burntmayonnaisesandwich
I've been toasting sandwiches with mayo on the outside, and Sir Kensington's Classic Mayonnaise leaves burnt/black stain on the toaster.
Other mayo (kraft…) leaves no black/burnt residue.
Sunflower Oil, Egg Yolks, Water, Lemon Juice, Salt, White Vinegar, Raw Sugar, Ground Mustard Seed, Black Pepper, Citric Acid.
Which ingredient is burning?
Starting from the basics: Mayonnaise, as you know, is a combination of water-based liquids, water-soluble ingredients, and lipids (fats/oils). Since water and lipids are immiscible, that makes mayonnaise an emulsion.
Because the droplets (of fat) suspended in an emulsion are not actually dissolved, the properties of that emulsion depend entirely on the size of those droplets and their dispersion. The most likely reason that your mayonnaise tasted like oil is that it actually was pure oil in spots.
The technical term for this is flocculation.
(source: Cube Cola)
This is probably what happened to you - it's possible that if you had really poor dispersion, you might have even been closer to the "coalescence" stage.
To use a more tangible example, consider what happens when you dissolve flour or corn starch in cold water, then heat it. The starch gelatinizes and you end up with a fairly uniform, thick paste. Now think of what happens if you toss it into hot water; you'll tend to end up with something that isn't uniform, instead you'll end up with big globs of cooked flour floating around in thin, cloudy water.
Keep in mind that the chemistry is completely different with an emulsion - in fact, there technically is no chemistry happening with an emulsion until emulsifiers come into the picture - but the concept is the same. You might not be able to see those globs of oil floating around in the water as well as you can see the globs of flour, but if you didn't get proper dispersion and suspension, they're there, and they will taste exactly how you'd expect a glob of pure oil to taste.
Traditional mayonnaise uses raw egg yolk (containing lecithin) and mustard (containing mucilage), both of which act as emulsifiers. These are called "emulsifiers" mainly because they help the emulsion to stay stable, which is why the store-bought mayonnaise doesn't separate (it also probably has a few extra additives). However, they aren't all that helpful for getting that initial dispersion; the most efficient way to do that is to let small drops of oil into a liquid that is being constantly and uniformly agitated.
You can do this by stirring, but an even better way is to use an immersion blender with emulsifying blade. Note that this is not the flat aerating blade that is often confused with the emulsifying blade, nor is it the star-shaped liquefying blade that is the default on most sticks and many manufacturers confusingly call an "emulsifying" blade. The one you want looks a bit like a hubcap; it's flat with several slits or holes and is sometimes also called a "smoothie blade" or "whisk blade":
or
(the one I'm talking about is the bottom left)
These things are perfect for preparations like mayonnaise, but if you don't have one, you can get halfway decent results with a wire whisk. You'll just need to use a lot of elbow grease.
If you get really good dispersion, and use sufficient emulsifiers such that the emulsion doesn't separate too fast, then I promise you, your mayo won't have that "fatty" taste and it will be 1000 times better than the store-bought goop.
Xanthan gum emulsifies by being a hydrocolloid, so agar and acacia have a chance of also working since they are also hydrocolloids. The amounts would have to be altered as agar sets much more solidly than xanthan gum, and gum acacia sets less solidly.
Lecithin is a commonly used emulsifier in mayonnaise (probably even more so than xanthan gum). It's normally obtained from soy, but sunflower lecithin is also available. Usually a fairly small amount is sufficient. My experience is with soy lecithin, but only a very small amount was needed: about a teaspoon to emulsify a quart of mayo.
Of course,t he most traditional emulsifier is simply egg yolks, which most homemade mayonnaise will already contain. Unless you need to keep it emulsified for really long periods of time, the yolks alone should suffice.
Even without the eggs, you can keep mayo emulsified for shorter periods of time using vegetables. Serious Eats did an article on it.
Best Answer
With the exception of the water and salt, any of those ingredients will burn if you heat them hot enough. If you're seeing it more with a particular brand, though, and if it's happening at temperatures below 200 degrees Celsius, it's probably the egg. Mass-produced commercial mayonnaise like Kraft has relatively little egg, while 'boutique' mayo (or homemade mayo) will have more.