Gelatin

Gelatin is a mixture of <a href="http://en.wikipedia.org/wiki/Peptide" title="Peptide" >peptides</a> and <a href="http://en.wikipedia.org/wiki/Protein" title="Protein" >proteins</a> produced by partial <a href="http://en.wikipedia.org/wiki/Hydrolysis" title="Hydrolysis" >hydrolysis</a> of <a href="http://en.wikipedia.org/wiki/Collagen" title="Collagen" >collagen</a> extracted from the skin, boiled crushed bones, connective tissues, organs and some intestines of animals such as domesticated <a href="http://en.wikipedia.org/wiki/Cattle" title="Cattle" >cattle</a>, chicken, horses hooves, and <a href="http://en.wikipedia.org/wiki/Pigs" title="Pigs" class="mw-redirect" >pigs</a>.

The natural molecular bonds between individual collagen strands are broken down into a form that rearranges more easily. 

Gelatin melts to a liquid when heated and solidifies when cooled again. Together with water, it forms a semi-solid <a href="http://en.wikipedia.org/wiki/Colloid" title="Colloid" >colloid</a> <a href="http://en.wikipedia.org/wiki/Gel" title="Gel" >gel</a>. Gelatin forms a solution of high <a href="http://en.wikipedia.org/wiki/Viscosity" title="Viscosity" >viscosity</a> in water, which sets to a gel on cooling, and its chemical composition is, in many respects, closely similar to that of its parent collagen.<a href="http://en.wikipedia.org/wiki/Gelatin#cite_note-Ward-1" >[2]</a> Gelatin is also soluble in most <a href="http://en.wikipedia.org/wiki/Solvent" title="Solvent" >polar solvents</a>.

Gelatin solutions show viscoelastic flow and streaming <a href="http://en.wikipedia.org/wiki/Birefringence" title="Birefringence" >birefringence</a>. If gelatin is put into contact with cold water, some of the material dissolves, but not all. The solubility of the gelatin is determined by the method of manufacture. Typically, gelatin can be dispersed in a relatively concentrated acid. Such dispersions are stable for 10–15 days with little or no chemical changes and are suitable for coating purposes or for extrusion into a precipitating bath.

Gelatin gels exist over only a small temperature range, the upper limit being the melting point of the gel, which depends on gelatin grade and concentration (but is typically less than 35°C) and the lower limit the freezing point at which ice crystallizes. The upper melting point is below <a href="http://en.wikipedia.org/wiki/Body_temperature" title="Body temperature" class="mw-redirect" >body temperature</a>, a factor which is important for <a href="http://en.wikipedia.org/wiki/Mouthfeel" title="Mouthfeel" >mouthfeel</a> of foods produced with gelatin.<a href="http://en.wikipedia.org/wiki/Gelatin#cite_note-Cole-2" >[3]</a>

Mechanical properties of gelatin gels (for example the gel strength, which is quantified using the <a href="http://en.wikipedia.org/wiki/Bloom_(test)" title="Bloom (test)" >Bloom test</a>) are very sensitive to temperature variations, previous thermal history of the gel, and time. The viscosity of the gelatin/water mixture increases with concentration and when kept cool (≈ 4 °C).