Gelatin

Gelatin is a collection of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, bones, and connective tissues of animals such as domesticated cattle, chicken, pigs, and fish. During hydrolysis, some of the bonds between and within component proteins are broken. Its chemical composition is, in many aspects, closely similar to that of its parent collagen.[2] Photographic and pharmaceutical grades of gelatin generally are sourced from cattle bones and pig skin. Gelatin is classified as a hydrogel.

Amino acid composition

Gelatin is nearly tasteless and odorless with a colorless or slightly yellow appearance.[3][4] It is transparent and brittle, and it can come as sheets, flakes, or as a powder.[3] Polar solvents like hot water, glycerol, and acetic acid can dissolve gelatin, but it is insoluble in organic solvents like alcohol.[3] Gelatin absorbs 5–10 times its weight in water to form a gel.[3] The gel formed by gelatin can be melted by reheating, and it has an increasing viscosity under stress (thixotropic).[3] The upper melting point of gelatin is below human body temperature, a factor that is important for mouthfeel of foods produced with gelatin.[5] The viscosity of the gelatin-water mixture is greatest when the gelatin concentration is high and the mixture is kept cool at about 4 °C (39 °F). Commercial gelatin will have a gel strength of around 90 to 300 grams Bloom using the Bloom test of gel strength.[6] Gelatin's strength (but not viscosity) declines if it is subjected to temperatures above 100 °C (212 °F), or if it is held at temperatures near 100 °C for an extended period of time.[7][8]

Gelatins have diverse melting points and gelation temperatures, depending on the source. For example, gelatin derived from fish has a lower melting and gelation point than gelatin derived from beef or pork.[9]

When dry, gelatin consists of 98–99% protein, but it is not a nutritionally complete protein since it is missing tryptophan and is deficient in isoleucine, threonine, and methionine.[10] The amino acid content of hydrolyzed collagen is the same as collagen. Hydrolyzed collagen contains 19 amino acids, predominantly glycine (Gly) 26–34%, proline (Pro) 10–18%, and hydroxyproline (Hyp) 7–15%, which together represent around 50% of the total amino acid content.[11] Glycine is responsible for close packing of the chains. Presence of proline restricts the conformation. This is important for gelation properties of gelatin.[12] Other amino acids that contribute highly include: alanine (Ala) 8–11%; arginine (Arg) 8–9%; aspartic acid (Asp) 6–7%; and glutamic acid (Glu) 10–12%.[11]

Hydrolyzed collagen, like gelatin, is made from animal by-products from the meat industry or sometimes animal carcasses removed and cleared by knackers, including skin, bones, and connective tissue.

In 1997, the U.S. Food and Drug Administration (FDA), with support from the TSE (transmissible spongiform encephalopathy) Advisory Committee, began monitoring the potential risk of transmitting animal diseases, especially bovine spongiform encephalopathy (BSE), commonly known as mad cow disease.[14] An FDA study from that year stated: "... steps such as heat, alkaline treatment, and filtration could be effective in reducing the level of contaminating TSE agents; however, scientific evidence is insufficient at this time to demonstrate that these treatments would effectively remove the BSE infectious agent if present in the source material."[15] On 18 March 2016, the FDA finalized three previously issued interim final rules designed to further reduce the potential risk of BSE in human food.[16] The final rule clarified that "gelatin is not considered a prohibited cattle material if it is manufactured using the customary industry processes specified."[17]

The Scientific Steering Committee (SSC) of the European Union in 2003 stated that the risk associated with bovine bone gelatin is very low or zero.[18][19]

In 2006, the European Food Safety Authority stated that the SSC opinion was confirmed, that the BSE risk of bone-derived gelatin was small, and that it recommended removal of the 2003 request to exclude the skull, brain, and vertebrae of bovine origin older than 12 months from the material used in gelatin manufacturing.[20]

If the raw material used in the production of the gelatin is derived from bones, dilute acid solutions are used to remove calcium and other salts. Hot water or several solvents may be used to reduce the fat content, which should not exceed 1% before the main extraction step. If the raw material consists of hides and skin; size reduction, washing, removal of hair from hides, and degreasing are necessary to prepare the hides and skins for the hydrolysis step.

After preparation of the raw material, i.e., removing some of the impurities such as fat and salts, partially purified collagen is converted into gelatin through hydrolysis. Collagen hydrolysis is performed by one of three different methods: acid-, alkali-, and enzymatic hydrolysis. Acid treatment is especially suitable for less fully cross-linked materials such as pig skin collagen and normally requires 10 to 48 hours. Alkali treatment is suitable for more complex collagen such as that found in bovine hides and requires more time, normally several weeks. The purpose of the alkali treatment is to destroy certain chemical crosslinks still present in collagen. Within the gelatin industry, the gelatin obtained from acid-treated raw material has been called type-A gelatin and the gelatin obtained from alkali-treated raw material is referred to as type-B gelatin.[23]

Advances are occurring to optimize the yield of gelatin using enzymatic hydrolysis of collagen. The treatment time is shorter than that required for alkali treatment, and results in almost complete conversion to the pure product. The physical properties of the final gelatin product are considered better.[24]

Extraction is performed with either water or acid solutions at appropriate temperatures. All industrial processes are based on neutral or acid pH values because although alkali treatments speed up conversion, they also promote degradation processes. Acidic extraction conditions are extensively used in the industry, but the degree of acid varies with different processes. This extraction step is a multistage process, and the extraction temperature usually is increased in later extraction steps, which ensures minimum thermal degradation of the extracted gelatin.

This process includes several steps such as filtration, evaporation, drying, grinding, and sifting. These operations are concentration-dependent and also dependent on the particular gelatin used. Gelatin degradation should be avoided and minimized, so the lowest temperature possible is used for the recovery process. Most recoveries are rapid, with all of the processes being done in several stages to avoid extensive deterioration of the peptide structure. A deteriorated peptide structure would result in a low gel strength, which is not generally desired.

The 10th-century Kitab al-Tabikh includes a recipe for a fish aspic, made by boiling fish heads.[25]

A recipe for jelled meat broth is found in Le Viandier, written in or around 1375.[26]

In 15th century Britain, cattle hooves were boiled to produce a gel.[27] By the late 17th century, the French inventor Denis Papin had discovered another method of gelatin extraction via boiling of bones.[28] An English patent for gelatin production was granted in 1754.[27] In 1812, the chemist Jean-Pierre-Joseph d'Arcet further experimented with the use of hydrochloric acid to extract gelatin from bones, and later with steam extraction, which was much more efficient. The French government viewed gelatin as a potential source of cheap, accessible protein for the poor, particularly in Paris.[29]

Food applications in France and the United States during the 19th century appear to have established the versatility of gelatin, including the origin of its popularity in the US as Jell-O.[30] In the mid-19th century, the American industrialist and inventor, Peter Cooper, registered a patent for a gelatin dessert powder he called "Portable Gelatin", which only needed the addition of water. In the late 19h century, Charles and Rose Knox set up the Charles B. Knox Gelatin Company in New York, which promoted and popularized the use of gelatin.[31]

Eggs in aspic

Probably best known as a gelling agent in cooking, different types and grades of gelatin are used in a wide range of food and nonfood products. Common examples of foods that contain gelatin are gelatin desserts, trifles, aspic, marshmallows, candy corn, and confections such as Peeps, gummy bears, fruit snacks, and jelly babies.[32] Gelatin may be used as a stabilizer, thickener, or texturizer in foods such as yogurt, cream cheese, and margarine; it is used, as well, in fat-reduced foods to simulate the mouthfeel of fat and to create volume. It also is used in the production of several types of Chinese soup dumplings, specifically Shanghainese soup dumplings, or xiaolongbao, as well as Shengjian mantou, a type of fried and steamed dumpling. The fillings of both are made by combining ground pork with gelatin cubes, and in the process of cooking, the gelatin melts, creating a soupy interior with a characteristic gelatinous stickiness.

Gelatin is used for the clarification of juices, such as apple juice, and of vinegar.[33]

Isinglass is obtained from the swim bladders of fish. It is used as a fining agent for wine and beer.[34] Besides hartshorn jelly, from deer antlers (hence the name "hartshorn"), isinglass was one of the oldest sources of gelatin.

In cosmetics, hydrolyzed collagen may be found in topical creams, acting as a product texture conditioner, and moisturizer. Collagen implants or dermal fillers are also used to address the appearance of wrinkles, contour deficiencies, and acne scars, among others. The U.S. Food and Drug Administration has approved its use, and identifies cow (bovine) and human cells as the sources of these fillers. According to the FDA, the desired effects can last for 3–4 months, which is relatively the most short-lived compared to other materials used for the same purpose.[35]

Capsules made of gelatin

• Certain professional and theatrical lighting equipment use color gels to change the beam color. Historically, these were made with gelatin, hence the term, color gel.
• Originally, gelatin constituted the shells of all drug and vitamin capsules to make them easier to swallow. Now, a vegetarian-acceptable alternative to gelatin, hypromellose, is also used, and is less expensive than gelatin to produce.
• Some animal glues such as hide glue may be unrefined gelatin.
• It is used to hold silver halide crystals in an emulsion in virtually all photographic films and photographic papers. Despite significant effort, no suitable substitutes with the stability and low cost of gelatin have been found.
• Used as a carrier, coating, or separating agent for other substances, for example, it makes β-carotene water-soluble, thus imparting a yellow color to any soft drinks containing β-carotene.
• Ballistic gelatin is used to test and measure the performance of bullets shot from firearms.
• Gelatin is used as a binder in match heads[36] and sandpaper.[37]
• Cosmetics may contain a non-gelling variant of gelatin under the name hydrolyzed collagen (hydrolysate).
• Gelatin was first used as an external surface sizing for paper in 1337 and continued as a dominant sizing agent of all European papers through the mid-nineteenth century.[38] In modern times, it is mostly found in watercolor paper, and occasionally in glossy printing papers, artistic papers, and playing cards. It maintains the wrinkles in crêpe paper.
• Biotechnology: Gelatin is also used in synthesizing hydrogels for tissue engineering applications.[39] Gelatin is also used as a saturating agent in immunoassays, and as a coat.[40] Gelatin degradation assay allows visualizing and quantifying invasion at the subcellular level instead of analyzing the invasive behavior of whole cells, for the study of cellular protrusions called invadopodia and podosomes, which are protrusive structures in cancer cells and play an important role in cell attachment and remodeling of the extracellular matrix (ECM).[41]