Re: How long does it take Gelatine/Jelly to freeze?

Jelly doesn't freeze, it sets. What is the difference I hear you ask?

Freezing refers to a "change of state" of matter. We have gases, liquids and solids and matter can often change from one state to another depending on the temperature (and pressure). The atoms, or molecules for compounds, are arranged differently in each phase but the chemical structure is the same.
http://www.grc.nasa.gov/WWW/k-12/airplane/state.html
http://www.chem4kids.com/files/matter_states.html

Setting usually refers to chemical changes in a substance that often changes its state. So for instance most glues don't dry, they sets. It undergoes a chemical reaction that makes it go hard. Concrete does the same thing, it is a chemical reaction making it hard, as opposed to a change of state with temperature.

What makes Jelly go hard is collagen. This is a protein found in the skin and bones of animals. It is a very long molecule with three strands sort of twisted along its long axis. It also binds well with water. Gelatin powder contains collagen.

When you mix the gelatin with hot water the strands of collagen come apart. Chemical bonds along the strands of the protein molecule break and the separate strands unravel. When you cool the mixture again the protein strands reform. They clump together with water bound along their strands into what we know a jelly.

Now this un-clumping and clumping of the collagen strands with water may sound a lot like what happens when you freeze something. Jelly goes soft again if you heat it up, and will turn watery.

However simply by definition it isn't. The name for this mixture is a hydrocolloid. Hydro simply means water, a colloid just means two things evenly mixed together. The substances in this colloid being water and a protein. When proteins unravel and come apart we call that "denaturing". This is a chemical reaction (chemical bonds between atoms break) as opposed to a phase change (which freezing and melting is).

The proteins in Jelly will start denaturing (or coming apart) at around about 35 degrees Celsius. Many people will call this "melting" but in fact it is chemical bonds within the protein molecules coming apart.

So what does this all mean?

Jelly doesn't freeze quicker, it undergoes a special type of chemical reaction, and the mixture "sets" rather than freezes. This chemical reaction can be reversed with temperature. The mixture comes apart if you heat it. But we don't call this freezing and melting even though it appears to behave very similar to the way water freezes and melts.
http://www.ifr.ac.uk/jellyvision/Jelly_facts.pdf
http://www.pitara.com/discover/5wh/online.asp?story=126
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/science-of-fruit-jellies/ http://ilovebacteria.com/jelly.htm