I found a chart that compares lots of bird eggs. I found that duck eggs have more protein but also more fat that chicken eggs, because ducks are all about oil for life in water. Mayonnaise is the egg protein holding the added oil in what is called an emulsion. The duck eggs have more fat to start, so they can take in less oil. Mystery solved!
So here is my recipe of mayonnaise adjusted for duck eggs. Mayonnaise usually calls for mustard, but since I am allergic to mustard and duck eggs are so flavorful, I left it out.
Duck Egg Mayonnaise
4 duck egg yolks
2 tablespoons lemon juice or about the juice of half a lemon
2 teaspoons onion salt
3/4 cup light olive oil
Place egg yolks in a blender cup with lemon juice and onion salt. Begin blending on medium speed for one minute to break up proteins. Remove the center of the blender cap if possible and dribble the olive oil drip by tiny drip until the mixture lightens and thickens. Increase adding olive oil to a very thin stream until it is completely added. The mixture will be thick and smooth. Adding more oil to thin or mayonnaise may cause the emulsion to break, becoming runny.
This mayonnaise is really great with sweet pickled peppers in duck egg salad!
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