Everyday Compounds 4 - Glycerol

Look on the ingredients label of many different cosmetic or personal care products, and glycerol (often also called glycerin or glycerine) is commonly present. It’s also found in a variety of food products, as well as in some pharmaceutical products, so what’s the reason for its inclusion?

Glycerol is a carbon-based compound, containing three hydroxyl (OH) groups, and at room temperature is a viscous liquid, colourless and odourless, with a sweet taste. It’s produced as one of the by-products of soap-making, which involves the treatment of vegetable or animal fats with strong alkaline solutions; the production of biodiesel is also a process which produces glycerol as a side product. Synthetically, glycerol can be produced from propene, a three carbon alkene. These three methods combined contribute to the estimated 950,000 tons of glycerol that are produced annually in the US & Europe alone.

The compound has a range of uses in the food industry. Due to its sweetness, it can be used as a sweetener in drinks – its sweetness is around 60% of that of normal white sugar (sucrose). Additionally, it can act as a humectant, a substance that absorbs and retains moisture from the surroundings. This means it finds use as an additive in baked goods, which it helps keep fresh. It’s also added to icing and confectionary to prevent sugar crystallisation, to liquors as a thickening agent, and can be used as a solvent for food colourings. When added to foods, it has the designated E number E422.

The properties noted above also make glycerol useful in pharmaceuticals and cosmetics. Amongst others it’s found in toothpastes, skin care products, shaving cream and mouthwash. It’s often added as a smoothing agent, and also as a humectant to prevent products drying out. As a sweetening agent, it’s sometimes added to pharmaceuticals, and is also utilised as a tablet-coating agent; additionally, it finds use as a levigating agent for drugs, used to reduce the particle size of drug powders.

A more archaic use of glycerol is in anti-freeze. Due to the presence of hydroxyl groups, glycerol can form hydrogen bonds with water molecules, with a composition containing 70% glycerol having a freezing point to as low as -38˚C (-36˚F). Since the 1920s, ethylene glycol has been used, however, since it can bring about an even lower freezing point of around -45˚C (-49˚F) with a 60% composition. This advantage is tempered by the fact that ethylene glycol is toxic, and around 225ml of 50% solution is the approximate lethal dose for an 80kg adult. This is simply the median lethal dose, and survival at much higher doses has been recorded when medical treatment has been received. Regardless, other chemicals are used where ethylene glycol use is inappropriate – for example, in water pipes in homes. Glycerol is one possibility in this case, with propylene glycol also utilised.

Glycerol also finds use as a precursor to explosives: it is the starting material in the manufacture of nitroglycerin, the explosive liquid commonly found in dynamite and some propellants. It’s produced by reacting glycerol with sulfuric acid and nitric acid. An oily liquid that can explode when exposed to heat, or even the dropping or bumping of its container, it’s mixed with absorbent substances, which reduce its shock sensitivity, to produce dynamite. Although its sensitivity has limited its usage for military applications, and it has been succeeded by explosives such as TNT, it’s still used in many other areas.

There are certainly other uses of glycerol than those listed here – it’s a very important chemical with a very wide range of possible applications due to its properties. A final example can be found right in your kitchen; the cellophane used in food packaging and sold in rolls for covering food, made primarily of cellulose, also has glycerin added to increase its plasticity – that is, to ensure the film is stretchy and to prevent it from becoming brittle. Due to the large surplus of glycerol produced from the biodiesel production process, research is continually looking at new uses for this versatile compound, or compounds that can be easily produced from it.

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