Pastry
Gelatine, agar and starch set a dessert in three incompatible ways
They are all called setting agents and they build entirely different structures, which is why one melts in the mouth, one does not melt until it is nearly boiling, and one cannot be unmoulded at all.
By Aditya Ramaswamy4 min read
A gel is a liquid held by a network
Every set dessert is the same arrangement at bottom: a small quantity of some long-chain molecule forms a three-dimensional mesh through a much larger quantity of liquid, and the liquid is trapped in the spaces. What varies is what the mesh is made of and what holds it together, and those differences produce gels that behave nothing alike despite looking similar in a bowl.
The three most common families in a sweet kitchen are protein gels, from gelatine; polysaccharide gels from seaweed, principally agar and carrageenan; and starch gels, which are what a pastry cream or a blancmange relies on. Pectin belongs to the second family and has its own conditions, which is a separate matter from the comparison here.
Gelatine melts at body temperature and that is its whole appeal
Gelatine is collagen broken into fragments that dissolve in hot water and, on cooling, partly reassemble into a network held by weak bonds. Those bonds come apart again at around thirty-five degrees Celsius, which is to say in the mouth. That is why a gelatine set is the one that dissolves as you eat it, releasing flavour cleanly, and why no other setting agent produces quite the same sensation.
The properties follow from that low melting point. Gelatine desserts must be kept cold, they can be melted and reset repeatedly without harm, and they set slowly enough that a mousse can be folded together before it firms. They also fail entirely in the presence of certain raw fruits — pineapple, kiwi, papaya, fresh figs — because those contain enzymes that cut protein chains, and a cut network cannot form. Cooking the fruit first destroys the enzyme and solves it.
Agar sets firm, holds heat, and doesn’t melt in the mouth
Agar comes from red seaweed and its network is held by different chemistry, with the practical consequence that a set agar gel does not liquefy until well above eighty degrees Celsius. It sets at a much lower temperature than it melts at, a large gap known as hysteresis, so an agar preparation can be poured warm and will hold at room temperature in a hot climate without refrigeration.
The texture is the trade. Agar gives a firm, brittle, clean-breaking gel that cuts sharply and does not melt on the tongue, so it eats as a solid rather than dissolving. It must be brought to a full boil to dissolve properly, which is the commonest reason it fails, and it needs proportionally far less material than gelatine for the same firmness. Strong acid degrades it during boiling, so acidic juices are added after dissolution rather than before.
Starch thickens and only sometimes sets
Starch works by a completely different route. Granules absorb water, swell and burst, and the released chains entangle into a viscous mass. Cornflour and wheat starch produce an opaque, spoonable set that holds a shape but isn’t truly elastic; potato and tapioca starches give clearer, stringier, more cohesive results that behave quite differently under a spoon.
The quantity of starch needed is far greater than for the other two, typically several percent of the mixture rather than a fraction of one, and it must be cooked to gelatinise. That gives starch a flavour and an opacity the others don’t have, which is sometimes a cost and sometimes the point. Nobody wants a transparent pastry cream.
Starch gels also change on standing, because the same retrogradation that stales bread makes them weep water over a day or two in the fridge. That is why a starch-set dessert is best eaten fresh and why many recipes combine a little gelatine with the starch, using one for immediate thickening and the other for keeping quality. Combining setting agents is normal professional practice rather than an admission of failure.
Choosing between them is a question about temperature and mouthfeel
If the dessert will be eaten cold and should dissolve as it is eaten, gelatine is the answer and there is no close substitute for the sensation. If it must survive a warm room, be served warm, or suit a vegetarian or vegan preparation, agar is the tool, and the firmer texture has to be accepted or engineered around by using less of it and accepting a softer, more fragile set.
If the dessert is served in the vessel it set in and wants richness rather than structure, starch is doing the job and neither of the others is needed. The mistake worth avoiding is treating the three as interchangeable by weight. They are not, they are not even the same order of magnitude, and a recipe rewritten by direct substitution will produce either a puddle or a rubber disc.
Common questions
Can I swap agar for gelatine one for one?
No. Agar sets far more strongly by weight and produces a completely different texture, firm and brittle rather than wobbly and melting. As a starting point it takes considerably less agar than gelatine for a comparable firmness, and the result will still not eat the same way. Test a small quantity before committing.
Why did my gelatine mousse stay liquid?
Either the gelatine was never fully dissolved, or it met an acid or an enzyme that broke it down, or the mixture was too warm when it was folded together and never got cold enough to set. Fresh pineapple, kiwi and papaya are the classic culprits and defeat gelatine completely unless cooked first.
Does gelatine need to be bloomed?
Soaking sheets or powder in cold water first lets the granules hydrate evenly before heat arrives, which prevents lumps of undissolved gelatine sealed in a shell of gel. With sheet gelatine it is essential; with some modern powders it is less critical but still the safer habit.
Contributing editor, The Last Bite
Aditya has written about baking basics, chocolate & sugar, frozen for most of the last decade and prefers a plain explanation to a clever one.
