Science Questions

What makes jam set, or not?

Sat, 11th Aug 2012

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Molly C asked:

I make jam at home, and the recipes I use are always very specific about the amount of sugar and fruit to be combined with the pectin.


While attempting to make jam a few weeks ago, I was short 1/2 a cup of fruit, but decided to go for it anyways.


Unfortunately, it never set, and is more of a peach sauce than a jam. Why is this?


Victoria -   It depends on what fruits you're using.  Different fruits have different amounts of pectin in them and the whole chemistry of jam making is all about making this pectin thatís in the fruit break down and become water soluble.  That then recombines, and all of those hydrogen bonds that are holding it together recombine in a chemical reaction with the fruit acid and with the sugar, and that makes a lovely network that forms a gel, and thatís the kind of jelly-like substance of jams.  So you need to get that chemical reaction right, the pectin amount right, the fruit acid right, and the amount of sugar right so that you make the right consistency of that network that will hold your jelly together, your jam together, so you donít get fruit sauce.

Chris -   What does the pectin do that enables that to happen?

Victoria -   The pectin actually is the backbone of this structure so that it holds together.  Itís a long chain carbohydrate, a polysaccharide.  Itís these long, interlinked, connected chains held together by hydrogen bonds that gives the fruit its rigidity and it will give your jam that gel-like semi-rigidity.  But you need to break it down so that itís not too stiff.  If you have too much pectin then it will recombine in this really, really solid sticky thick form, so you have too much pectin.  And if you donít have enough of it or you havenít boiled it for long enough, or you havenít got the balance right to achieve that chemical reaction, you get this flimsy structure within your gel.  Itís basically a lattice structure that forms within your gel that will hold it down together nicely.  So, it sounds like sheís using commercial pectin as well, which is an extra cheat which you can use, but you do need to get those amounts quite right.  There are other cheats you can use like you can re-boil up and throw in a bit more fruit acid, so it's just got a little bit more of that chemical reaction going. So you can fix it.


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peaches are not acidic, like say raspberries, so need extra acid ...,1723,157165-239195,00.html RD, Sat, 14th Jul 2012

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