, resulting in a decrease in viscosity. Since the isoelectric point of gelatin produced by the acid method is higher than that of the alkaline method, the viscosity change of acidic gelatin caused by standing is very small at the isoelectric point and can be almost ignored. Experiments have shown that when pH>8, this change is not significant; when pH<8, it becomes very obvious.
2-5 Swelling of gelatin
The swelling of gelatin in water changes with the change of pH value. There is a minimum value at the isoelectric point, that is, pH=4.7~5. In the presence of strong acids such as hydrochloric acid, gelatin swells particularly quickly. When pH=2.5, the swelling rate will reach the maximum value.
Gelatin is insoluble in cold water, but it can swell. It can absorb 5 to 10 times or even more water to form a firm and elastic jelly, which can be turned into a solution when heated.
| ITEMS | STANDARD | RESULT | METHODS |
| COLOR | LIGHT YELLOW OR YELLOW | LIGHT YELLOW | --------------------- |
| ODOUR | NORMAL | NORMAL | --------------------- |
| TASTE | NORMAL | NORMAL | ------------------------ |
| TEXTURE | DRIED GRANULES | GRANULES | ------------------------ |
| JELLYSTRENGTH | 180-240BLOOM.G | 200BLOOM | 6.67% AT 10°C FOR 18 HOURS |
| VISCOSITY | 3.5MPa.S ±0.5MPa.S | 3.6Mpa.S | 6.67% AT 60°CAMERICAN PIPETTE |
| MOISTURE | ≤12% | 11.1% | 24 HOURS AT 550°C |
| ASH CONTENT | ≤1% | 1% | COLORIMETRIC |
| TRANSPARENCY | ≥300MM | 400MM | 5% SOLUTION AT 40°C |
| PH VALUE | 4.0-6.5 | 5.5 | SOLUTION 6.67% |
| SO2 | ≤30PPM | 30PPM | DISTILLATION-LODOMETRY |
| HEAVY METAL | ≤30PPM | 30PPM | ATOMIC ABSORPTION |
| ARSENIC | <1PPM | 0.32PPM | ATOMIC ABSORPTION |
| PEROXIDE | ABSENT | ABSENT | ATOMIC ABSORPTION |
| CONDUCTIVITY | PASS | PASS | SOLUTION 6.67% |
| TURBIDITY | PASS | PASS | SOLUTION 6.67% |
| INSOLUBLE | <0.2% | 0.1% | SOLUTION 6.67% |
| TOTAL BACTERIA COUNT | <1000/G | 285/G | EUR.PH |
| E.COLI | Absence in 25 gram | Absent | ABS/25G |
| CLIPBACILLUS | Absence in 25 gram | Absent | EUR.PH |
| SALMONELLA | Absence in 25gram | Absent | EUR.PH |
When gelatin expands and absorbs water, it is in two states:
(1) Water that is combined with the gelatin colloid particles by attraction. This water is not easy to evaporate from the jelly, so it is called "hydrated water".
(2) Water in a free state, which exists between the molecules of gelatin, is called "swelling water". This water is easy to evaporate. The amount of water absorbed by gelatin when it swells increases with the increase in temperature and the pressure difference between the internal osmotic pressure and the external osmotic pressure. The higher the internal osmotic pressure, the greater the swelling. When gelatin forms salts that can dissociate into ions in a strong acid medium, or when alkali is added to a neutral gelatin solution, the internal osmotic pressure will increase and the swelling will deepen. If the acid or alkali is excessive, the swelling will decrease instead. When a neutral salt (such as NaCl) is added to an acidic gelatin solution, the swelling can be reduced because the external osmotic pressure will increase. At the same time, the internal osmotic pressure may also decrease due to the decrease in the ionization coefficient of the gelatin salt. This effect is most significant in sodium sulfate among sodium salts, and this property has been used in the process of temperature-increasing imaging.
2-6 Surface tension of gelatin solution
The surface tension of gelatin solution is related to pH value. It has a maximum value in acidic and alkaline solutions respectively. It is the lowest at the isoelectric point. At 16°C, gelatin will temporarily flocculate, resulting in a sudden increase in surface tension. Surfactants can reduce the surface tension of gelatin solution.
2-7 Gelatin gel
When the various protein chains that make up the micelle associate with each other through side chains, an insoluble solid lattice will be formed, which is the gel. Melting point and hardness are the performance indicators of gelatin gel. Gelatin used to prepare gel is generally divided into hard gelatin and soft gelatin. The raw materials of hard gelatin have not undergone much chemical changes, and the substances contained in its solution have a higher molecular weight. Therefore, the resulting gel has their original mechanical strength and strong affinity for external substances. However, when the molecular weight reaches a certain limit, the rigidity of the gel no longer increases. The raw materials of soft gelatin have been subjected to strong attack and hydrolysis, so they contain some degradation products. Gelatin gel consists of two parts: one part is solid, that is, the lattice of composite micelles, which has a directional structure and is composed of simple chains; the other part is liquid, which impregnates the lattice of composite micelles and puts it in a tense state by osmotic pressure.
Some physical and chemical effects can destroy the property of gelatin to become gel. If the gelatin solution containing sodium carbonate is neutralized and then the gelatin is precipitated with ammonium sulfate, the resulting gelatin will no longer be able to gel. Some substances, such as urea and guanidine, have an inhibitory effect on the gelation process. Gelatin irradiated with ultrasonic waves can no longer become a gel.
2-8 Freezing point and melting point of gelatin
Gelatin solution can be condensed into jelly when cooled. The highest temperature at which the gelatin solution with a concentration of 10% begins to condense is called the freezing point of gelatin. The lowest temperature required for the gelatin to melt is called the melting point of gelatin.
The melting point of gelatin gel (i.e. jelly) is the temperature at which the gel loses its strength, i.e., changes from the gel state to the sol state. At this temperature, the lateral bonding forces connecting the parallel molecular layers will completely disappear. The melting point is highest at the isoelectric point (pHi=4.7 to 5). The gelatin that has been less hydrolyzed and thus less degraded has a higher melting point. The high melting point property can indicate that the purity of the gelatin is relatively high. Adding a small amount of chromium salt (such as chromium alum) or aluminum salt (such as alum) to the gelatin solution can increase the melting point. In the case of high solution concentration, the viscosity will also increase. At pH=2.7, the melting point is the lowest (but the swelling degree is the highest at this time), and there is a lower peak at pH=2. Between the isoelectric point and pH=8, the melting point changes little, and this property is often used in the use of commercial gelatin. Adding potassium salt to the gelatin solution can lower its melting point. The effect caused by adding different types of potassium salts is also different. Generally speaking: sulfate < gas < bromide < nitrate < iodide < thiocyanate.
2-9 Foaming ability of gelatin
Put the gelatin solution in a test tube and shake it up and down at a certain amplitude, and part of the gelatin in the test tube will form foam. This is the foaming ability of gelatin. The foaming ability of over-hydrolyzed gelatin is greater. Therefore, low-grade gelatin has more foam than high-grade gelatin. Adding insoluble substances (such as rosin, carbon black, zinc oxide, glass powder, sulfur, etc.) can increase the foam. The higher the fineness, the greater the effect; adding linseed oil, oleic acid, fish oil and lubricating oil can reduce the foaming performance.
Chemical properties of gelatin
3-1 The effect of gelatin on acid, alkali and salt
Gelatin can form compounds with acid, alkali and salt. When it reacts with boiling acid and alkali, gelatin will gradually decompose and form simple substances such as sarcosine, peptone, polypeptide and amino acid. If gelatin is reacted with boiling dilute sulfuric acid for several hours, then neutralized with calcium carbonate and the resulting filtrate is evaporated, glycine crystals can be obtained
3-2 Effect of bacteria on gelatin
Gelatin solution is a bacterial culture medium. Many bacteria such as pathogenic Staphylococcus, obligate anaerobes, facultative anaerobes, and Rhizoctonia solani, aspergillus, and Mucor among fungi can liquefy gelatin.
3-3 Molecules and molecular weight of gelatin
Proteins, especially gelatin, are composed of various amino acids. For example, after two glycine molecules are dehydrated, a mono-diglycine is produced; such a reaction is called condensation. These amino acids are condensed in large quantities in a certain proportion to become gelatin (according to many people's X-ray research, a peptide chain composed of 288 amino acid rings is formed, and gelatin is formed by the high polymerization of many directional or disordered peptide chains).
Therefore, gelatin molecules have neither a fixed structure nor a fixed molecular weight. But their molecular weights are integer multiples of the molecular weight of simple proteins, and are often multiples in geometric series. Therefore, the commercial gelatin we are talking about is a mixture of many gelatin substances, and their molecular weights vary from 15,000 to 250,000. The amount of various components depends on the properties of the raw materials on the one hand, and on the manufacturing method on the other. Among commercial gelatin, those with a lower degree of degradation have a relatively large molecular weight, about 55,000 on average. In order to produce more and better gelatin, it is required in production to hydrolyze collagen into gelatin as much as possible, and to prevent gelatin from being hydrolyzed further. From the perspective of molecular structure, the isoelectric point of gelatin: Since gelatin contains carboxyl and amino groups in its molecular structure, they have dual chemical properties. In alkaline solutions, the carboxyl groups in gelatin molecules lose hydrogen ions and become anions, and they will move to the cathode; in acidic solutions, the amino groups in gelatin molecules gain hydrogen ions and become cations, and they will move to the cathode; this Pl value in equilibrium with each other is the isoelectric point of gelatin.
3-4 Hydrolysis of gelatin
In practice, there are three ways to hydrolyze gelatin:
(1) Let gelatin react with boiling dilute sulfuric acid for several hours or azeotropize with hydrochloric acid;
(2) Treat with lime milk or barium hydroxide at elevated temperature;
(3) Use protease enzymes such as bacteria, fungi or alkaline proteases to treat in a medium at 55°C and pH=6.5~7.
If you want the hydrolysis process to proceed gradually and produce various peptides with smaller and smaller molecular weights, the temperature should be maintained at a lower level or the concentration of the reagents used should be lower, so that the various variants can be clearly distinguished. As the hydrolysis proceeds, the structure of gelatin becomes simpler and the viscosity becomes lower. Since the hydrolysis process is a complex and difficult process to master, and the results of each experiment are different, there are also differences in the reports on the content of various amino acids in gelatin.
Raw materials of gelatin
Gelatin is made from organic materials containing raw gelatin, such as bones, skins and tendons. In addition to collagen (raw gelatin) that can produce gelatin, these raw materials also contain a variety of proteins and fats. Bones also contain various mineral salts.
Raw gelatin that can produce gelatin is called collagen. It is a substance that constitutes the connective tissue, skin, bone and tendon cell tissue of animals. According to the chemical composition and structure of the tissue, there are several types of collagen, such as fibrous collagen (such as tendons and raw skin dandruff), glassy collagen (such as bone tissue, i.e. osteoin), cartilaginous collagen (cartilage), elastic collagen (such as shark fins) and fish egg phosphoprotein collagen (such as fish bladder, i.e. fish glue).
Collagen is a fibrous protein, and its final structural unit is a fibril type. Using acute angle X-ray diffraction and electron microscopy, it can be found that collagen is a periodic band structure, showing 640A along the main axis. Collagen is colorless and insoluble in cold water, dilute acid, and dilute alkaline solutions. Collagen is insoluble in water because of the bonds between molecules in its oriented structure, and to some extent, because the fibril bundles are covered with a film that reduces expansion, which hinders the destruction of intermolecular bonds.
Collagen's hydration ability is not strong, but after treatment (such as liming), with the help of hydrogen bonds, the amount of water it can fix is about 35% of its own weight. Increasing the water content of collagen can reduce the temperature of gelatin production.
Collagen can be hydrolyzed. The hydrolysis process is actually the process of extracting gelatin from collagen. There are many ways of hydrolysis, such as acid hydrolysis, alkaline hydrolysis (liming), the use of liquefiers such as potassium thiocyanate and urea, and the use of pepsin to promote hydrolysis. Collagen swells in water differently from other proteins with fibrous structures. Collagen can melt when heated to 60~70'C℃. Although the chemical composition of melted collagen is the same, its structure has the following changes:
1. The length of collagen bundles is drastically shortened and thickened;
2. The regular and neat arrangement of molecular chains in the structure disappears, thus destroying hydrogen bonds and moving the isoelectric point to a higher p value;
3. It changes from low elasticity to high elasticity, but this state disappears after losing water;
4. The strength of collagen has changed;
5. Fermentation can increase the digestibility of collagen. After collagen becomes gelatin, it can no longer return to its original structure.
Gelatin can be infinitely dissolved in water at 40°C, while gelatin cannot be dissolved. When boiling collagen, it absorbs heat, just like the heat absorbed by crystals during the melting process. However, the melting and solidification of crystals are reversible, while the boiling of collagen is irreversible. During boiling and dissolving, not all but only part of the bonds between the elongated molecules of collagen are destroyed. In order to make the molten collagen disperse well in water, the bonds that still exist should be further destroyed. This goal can be achieved by heating for a long time.
After chemical treatment with lime, the dissolution of collagen is accelerated due to the destruction of molecular bonds. However, when such collagen is dissolved in hot water, the molecules will be destroyed. In order to avoid this, that is, to reduce hydrolysis, it must be neutralized with hydrochloric acid, phosphoric acid or sulfurous acid to a pH value of 5.5~6.
When cooking raw collagen that has not been chemically treated or fermented in advance, if the temperature is low, the process of collagen dissolving in water proceeds very slowly. Experiments have shown that it can only be carried out at 2~2.5 atmospheres and a temperature of 120℃.
Other proteins
In addition to collagen, the raw materials also contain a variety of prion compounds, such as albumin, keratin, elastic hard protein, viscose, pseudomycose, melanin and cartilage. Albumin is present in the cortex, inner fibrous tissue, blood, lymph of hides and porous parts of bones. Albumin can dissolve in dilute solutions such as water, salt, alkali and acid. Albumin can be precipitated in concentrated acid and salt. When heated in an acidic medium, albumin can curl and precipitate.
Viscosity and pseudomyxoprotein (glycoprotein) are contained in the substance covering the mucous membrane and between the interfibrous spaces of the hide. Viscosity is composed of prion compounds and carbon. In addition to producing protein, it also contains glucosamine or lactose when it decomposes. These substances are soluble in weak bases. Viscosity can be precipitated with acid in alkaline solutions, while pseudomyxoprotein cannot be precipitated.
Keratin is contained in the epidermis of hair, feathers, claws, hoof horns and other keratinized parts of organisms. Keratin contains a large amount of weakly bound sulfur. The sulfur content in keratin is up to 35%, which can be removed with concentrated alkali or water at high temperature, especially under pressure. Sulfur combines with alkali to form sulfites; when combined with water, it forms hydrogen sulfide and mercaptans.
Elastic drusen is the main part of the elastic fibers of connective tissue. All soft raw materials for making gelatin contain a large amount of elastic drusen, and its content in the ox spinal cord (ligament of the neck) is the highest (up to 74.5%).
Elastic drusen is extremely stable to the effects of acid and alkali, and is insoluble in water when boiled. When boiling gelatin raw materials, the remaining part of about 10-12% of the original raw material weight is mainly elastic drusen. Melanin is a reddish brown, brown or black pigment that exists in the epidermis and dark hair of animals. It is soluble in dilute alkali solutions but insoluble in water and dilute acid. The presence of chromatophore can affect the color of gelatin and reduce its transparency. Cartilage exists in cartilage connective tissue, closely interwoven with elastic fibers and collagen fibers, and is a mixture of gelatin and mucin. Under the action of acid and alkali, cartilage can be hydrolyzed into protein, peptone and protein. When boiled in water, it can gradually dissolve to form inferior coarse glue with poor viscosity. Cartilage glue has a strong ashing effect and can cause gelatin to flocculate quickly under the action of chrome alum.
4-3 Fat in raw materials
Different gelatin raw materials contain different amounts of fat. Subcutaneous tissue, tubular bones and spongy bones contain the most fat, while raw hides contain less fat. The presence of fat in raw materials is a serious obstacle to the manufacture of gelatin.
4-4 Minerals in raw materials
The raw materials contain minerals: bones contain more, about 50% of the bone weight; skin, tendons and other raw materials contain less, almost none. When making gelatin, except for preserving only collagen that can produce gelatin, other substances are removed according to their physical and chemical properties. The method is introduced in the acid and liming process.
4-5 Composition and types of bones
The main components of bones are bone protein (including collagen and other non-collagenous proteins) and calcium phosphate. Bone protein accounts for 18%, inorganic matter accounts for 71%, and water accounts for 8%.
The types of bones, according to the type of livestock, include cattle bones, horse bones, pig bones, sheep bones...; according to their shape, they can be divided into three categories: tubular bones, flat bones, and short bones:
Tubular bones are bones whose length far exceeds their width and thickness. They are tubular in the middle, thick and closed at both ends, and filled with bone marrow. Bones belonging to this category include the humerus, metacarpal bones and forearm bones of the forelimbs, and the femur, tibia and metatarsal bones of the hind limbs; flat bones are flat, slightly curved and wide. The skull, frontal bone, hip bone, ribs, scapula, etc. all belong to this category;
Short bones are bones whose length, width and thickness are similar, and are mostly round or polygonal. Bones belonging to this category include cervical vertebrae, spine, sacral vertebrae, wrist bones, kneecaps, hoof bones, etc.
The production of gelatin mostly uses leg bones, scapulae, mandibles, horn cores, frontal bones, ribs, ischial bones, etc. of large livestock, mainly cattle bones. Pig and sheep bones can be used to make edible and industrial gelatin. Leg bones and jawbones are the best for making photographic gelatin. Although the frontal bones, shoulder blades and ribs of cattle can also be used to make photographic gelatin, the physical and chemical properties and photosensitivity of the finished gelatin are poor. Bones that cannot be used to produce gelatin can be used to produce bone glue.
4-6 Composition and types of skin
1. Physiological composition of skin
The skin of livestock can be divided into three layers according to its physiological structure: epidermis, dermis and subcutaneous layer. Epidermis The epidermis is the outermost layer of the skin, divided into two layers, the outer layer is called the stratum corneum, and the inner layer is called the basal layer. The basal layer is connected to the dermis and is an important part of the epidermis. Hair, hair, sebaceous glands and sweat glands are all products of the epidermis. The epidermis can dissolve alkali, and the ratio of epidermis to total skin thickness is 5% for pig skin and 0.5% for cowhide. The epidermis has no use value in gelatin production.
Dermis is the main part of the skin, accounting for about 85% of the thickness of the skin. The dermis is also divided into two layers, namely the papillary layer and the reticular layer. The papillary layer has the function of maintaining the body temperature of animals, so it is also called the constant temperature layer. It is connected to the epidermis. The reticular layer is inside, which is about 4 to 5 times the thickness of the papillary layer. The dermis is woven by collagen connective tissue fibers. Among them, the collagen fibers in the papillary layer are relatively fine, and the collagen fibers in the reticular layer are much stronger and thicker. The dermis also contains a part of elastic connective tissue, nerve tissue, muscle, fat, blood vessels, etc. Collagen fiber is the most important component of the dermis. In the production of gelatin using skin as raw material, cowhide and pig skin are often used. For cowhide, the papillary layer and reticular layer can be clearly divided, and they are bounded by the roots of the hair glands. The hair glands of the hair glands are embedded in the fat tissue of the subcutaneous layer, so the papillary layer and reticular layer of pig skin are not as easy to divide as cowhide. The dermis of cowhide contains less fat, while pigskin contains a lot of fat not only in the reticular layer, but also in the papillary layer. In the physiological tissue of the skin, the dermis is the raw material for making gelatin.
Subcutaneous layer The subcutaneous layer is the transition layer where the skin and the animal body are combined, not the real skin. Its thickness varies with the animal species, and even on the same skin, it varies with different parts. Generally speaking, animals with less hair (such as pigs) have thicker subcutaneous layers. The subcutaneous layer is composed of extremely irregular and loose connective tissue fibers, which are also mixed with a large amount of muscle tissue, vascular nerve tissue and adipose tissue (the subcutaneous layer of pigskin is all adipose tissue).
The subcutaneous layer contains fewer collagen fibers, and the yield of making gelatin with it is low, there are many impurities, and the quality is poor. In gelatin production, the subcutaneous layer is commonly known as the membrane layer or membrane gelatin.
2. Chemical composition of skin
The chemical composition of the skin of various animals is roughly: about 65% water, 33% protein, 2% fat, and 0.5% minerals. Fresh skin contains more water, about 60~70%. The skin of young animals contains more water than that of old animals, and the outer part of the skin contains more water than the center. The expanded collagen fibers contain the most water, and the muscle tissue contains the least water.
The skin contains a small amount of potassium, sodium, calcium, magnesium, iron and phosphorus, and the upper part of them exists in the form of chlorides, sulfates, phosphates and carbonates. There are also trace amounts of silicon, zinc, nickel, arsenic, fluorine, iodine, sulfur and uranium. The ash, which accounts for about 0.5%, mainly comes from the above minerals.
The protein content in animal skin varies with the type, race and feed of the animal, and is also related to the age of the animal. Dry skin contains 90~95% protein, and fresh skin contains about 35% protein. Generally speaking, the skin of young animals contains more protein than that of old animals. The proteins in the skin can be divided into two categories: structural proteins and non-structural proteins. The former include: collagen, elastin, keratin, etc.; the latter include: albumin, globulin, mucus, melanin, etc. Structural protein is the main protein in animal skin and also the support of animal skin; non-structural protein exists in the gaps between the structural protein fiber tissue. Collagen (collagen, commonly known as raw colloid) is a substance that can be hydrolyzed into gelatin, so collagen is the real raw material needed in gelatin production.
3. Types of skin
As the raw material for gelatin production, there are several types according to the variety and source:
Fresh pig skin is fresh or refrigerated pig skin provided by slaughterhouses or meat processing plants, which can be used to make high-grade gelatin. Cow block skin is cow head skin, cow foot skin and scrap skin that the tannery believes has no leather value. These skins have a thick dermis and a thin subcutaneous layer. If they are well processed, they can be used as raw materials for making high-grade gelatin. Raw cowhide is the scraps cut off when making various raw leather products and the thin slices cut off by machines. This kind of leather is of good quality and is the raw material for making high-grade gelatin. Cow split hide is the center layer of cowhide cut off by the tannery. The upper part is called the upper split hide, which is smooth on both sides, without the subcutaneous layer, and is all dermis. It is the raw material for making Wang and other gelatins. The lower part is called the lower split hide, which is smooth on one side and slightly with the subcutaneous layer on the other side. It can be used to make general gelatin. Wet cow split hide is dried in the sun to become dry cow split hide. Pig split hide is also called "pig Sanqing", which is the dermis layer of pig skin cut off by the machine with smooth sides. It can also be used to make high-grade gelatin after careful treatment. Wet pig split hide is dried in the sun to become dry pig split hide.
Dry cowhide is a large piece of hairy cowhide that is no longer suitable for leather making due to poor preservation. Wet sheepskin is the broken skin cut off when making sheepskin, which is mixed with scattered pieces of skin. Wet mixed leather is the scraps of leather shaved off when making horse and donkey leather, and the quality is poor. Heavy wet leather is the scraps of cowhide shaved off when making bottom leather, in strips. This kind of leather has a thin dermis, poor quality, and high alkalinity, and can only be used to make industrial gelatin. Pig skin is the scraps cut off from the tanneries. Some of them are directly dried in the sun, and some are salted and then dried.
4-7 Source of raw materials
The raw materials for making gelatin mainly come from:
1. A large amount of bones contained in meat products consumed by the vast majority of people in rural and urban areas;
2. Bones, skins and tendons provided by slaughterhouses, meat product joint enterprises and other meat processing enterprises;
3. Waste materials in the leather making process of leather factories;
4. Waste skins that are damaged, moth-eaten or moldy and cannot be used for leather making. These raw materials are purchased by gelatin manufacturers according to directive plans or by certain economic means for production.
Production of Gelatin
There are several ways to make gelatin from raw materials, including heat treatment, acid method, salt-alkali method, enzyme method, and alkaline method. The differences between these methods are only reflected in the stage before the formation of gelatin:
1. Heat treatment method
The method of converting collagen into gelatin by simply extracting it with hot water for a long time is called heat treatment method. Aggregates can be extracted from rice in the form of gelatin solution after 12 hours at 90°C and neutral pH (the yield is closely related to the size of the raw material particles, among which particles with a diameter of about 3 mm at 100°C can also obtain the same yield after 3 hours); skin powder can extract more than 90% of collagen after 5 hours at 100°C. Some manufacturers wash fresh pig skin, add water to boil, scrape off subcutaneous fat after cooling, and then wash with warm water. According to the ratio of water to pig skin of 2:1, the skin collagen is hydrolyzed at 90~95°C for 8~9 hours, and the gelatin concentration will reach 18~20%. This gelatin can be used as a thickener for canned products, and the water in it can also be evaporated to make gelatin. However, the gelatin produced by this method is of poor quality. The commercial value is not great.
2. Acid method
The method of making gelatin by hydrolyzing collagen with acid is called the acid method. The acid method is most suitable for making gelatin from bones and pig skin. The whole process of acid method gelatin making is much faster than that of alkaline method, and the properties of gelatin made by acid method are very different from those made by alkaline method. Since the former is not treated with lime, it contains more undamaged amide groups and aldehyde protein degradation products, so the acid method gelatin inhibitor and sensitizer content is higher. In addition, the acid method gelatin has more contrast improvers generated by the reaction of aldehydes and cysteine, generally 1.8 (alkaline method gelatin is 1.5~1.6), so its isoelectric point is between 7 and 9. In terms of photographic performance, acid method gelatin is suitable for high-contrast positive film and diffusion transfer process.
Acid method preparation There are generally three types of gelatin:
1. Chronic hydrochloric acid acidification method
The acid-based gelatinization process using bones as raw materials is to degrease the bone powder and extract the oil (or extract the oil without crushing) by hydraulic degreasing and washing to remove the residual acid, and then soak it in 2.5% hydrochloric acid at a temperature of 10~20℃ for 8~10 days, and then wash and extract it. When producing high-viscosity acid-based gelatin, wash it with water to pH5.5: when producing low-viscosity acid-based gelatin, wash it with water to pH4. The raw materials treated with acid are harder, and the rate of collagen hydrolysis to gelatin is slower than that of alkali. In order to speed up the extraction speed, a lower pH value is usually used.
2. Rapid and highly active acidification method
When producing positive film emulsion glue, the washed bone can be acidified with sulfur dioxide at a concentration of The key is to strictly control the temperature not higher than 15℃. Otherwise, the temperature is too high, the bone hardens, and the extraction becomes more difficult.
3. Animal skin acidification method
The acid method glue manufacturing process using skin as raw material is to cut the skin, degrease it by hydraulic pressure, wash it with water several times, and then soak it in 0.5-5% hydrochloric acid (sulfite, sulfuric acid or phosphoric acid can also be used) at a temperature of 10~20℃ for 10~24 hours, drain the waste acid water, and then wash it with water to pH4~5. Considering that the isoelectric point of most non-collagenous proteins is within the range of pH4-5, they are not dissolved during acid immersion. In order to make these non-collagenous proteins easy to condense and rarely dissolve in the glue solution, the pH value during extraction should be controlled at 4-3. The isoelectric point of pig skin gelatin produced by the acid method is as high as 4-3. It can reach pH9. Prolonging the acid soaking time can reduce its isoelectric point. For this reason, the acid soaking time can be up to 7 days.
3. Salt-alkali method
The method of producing gelatin by using a mixture of sodium sulfate and sodium hydroxide instead of lime milk to soak the raw materials is called the salt-alkali method. The process is to soak the raw materials at 0~20℃ for 1~4 days with a mixture of 10% sodium sulfate aqueous solution and 2~5% sodium hydroxide. After washing and neutralization, it can be loaded into the pot for extraction. The sodium hydroxide used to soak the raw materials can fully expand the collagen fibers, which is beneficial to increase the gelatin output speed. However, when the concentration of sodium hydroxide is high, the collagen fibers will change from maximum expansion to dissolution, causing the raw materials to rot; while sodium sulfate has a dehydration and salting-out effect on protein, causing the collagen fibers to shrink and prevent it from reaching maximum expansion. The mixed solution can be used for impregnation, which can not only make the collagen swell, but also prevent it from rotting. The salt-alkali method is suitable for producing gelatin with skin as raw material, and is more effective for dry skin that is difficult to handle with lime. Wet cowhide, dry pig skin, and dry cowhide are impregnated with the salt-alkali method, which can not only increase the gelatin production rate; the main quality indicators of gelatin such as freezing force and viscosity are also improved.
The large amount of high-concentration strong alkali solution produced by the salt-alkali method for producing gelatin is easy to pollute the environment and should be recycled or neutralized with waste acid.
4. Enzyme method
The method of using enzyme treatment to dissolve collagen and heat denature it to become gelatin is called the enzyme method. The process is to hydrolyze collagen into gelatin protein with protease, add dilute acid to dissolve the gelatin protein, and then use acetone, sodium sulfate or sodium chloride to precipitate the gelatin protein. In this way, 100% gelatin can be obtained. The use of enzyme method to produce gelatin eliminates the processes of extraction and evaporation, shortens the production cycle, and reforms the traditional process. It is a direction for the development of gelatin production. However, due to the difficulty in screening enzymes and controlling the degree of hydrolysis, it still needs further exploration.
5. Alkali method
The method of producing gelatin by pretreating raw materials containing collagen with lime suspension, sodium hydroxide solution, etc. is called alkaline method. In terms of product yield, properties and purity, this is a successful method that can produce high-quality gelatin. At present, more than 80% of domestic gelatin is produced by alkaline method, and countries around the world also generally use alkaline method to produce gelatin. Therefore, this article introduces the whole process of producing gelatin by alkaline method from Chapter 5. In view of Most of the process before liming and after extraction belongs to the physical processing of raw materials and glue liquid, which will not change the properties of gelatin. Therefore, the corresponding parts of the alkaline production process can be used as reference for the parts not mentioned in the other methods introduced above.
After the selection and classification of skins and bones, gelatin is produced by the alkaline method. Except that bones can be directly acid-soaked or crushed first, oil extracted and then acid-soaked, and skins need to be pre-limed and cut, the process flow from then on is basically the same.
After preservation and bleaching, the concentrated glue liquid can be directly used as a finished product and used as a thickener for canned food. In addition, it can be processed into gelatin products in various shapes such as film, glue powder, glue beads, and glue particles through various ways using different processes and equipment.
Post time: Jul-18-2025