October 15, 2011
In this post I noted that I found some strange similarities between avocados and eggs. Avocados can be subsituted for eggs in some recipes (especially when not much egg is called for); they seize up and get rubbery just like eggs when they’re overcooked.
I also note, not that it’s relevant, that they’re constructed very similarly. Think about the inside of each - an embryo and its food source, suspended within the fruit by some other filling (egg white or avocado flesh). The possibility that avocados are some kind of “vegan egg” makes it tempting to draw as many parallels as possible.
So, I did a fair amount of research to try to understand whether avocados and eggs are really similar, chemically or in any other way. The answer, I’m afraid, is “not really”.
Avocados can do some neat baking tricks. You can make frosting, or substitute avocado for much of the butter in a brioche. The use of avocados in baking was the first thing that made me wonder if they have egg-like properties.
It is also true that avocados can substitute for eggs in baking occasionally, but generally only for a small amount of eggs - just one or 2. In this case, it’s also the fat that the avocado is taking the place of.
More generally, it turns out that in recipes like these, avocado is almost always substituting for butter, in order to add the fat that makes baked goods more tender and moist. This is a very similar technique to simply adding fruit puree, such as a blended banana, or applesauce, as a substitute. The difference with avocados is that they’re uniquely fatty - which is part of what makes them extra-delicious. As a result, they contribute more of what butter does than a banana or apple would. Just to be clear, however, it’s not really comparable. Butter is about 80% fat by weight, avocados 15%, and bananas less than 1%.
The other thing I wondered about avocados is whether they might be like eggs because they form emulsions. Before I go any further, let me explain what an emulsion is.
Normally, when you try to mix oil and vinegar together, or oil and water, you can’t. No matter how hard you agitate the mixture, all the little oil blobs inevitably find each other and then link up into one giant blob. There are lots of exceptions to this, however - mayonnaise, for example, contains water components (such as lemon juice and vinegar) and also lots of oil. And yet the mixture doesn’t separate.
This state is called an emulsion, and it’s brought about by emulsifers. Emulsifers are long molecules with one side that likes water and dissolves readily in it, and another side that likes oil and dissolves readily in it. When the molecule is simultaneously dissolved at both ends, oil and water molecules are linked together. One emulsifier you know is soap, which helps water remove grease from your hands. Another is the crema you sometimes get on good espresso, which is apparently an emulsion of coffee oils with water. Yum!
What keeps mayonnaise (our original example) from falling apart is a very common food emulsifier called lecithin. The readiest source of lecithin in a kitchen is the egg yolk, hence lecithin’s name from Greek lekithos (λέκιθος), “egg yolk”. The lecithin molecules from the yolk dissolve in the oil and water-based ingredients in mayonnaise, binding them together so the sauce is stable. There are lots of other emulsifiers too, including mustard, and there are even other common sources of lecithin, such as soy. But generally it’s egg yolks that play this role in the kitchen.
What does this have to do with avocados? If avocados are anything like eggs, you would expect them to have some emulsifying powers, like eggs. I think what made me suspect this was this Alton Brown recipe, in which he uses avocados instead of eggs to make ice cream. Eggs play a critical role in ice cream, which is to emulsify the fat and water present in the ingredients. How can you do away with them unless avocados also play this role?
It turns out that the fat in avocados is already emulsified within the fruit. This is the reason you can handle avocado flesh without getting your hands greasy. So, as far as I can tell, what you’re doing by adding avocado to ice cream is a little like adding butter (but much tastier and less weird), which shouldn’t really be a problem. And indeed, it turns out that avocados are a pretty weak emulsifer of anything else. You certainly can’t make mayonnaise with ‘em.
There are lots of other wonderful and delicious things you can do with eggs, too, like whipping them into a foam to make meringues, or making a delicious zabaglione or souffle. These are also things that avocados can’t do, though you may be able to find other egg substitutes for these contexts.
So, even though they look similar (at least to me), and poetically I would sort of like the avocado to be a vegan egg, it isn’t.