Soft matter / 01
What is jelly, really?
Your jelly is mostly water. The protein that makes it solid is usually only about 1.5 to 3% of it. The other 97% is liquid that has forgotten how to flow.
Step 1: take collagen apart
Gelatin doesn't grow anywhere. Collagen does. It's the main building protein in skin, bone, tendon and connective tissue. Picture three long protein chains twisted into a rope: a triple helix. That rope gives tendons their tensile strength.
Now heat it in water. The bonds holding the three chains give way, the rope unwinds, and you get a tangle of loose strands. Those loose strands are gelatin. Industry soaks the collagen in acid or alkali first, pulls the gelatin out with hot water, then dries it into sheets or powder.
So gelatin is unwound collagen. Everything fun about jelly comes from what those strands do when they cool down again.
Step 2: cool it and it re-tangles
Dissolve gelatin in hot water and the strands swim about freely. As it cools, they try to rebuild their triple helix. They fail. Instead of rewinding neatly end to end, short bits of different strands twist together into tiny helices called junction zones. The rest of each strand stays loose.
The result is a 3D net. Each strand is tied into several junction zones, each zone ties together strands that started far apart, and the whole lot links into one mesh that fills the bowl.
The water can't get out. It isn't chemically stuck, just trapped: capillary forces hold it in pores far too small to drain. So the mix can't flow any more, because flowing would mean the net passing through itself.
That's why a gel is a solid made mostly of liquid. The net is a tiny slice of the mass, and it does all the holding up.
Weak bonds, big consequences
Junction zones are held by hydrogen bonds, not covalent ones. Nothing is welded for good. That one fact gives you three things you can test in your kitchen.
You can undo it. Warm the jelly and the junction zones let go. It melts back into liquid, and cooling sets it again. Over and over. A cake is different: its chemistry is one-way, and no fridge puts it back in the bowl.
It keeps getting firmer. Junction zones keep forming and growing long after the jelly looks set. Yesterday's jelly is noticeably firmer than one made two hours ago. This is called maturation, and it's why recipes say set it overnight, not just until cold.
It melts in your mouth. Gelatin gels melt somewhere in the low 30s °C, depending on how much gelatin and which grade. Your mouth is warmer than that, so a gelatin sweet really melts on your tongue instead of just dissolving. No other common gelling agent does that.
The other gels (and why they feel off)
Loads of things can gel water. None of them act like gelatin.
Agar comes from red seaweed. It sets when it cools, but only melts again much hotter, usually above about 80 °C. That gap between setting and melting is called hysteresis, and agar's is famously wide. So a vegan agar jelly never melts in your mouth. It also snaps instead of giving, so it's short and brittle, not chewy.
Pectin comes from fruit, and it needs help. High-methoxyl pectin only gels with lots of sugar and enough acid, which is why jam recipes are so fussy about both. Low-methoxyl pectin uses calcium instead and copes with less sugar. Pectin gels tend to be firm and a bit short: eat a pâte de fruit next to a wine gum and you'll feel it.
Carrageenan is also from red seaweed, and it comes in two types. Kappa sets firm and brittle with potassium. Iota sets soft and stretchy with calcium. Makers often blend them, because neither one alone gives the texture they want.
So why does it wobble?
A net of loose strands tied together here and there is, physically, a very soft, springy solid. It pushes back when you shear it out of shape, because that stretches the strands between junction zones. It pushes back much harder when you squash it, because that would mean forcing water out through pores too small to let it through.
Easy to shear, almost impossible to squash: that combo is the wobble. Our jelly runs on exactly those two rules, and the wobble read works out how fast they make it shake.