Royce Madigan of Bendigo Valley asks :-

I tried growing crystals of sugar, glucose and gelatine using the same method as growing salt crystals, ie dissolving the material in water until no more could be and then hanging a string in the liquid to allow large crystals to grow. It worked for sugar but not for glucose or gelatine. Why not?

Kathryn McGrath, a chemist at Otago University, responded.

Sweetness is due to the presence of sugars. Two of the most common are glucose and sucrose. Glucose, also known as grape or corn sugar, exists in two forms: D- and L-glucose. Natural glucose is of the D form.

In solution two structures predominate: alpha-D-glucose (36 per cent) and beta-D-glucose (64 per cent); these differ only by the positioning of one of the OH groups.

Sucrose is common household sugar whether white, brown or raw. It is extracted from both sugar cane (20 per cent by weight) and sugar beet (15 per cent by weight). Sucrose exists in a single form.

Both glucose and sucrose should be able to be crystallised readily to give large crystals from a supersaturated solution (a solution that has been heated and sugar added until no more will dissolve and then cooled down) in water. Reasons as to why large crystals may not grow include the presence of impurities in the original sugar or, for glucose, competition between the different forms. This can lead to the formation of small powder-like crystals.

Gelatine, unlike both glucose and sucrose, is an amino acid based polymer, derived from the skin and bones of animals. The predominant precursor of gelatine is collagen. Gelatine forms a stiff gel when dissolved in water so is the major component of jelly.

As such, gelatine cannot be crystallised out from a solution in water. The structure of gelatine is complex but information can be gained on it and its collagen precursor by growing crystals from liquids other than water and performing studies such as X-ray diffraction.