Overview
Glycerin, also known as glycerol, is an organic compound of carbon, hydrogen and oxygen with the chemical formula C3H8O3. It is produced industrially, usually as a by-product of soap manufacture, from oils and fats. It can be made from animal fat or, in the case of vegetable glycerin, vegetable oil. The source of the raw material does not affect the chemistry of the final product but, since glycerin is widely used in foods and medications, this distinction is important for vegetarians. It is also used as a sweetener and as aningredient in a number of cosmetic products.
Structure and Properties
The compound consists of a chain of three carbon atoms, to which are attached hydrogen atoms on one side and hydroxyl (OH) groups on the other. The three OH groups form hydrogen bonds between molecules giving the compound a syrup-like viscosity and allowing it to dissolve easily in water. Chemically speaking, glycerin is an alcohol, but for food purposes, it is classed as a carbohydrate because it provides calories and is not a fat or a protein.
Pure glycerol does not crystallize easily, but it can be chilled to form a solid that melts at about 18°C. It lowers the freezing point of water, however, by anamount that depends on the concentration. For example, a 66.7% solution freezes at -46.1°C. For this reason, it can be used as non-toxic anti freeze and for storing sensitive liquids, such as enzymes, in laboratory freezers.
Manufacturing
Glycerol forms the “backbone” of many lipids, or oils and fats, and there are various processes that can be used to extract it from these substances. Most glycerin is made as a by-product of the manufacture of soap. In this process, either animal fat or vegetable oil can be used. It is heated with a strong alkali, usually caustic soda (sodium hydroxide), which produces soap and a solution of glycerin in water. This solution is then purified by distillation. Vegetable glycerin can also be made directly from vegetable oil, often coconut or palm oil, by heating it to a high temperature under pressure with water. The glycerin backbone splits off from the fatty acids, and is absorbed by the water, from which it is then isolated and distilled to obtain the pure product. Food-grade vegetable glycerin is 99.7% pure, with the remaining 0.3% being water. The interest in biodiesel fuel has resulted in the production of large amounts of low quality, non-food grade vegetable glycerin as a by-product. Purifying the liquid is not considered economically viable and it cannot be casually disposed of as it contains toxic methanol, which is used in the manufacturing process. As of 2013, there is much research into finding a use for this substance, something that does not require high purity glycerin. One promising possibility is to use it as a precursor in the production of some useful plastics.
Main Uses
Glycerin is widely used in the food industry for two main reasons: it has a sweet taste, but has fewer calories than sugar; and it is hygroscopic, that is, it absorbs moisture from the air. It is therefore used both to sweeten foods and to keep them moist. The compound is metabolized more slowly than sucrose, the type of sugar most commonly found in candy and in processed foods, and therefore does not have such a dramatic effect on blood sugar levels. It also does not contribute to bacterial tooth decay. Foods marketed as being low in carbohydrates are often sweetened with glycerin.
Another major use is in the cosmetics industry. Due to its hygroscopic properties, it is used in many moisturizing skin products, as it seems to help relieve dry skin problems by drawing water up from the lower layers. It is also a component of glycerin soap, which is often used by people with sensitive skin.
Lotions containing this compound are also popular. Vegetable glycerin can be used as a substitute for ethanol, the chemical commonly called “alcohol”, in making botanical extracts, such as herbal essences.
It acts as a solvent that dissolves the substances of interest from the raw plant material. The advantage of this is that people who do not want to be exposed to alcohol can still have access to the botanical products. The disadvantage is that the resulting products have a much shorter shelf life.
There are also medical uses for vegetable glycerin. It is a common ingredient in cough mixtures, due to its soothing properties. Other applications are as a topical remedy for a number of skin problems, including psoriasis, rashes, burns, bedsores and cuts; as a laxative, in the form of suppositories; and to treat gum disease, as it inactivates the associated bacterial colonies.
Glycerol
FromWikipedia, the free encyclopaedia
"Glycerine"and "Glycerin"
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Glycerol |
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IUPAC name propane-1,2,3-triol |
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Other names: glycerine, glycerine, propanetriol
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Identifiers |
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CAS number |
56-81-5 |
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Pub Chem |
753 |
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ChemSpider |
733 |
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UNII |
PDC6A3C0OX |
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Drug Bank |
DB04077 |
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KEGG |
D00028 |
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ChEBI |
CHEBI:17522 |
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ChEMBL |
CHEMBL692 |
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ATC code |
A06AG04, A06AX01, QA16QA03 |
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Properties |
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Molecular formula |
C3H8O3 |
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Molar mass |
92.09 g mol−1 |
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Appearance |
colourless liquid |
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Odour |
odourless |
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Density |
1.261 g/cm3 |
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Melting point |
17.8 °C, 291 K, 64 °F |
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Boiling point |
290 °C, 563 K, 554 °F |
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Refractive index (nD) |
1.4746 |
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Viscosity |
1.412 Pa·s |
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Hazards |
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MSDS |
External MSDS |
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NFPA 704 |
1 1 0 |
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Flash point |
160 °C (320 °F) (closed cup) |
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Supplementary data page |
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Structure and |
n, εr, etc. |
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Thermodynamic |
Phase behaviour |
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Spectral data |
UV, IR, NMR, MS |
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Glycerol (or glycerine, glycerin) is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids known as triglycerides. Glycerolis sweet-tasting and of low toxicity.
From fats and oils
Triglycerides found in fats and oils are by definition esters of glycerol with long-chain carboxylic acids; the hydrolysis (saponification) or transesterification of these triglycerides produces stoichiometric quantities of glycerol. In this scheme, glycerol is produced as a co-product in the production of long-chain carboxylate salts used as soaps
It is also a by product of the production of biodiesel via transesterification. This form of crude glycerin is often dark in appearance with a thick, syrup-like consistency. Triglycerides (1) are treated with an alcohol such as ethanol (2) with catalytic base to give ethyl esters of fatty acids (3)and glycerol (4):
Glycerol from triglycerides is produced on a large scale, but the crude product is of variable quality, with a low selling price of as low as 1-8 U.S. cents perpound in 2011. It can be purified, but the process is expensive. As a result, a good fraction of crude glycerol is disposed of as waste. Some glycerol is burned for energy, but the heat value is low. That said, crude glycerol from the hydrolysis of triglycerides can be purified by treatment with activated carbon to remove organic impurities, alkali to remove un-reacted glycerol esters, and ion exchange to remove salts. High purity glycerol (> 99.5%) is obtained by multi-step distillation; vacuum is helpful due to the high boiling point of glycerol (290 °C).
Synthetic glycerol
Synthetic glycerol refers to material obtained from non-triglyceride sources. Glycerol may also be produced by various routes from propylene. The epichlorohydrin process is the most important; it involves the chlorination of propylene to give allyl chloride, which is oxidized with hypochlorite to dichlorohydrins, which reacts with a strong base to give epichlorohydrin. Epichlorohydrinis then hydrolyzed to give glycerol. Chlorine-free processes from propylene include the synthesis of glycerol from acrolein and propyleneoxide.
Because of the emphasis on biodiesel, where glycerol is a waste product, the market for glycerol is depressed, and these old processes are no longer economical on a large scale. In fact, due to the glycerol glut, efforts are being made to convert glycerol to its precursors, such as acrolein and epichlorohydrin.
Applications
Food industry
In foods and beverages, glycerol serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g biscuits) and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of leaves. As a sugar substitute, it has approximately 27 kilocalories per teaspoon (sugar has 20) and is 60% as sweet as sucrose. It does not feed the bacteria that form plaques and cause dental cavities. As a food additive, glycerol is labeled as E number E422. As used in foods, glycerol is categorized by the American Dietetic Association as a carbohydrate. The U.S. Food and Drug Administration (FDA) carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerol has a caloric density similar to table sugar, but a lower glycemic index and different metabolic pathway within the body, so some dietary advocates accept glycerol as a sweetener compatible with low carbohydrate diets.
Pharmaceutical and personal care applications
Glycerol suppositories used as laxatives. Glycerolis used in medical and pharmaceutical and personal care preparations, mainly as a means of improving smoothness, providing lubrication and as a humectant.
It is found in allergen immune therapies, cough syrups, elixirs and expectorants, toothpaste, mouth washes, skin care products, shaving cream, hair care products, soaps and water based personal lubricants. In solid dosage forms like tablets, glycerol is used as a tablet holding agent. For human consumption, glycerol is classified by the U.S. FDA among the sugar alcohols asa caloric macronutrient. Glycerolis a component of glycerin soap. Essential oils are added for fragrance. This kind of soap is used by people with sensitive, easily irritated skin because it prevents skin dryness with its moisturizing properties. It draws moisture up through skin layers and slows or prevents excessive drying and evaporation. With similar benefits, glycerin is a common ingredient in many bath salts recipes. However, some assert that due to glycerin's moisture absorbing properties, it can be more of a hindrance than a benefit. Glycerol can be used as a laxative when introduced into the rectum in suppository or small-volume (2–10 ml) (enema) form; it irritates the anal mucosa andinduces a hyperosmotic effect. Taken orally (often mixed with fruit juice to reduce its sweet taste), glycerol can cause a rapid, temporary decrease in the internal pressure of the eye. This can be a useful initial emergency treatment of severely elevated eye pressure.
Botanical extracts
When utilized in 'tincture' method extractions, specifically as a 10% solution, glycerol prevents tannins from precipitating in ethanol extracts of plants (tinctures). It is also used as an 'alcohol-free' alternative to ethanol as a solvent in preparing herbal extractions. It is less extractive when utilized in a standard tincture methodology. Glycerol is approximately 30% more slowly absorbed by the body resulting in a much lower glycemic load. Alcohol based tinctures can also have the alcohol removed and replaced with glycerol for its preserving properties. Such products are not 'alcohol-free' in either a scientific or consumable sense, but should in all instances more accurately be referred to as"Alcohol Removed" products. Fluid extract manufacturers often extract herbs in hot water before adding glycerin to make glycerites. When used as a primary 'true' alcohol free (e.g.no alcohol, i.e. ethanol) ever being used) botanical extraction solvent in innovative non tincture based 'dynamic' methodologies, glycerol has been shown, both in literature and through extraction applications, to possess a high degree of extractive versatility for botanicals including removal of numerous constituents and complex compounds, with an extractive power that can rival that of alcohol and water/alcohol solutions, that glycerol possess.
Such high extractive power assumes that glycerol, with its tri-atomic structure, is utilized with dynamic methodologies as opposed to standard passive 'tincturing' methodologies that are better suited to alcohol's di-atomic structure.
Glycerol possesses the intrinsic property of not denaturing or rendering a botanical's constituents inert (as di-atomic alcohols, i.e. ethanolic (grain) alcohol,methanolic (wood) alcohol, etc., do). Glycerolis a stable preserving agent for botanical extracts that, when utilized inproper concentrations in an extraction solvent base, does not allow inverting or reduction oxidation of a finished extract's constituents, even over several years. Both glycerol and ethanol are viable preserving agents. Glycerol is bacteriostatic in its action, and ethanol is bactericidal in its action.
Antifreeze
Like ethylene glycol and propylene glycol, glycerol is a non-ionic kosmotrope that forms strong hydrogen bonds with water molecules, competing with water-water hydrogen bonds. This disrupts the crystal lattice formation of ice unless the temperature is significantly lowered. The minimum freezing point temperature is at about −36 °F / −37.8 °C corresponding to 70% glycerol in water. Glycerol was historically used as an anti freeze for automotive applications before being replaced by ethylene glycol, which has a lower freezing point. While the minimum freezing point of a glycerol water mixture is higher than an ethylene glycol water mixture, glycerol is not toxic and is being re-examined for use in automotive applications. In the laboratory, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C due to the depression of the freezing temperature of solutions with high concentrations of glycerol. It is also used as a cryoprotectant where the glycerol is dissolved in water to reduce damage by ice crystals to laboratory organisms that are stored in frozen solutions, such as bacteria, nematodes, and mammalian embryos.
Chemical intermediate
Glycerol is used to produce nitroglycerin, which is an essential ingredient of various explosives such as dynamite, gelignite, and propellants like cordite. Reliance on soap making to supply co-product, glycerine made it difficult to increase production to meet wartime demand. Hence, synthetic glycerin processes were national defense priorities in the days leading up to World War II. Nitroglycerin, also known as glyceryl trinitrate(GTN) is commonly used to relieve angina pectoris, taken in the form of sub-lingual tablets, or as an aerosol spray. A great deal of research is being conducted to try to make value-added products from crude glycerol (typically containing 20% water and residual esterification catalyst) obtained from biodiesel production. The use of crude glycerin as an additive to biomass for a renewable energy source when combusted or gasified is also being explored.
- Hydrogen gas production unit
- Glycerine acetate (as a potential fuel additive)
- Conversion to propylene glycol
- Conversion to acrolein
- Conversion to ethanol
- Conversion to epichlorohydrin, a raw material for epoxy resins
Metabolism
Glycerol is a precursor for synthesis of triacylglycerols and of phospholipids in the liver and a dipose tissue. When the body uses stored fat as a source of energy, glycerol and fatty acids are released into the bloodstream. In some organisms, the glycerol component can be converted into glucose by the liver and, thus, provide energy for cellular metabolism. Before glycerol can enter the pathway of glycolysis or gluconeogenesis (depending on physiological conditions), it must be converted to their intermediate glyceraldehyde3-phosphate in the following steps:
The enzyme glycerol kinase is present only in the liver. In adipose tissue, glycerol 3-phosphate is obtained from dihydroxyacetonephosphate (DHAP) with the enzyme glycerol-3-phosphate dehydrogenase. Glycerol has very low toxicity when ingested; its LD50 oral dose for rats is 12600 mg/kg and 8700 mg/kg for mice.
Historical cases of contamination with diethylene glycol
Glycerine and diethylene glycol are similar in appearance, smell, and taste. The US Federal Food, Drug, and Cosmetic Act was passed following the 1937 "Elixirsulfanilamide" incident of poisoning caused by diethylene glycolcontamination of medicine. On May 4, 2007, the US Food and Drug Administration advised all US makers of medicines to test all batches of glycerine for the toxic diethylene glycol. This followed an occurrence of hundreds of fatal poisonings in Panama resulting from a Chinese factory deliberately falsifying records in order to export the cheaper diethylene glycol as the more expensive glycerol.