In short: Pastry powders are thickeners and stabilisers: albumen, agar, pectin, starches, gums and milk powder. Each has its own job and its own dosage, and swapping one for another at the same weight does not work.
Too much choice
The market now offers an enormous variety of pastry powders that help beginners and professionals alike make good-looking, good-tasting desserts. The difficulty is standing in front of the shelf and knowing which of them you actually need.
Albumen
Dried, pasteurised egg white. It replaces raw white in meringue, marshmallow and royal icing, removing the risk that comes with uncooked egg, and it often whips into a more stable foam than fresh white.
Agar
A seaweed-derived gelling agent, several times stronger than gelatin and setting at around 40 °C rather than in the fridge. It gives a firm, brittle set rather than an elastic one, which is why marshmallow made on agar behaves differently from marshmallow made on gelatin.
Pectin
Used for confit, jellies and fruit layers. It needs sugar and acid to set, which is why a low-sugar recipe often fails with it. Different types exist for different jobs, and they are not interchangeable.
Starches
Corn and potato starch thicken but do not gel: the difference matters because a starch-thickened filling will not hold a clean edge in a cut section.
Gums and milk powder
Gums stabilise emulsions and prevent ice crystals in frozen desserts, at very small dosages. Milk powder adds body and browning without adding liquid, which is useful in ganache and in enriched doughs.
Frequently asked questions
How does agar differ from gelatin
Agar is several times stronger and sets at room temperature; gelatin only in the fridge. Amounts must always be recalculated.
What is albumen for
It is dried egg white. It replaces raw white in meringue, marshmallow and icing and removes the risk of unpasteurised egg.
Can I replace pectin with starch
No. Pectin forms a gel, starch only thickens — the texture ends up quite different.