Here is a recipe for homemade mayo that is perfectly safe. The acidification of the yolks is all that is really necessary--but this recipe adds another step in order to pasteurize the yolks and enable the mayo to be used immediately: heat.
Normally, heating eggs yolks to the point where pasteurization occurs cooks them to a point where they won't work for things like fresh mayo and egg-based sauces like Hollandaise or Béarnaise because the proteins in the yolks harden as their temps exceed 150. However, if one acidifies the egg yolks and dilutes it by the addition of water, the temps needed for protein coagulation are raised past the point of pasteurization. The diluted yolks can be effectively pasteurized by gentle heating and this will occur before the proteins harden making it possible to make the mayo both safe and creamy and, because of the acidification and resulting low Ph, one with excellent shelf-life. In fact, the Ph is low enough (3.5) that federal regulations would allow its storage on the counter instead of the fridge(!)--but for best quality and flavor it should be fridged.
This recipe is based on and adapted from the science and methods of Harold McGee and Pete Snyder.
3 egg yolks from large eggs
2 T white wine vinegar
2 T lemon juice
2 T water
2 t ground dry mustard or 1-2 t prepared mustard (I use Dijon)
1/2 t salt
ground white pepper to taste (optional)
up to 3 c oil (see note)
Using the top section of a small double boiler or a small stainless steel bowl (but one large enough to comfortably whisk/stir the mix), combine the egg yolks, white wine vinegar, lemon juice and water.
In the bottom of the double boiler (or in a pot sized to accomodate the small stainless steel bowl), bring a small quantity of water to a simmer (180-190F) and place the egg mixture over the simmering water.
Whisking or stirring the egg mixture gently but constantly, heat the mixture to a temp of 150. (This happens fairly quickly.) It is essential to temp frequently and to use an accurate tip-sensitive thermocouple thermometer. (Do not use a bimetal analog thermometer!) If you wish, tape the therm probe to the whisk to get a constant reading while stirring.
When the egg mixture hits 150, immediately remove the bowl from the bottom of the double boiler or pot and place on the counter to cool to room temp. (The pasteurized mix is stable and can be fridged for several days before continuing. The Ph is 3.5, well below the 4.1 necessary.)
Place the mix in a larger stainless steel bowl and add the dry or prepared mustard, salt and optional white pepper. Using a balloon whisk or an electric hand mixer, whisk the mixture till just combined and then, very slowly, just a t at a time, begin adding the oil. Once the emulsion forms the oil can be added more rapidly but try to keep it no faster than a slow steady stream.
I egg yolk can emulsify 1 c of oil so up to 3 c can be used. Quit the oil addition when the mayo looks right to you. The mayo will become quite thick and, if desired, can be thinned a bit with additional wine vinegar or lemon juice. Taste and adjust seasoning if necessary.
Put the mayo in a sanitized container and cover. As noted above, federal regs state that a mix this acidified and stable need not be fridged but for best quality do so. It should have a shelf-life of 3-5 weeks.
Note: For typical mayo use a neutral-tasting vegetable oil. While olive oil makes a distinctive mayo (and is essential for aïoli and its variations), mayo made with olive oil should be used immediately as chilling it to frisge temp solidifies the olive oil causing the mayo to break.