Why doesn't a flax egg work everywhere?
"1 tbsp ground flax + 3 tbsp water = 1 egg" is true for one of egg's several jobs, and silent about the rest.
The kitchen answer
A flax egg replaces moisture-binding -- it doesn't replace egg's ability to set solid with heat, or the richness egg's fat contributes. It works well in cookies and quick breads, and poorly in anything relying on egg to actually set or enrich.
What's actually happening
Ground flaxseed mixed with water forms a thick, gel-like slurry because flax's outer layer contains mucilage -- soluble fiber that absorbs water and swells into a viscous gel. That gel binds moisture and adds a sticky, cohesive quality to a batter, which covers a real (but limited) part of what egg does in most recipes.
What it doesn't do is coagulate the way egg protein does. Egg's set-when-heated structural role, and the richness carried by egg's own fat, both come from egg's actual biology -- a plant mucilage has neither, no matter how well it mimics egg's binding texture in a raw batter.
What does that mean for your recipe
Before reaching for a flax egg, it helps to ask which job the egg is actually doing in that specific recipe. If it's mostly binding moisture in a cookie or quick bread, a flax egg is a genuinely good match. If the recipe depends on egg setting solid (a custard, a quiche, a soufflé) or on egg's fat for richness (a rich pound cake), a flax egg is solving the wrong problem, no matter how correctly the ratio was followed.
What can go wrong
- Custard or quiche that never sets: nothing in the flax egg coagulates under heat -- there's no protein network to lock the structure in place.
- Cake noticeably less rich or moist over time: egg's fat and its role in a stable emulsion aren't present in a flax substitute.
- Assuming "the ratio is right" means "the job is done": matching the standard substitution amount only guarantees similar moisture-binding, not similar behavior overall.
- Blaming the recipe instead of the substitution's scope: a poor result in a coagulation-dependent recipe reflects a mismatch, not a failed recipe.
Try it yourself
Same base custard, egg vs. flax egg:
- Make a small custard base with real egg.
- Make an identical base with a flax egg substituted in.
- Bake or heat-set both the same way.
Watch for: the real-egg version setting into a sliceable custard, while the flax version stays a thickened, gel-like liquid no matter how long it's heated -- direct, unambiguous evidence of the missing coagulation step.
Go deeper
This is the exact same "different molecule, same narrow job" idea as aquafaba replacing egg foam -- flax's mucilage and aquafaba's saponins are both solving one specific piece of what egg does, via a completely different mechanism, which is why neither one is a universal egg replacement despite both being called "egg substitutes."