On the past three attempts on making Jelly (Hartley's from the uk), the jelly has not set very well. Much to my children's disappointment - the jelly came out of the mould and just spread out on the plate.
We only live at 600m so not very high, but I never had any problem in low-lying London.
Any tips?

I'm not really an expert but i don't see a reason why high altitude might affect Jelly?

Make it again with less water and see what happens.

Yes that is my plan for next time, just wondered if anyone else had this problem and if there any science to it.

At 700m+, we've had no problems. My advice is to reduce the quantity of water slightly (as already given), and make sure it is properly chilled, before turning out. However, I can never recall (from childhood in UK 'til now), that a jelly ever came out of the mould like the pic. on the box, and didn't collapse a bit. I would imagine to accomplish this you would have to reduce the water considerably.

We are at 550M and jelly works just fine for us .. we use a rabbit mould and reduce the water slightly .. I admit the rabbit does "flop" a little but it certainly wobbles in a very admirable fashion. I do have childhood memories of jelly rabbits always standing to attention (so to speak ..) but that was in Cheshire at perhaps 20m above sea level? and anyway my sister says I am confusing them with blancmange rabbits which always stood very proud.

We live at 950% and no problem. I know recipes for cakes have to be adjusted for higher altitudes, or they won't rise.

Yes, it does.

Boiling point of the mix is lower at altitude - therefore, for a colloidal suspension like jelly, an increased cooking time is required for the protein chains to break and reform to useful lengths on cooling.

The weather (especially when there's a really deep low pressure system prevailing) will also influence the results, for the same reasons...

HTH

With this in mind, perhaps it would be a good idea to get a candy thermometer to check the temperature of the water? Not just visibly boiling but make sure it is 100°C / 212°F... maybe that would help?

Nice idea, but it won't help any. Physical laws say that the temperature of a boiling liquid does not rise beyond the boiling point until the entire liquid has been vaporized.

In other words, when water, at a given altitude, boils at, say, 96°C, you can heat it as much as you like, even with the exhaust of a jet engine, it won't get any hotter until it's all steam. You will never get 100°C hot water at that altitude without a pressure cooker or the like.

This is completely wrong - sorry.... Gelatin dissolves at about 40°C. Altitude does change the boiling point of water, due to the thinner air, but has no effect on the melting and gelation of gelatin.

What was the use by date ???

You don't need to apologise - I'd agree with you iof you were correct...

Your reference to "melting point" shows that you're looking in entirely the wrong direction - the key point is not the melting point of gelatine, it's the length of the protein (gelatine) molecules when the mix cools down and changes from being solids in suspension in liquid, to being liquid in suspension in solid, and whether these molecules are sufficiently long to "hold" the water within the solid.

Like I said, jelly (jello) is a colloidal suspension - IIRC, it's a reversible hydrocolloid, with a solid continuous medium and a liquid dispersed phase.

Feel free to post some authoritative references to the contrary - to be fair, I was once wrong about something a few decades ago...

Thank you Weejeem for that information. So should we use less water? 5% or 10% ? Or just simply cook it longer as you said.

Working the physics backward, I'm pretty sure that the "right" answer is to start with a bit more water, and then cook longer.

That is, you cook longer to get the same length of gelatin chain as normal. But because you cook it longer, more of your initial water evaporates. So you need a bit of extra water to make up for this.

Not that I can claim to have tried this.

Wow. This is a little over my head as to the science but as to the solution .... the problem is that the jelly we are talking about isn't "cooked" just mixed with a bit of boiling water from the kettle, then topped up with cold water.

Maybe I should try putting it all in a pan and boiling it for a little while? Suggestions anyone?

Please just read the post before yours.

Ahh, thanks. Times like this show that science wasn't my strongest subject in school.

According to what ThomasSSS has said, it does sound like that could be a workable solution.

My hubby made pannacotta for my birthday dessert, using gelatin "sheets" which I am fairly certain required some cooking as opposed to simply being a matter of combining hot (boiling) water with the ingredients.

Perhaps if you're not getting your boxed jelly / jell-o to come out properly, it may work better for you in the end to go the more complicated route of using fruit juice, sugar and gelatin sheets.

__________________
The Joys of Opticianry

The lengths of the protein molecules in gelatin don't change with heating or boiling, it takes a lot more than 100 degrees to break apart a protein molecule. To make jelly from gelatin, you need to denature the proteins in gelatin; in other words, heat them up so they change their conformation and essentially melt or unfold into long strings. The denatured gelatin has a much higher affinity for water than undenatured gelatin. As the denatured gelatin slowly cools, water molecules are trapped by these strings of gelatin, and you end up with jelly. (Yes weejem, it is a hydrocolloid.)

The process of protein denaturation is dependent on both temperature and time. The less hot your water is, the longer it will take for gelatin to fully denature. So if your water is boiling at 95 instead of 100, I'd advise, as others suggested, simply boiling the mixture for an extra minute or two before allowing it to cool. That should work fine. If you're worried about loss of liquid by boiling it in a pan, simply measure the volume before putting it in the pan. Once you've finished boiling it, measure it again, and make up the difference with water.

Regarding Mr. Zappa's assertion that gelatin will melt at 40 degrees, this may be true, but at that temperature the proteins will not be sufficiently denatured to trap enough water to make jello.

(And for the record, I do actually do this at work. Denature proteins that is, not make gelatin. Tomorrow they're getting five minutes at 95 degrees C, which should be more than sufficient.)

Now there's one for "What's My Line?" !