Ground flaxseed
Milled linseed which forms a mucilaginous gel with water — a capable binder that supplies no aeration and no structure whatever.
Also called linseed (UK); flax egg (US) — The name given to the gel, not to the seed.
Ground flaxseed mixed with water forms a viscous gel, because the seed coat contains soluble mucilage polysaccharides that hydrate readily. That gel binds: it holds a mixture together and stops it crumbling, which is the job egg does in a dense bake such as a brownie or a flapjack.
It cannot aerate. There is no protein network capable of holding a foam, so a batter relying on whisked egg for its lift will not rise. It cannot emulsify to any useful degree either. It also carries a distinct nutty flavour and visible dark flecks, and it darkens the crumb — all of which are acceptable in a wholemeal or chocolate bake and obvious in a pale sponge.
What changes if it is left out
In an egg-free formulation relying on it, the mixture loses cohesion and crumbles when cut. Nothing else in a typical egg-free bake performs the binding job.
Unsourced at passage level. The mucilage mechanism is established plant chemistry; the baking consequence is professional and domestic consensus.
RecordedEstablished practice, not a measured result
Related
What it does
Binding
Mucilage polysaccharides in the seed coat form a gel with water that bridges solid particles. It binds well and does nothing else.
Can stand in for
Eggs— for binding only — it cannot aerate or set
Works well where the egg was holding the mixture together: brownies, flapjacks, dense cakes. It supplies no foam and no coagulating protein, so a sponge relying on whisked egg will not rise. It also adds a nutty flavour and dark flecks, which disappear in a chocolate bake and are obvious in a pale one.
Can be replaced by
Chia seed— for binding, differing in texture and flavour rather than in function
Interchangeable for what they do. Chia’s gel is somewhat more viscous at comparable hydration, chia carries less of flax’s nutty flavour, and whole chia remains visibly seeded. None of those is a difference in function.