Spoilage vs. Safety
It's important to understand that food safety and spoilage are not exactly the same thing. Spoilage occurs when micro-organisms, harmful or otherwise, begin to break down a food. This is hard to miss, because it usually changes the appearance of your food and creates unpleasant smells. In the case of mayonnaise, it'll start to smell unpleasant -- like cheese that's been left in the sun -- and will begin to separate, with liquid oozing into the spoon depressions in your jar.
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Mayonnaise and Pathogens
Micro-organisms that can make you seriously ill, collectively known as pathogens, are sneakier. Salmonella, listeria, yersinia, staphylococcus and E. coli leave no visible signs of their presence. You can't really know if they're currently in your mayonnaise jar, but don't panic over the idea of invisible assailants hiding there. Commercial mayonnaise is manufactured with pasteurized eggs and a high level of acidity, so it's resistant to bacteria. In fact, in controlled experiments, pathogens injected directly into mayonnaise have simply died.
Mayonnaise and Basic Food Safety
Unfortunately, this built-in level of safety only lasts until you mix mayonnaise with something else. Then, the combined food is as perishable as its most perishable ingredient. If you've made chicken salad, for example, it's as perishable as the chicken. More worryingly, if your knife or spoon has gone back into the jar, your mayonnaise will contain traces of perishable food. To minimize the risk of illness, follow these simple guidelines:
- Never let your foods sit unrefrigerated for more than two hours, or one hour if the temperature is above 90 degrees Fahrenheit.
- The two-hour total includes preparation time, so keep track of how long your ingredients are on the counter as you work.
- Extend your food's life at picnics and parties by keeping it on ice or in a cooler.
- Buy mayonnaise in a squeeze bottle, rather than the usual wide-mouthed jar, so you don't contaminate it with your knife or spoon.
Homemade mayonnaise is an altogether different thing, and is much riskier. Mayonnaise and its close cousins -- homemade Caesar dressing, for example -- vary widely in their acidity, and are usually made with unpasteurized eggs. Foods containing homemade mayonnaise should be kept refrigerated, and should be eaten on the same day. The two-hour rule applies here too, and any food that's been left out for two hours should be discarded. To make mayonnaise safely at home, buy pasteurized eggs from the refrigerator section of your supermarket.