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More about Mayonnaise from Wikipedia
Homemade mayonnaise can approach 85% fat before the emulsion breaks down; commercial mayonnaises are more typically 70-80% fat. "Low fat" mayonnaise products contain starches, cellulose gel, or other ingredients to simulate the texture of real mayonnaise.
Homemade mayonnaise can also be made using raw egg whites, with no yolks at all, at least if it is done at high speed in a food processor. The resulting texture appears to be the same, and – if properly seasoned with salt, pepper, mustard, lemon juice, vinegar, and a little paprika – it tastes similar to traditional mayonnaise made with egg yolks.
Since homemade mayonnaise contains raw egg yolks, it subjects the consumer to the small risk of infection with Salmonella enteriditis (the risks of infection from using eggs in the USA is detailed in). Commercial producers either pasteurize the yolks, freeze them and substitute water for most of their liquid, or use other emulsifiers. At home, be sure to use the freshest eggs possible. Some stores sell pasteurized eggs for home use. The eggs can also be coddled in 170°F water, after which the hot yolks, now slightly cooked, are removed from the whites. Homemade mayonnaise will only keep under refrigeration for three to four days. A lower-fat version can be made with silken tofu.