First question: Mayonnaise is primarily comprised of eggs and oil. Therefore, can mayonnaise have salmonella? Assume it can not, how does the manufacturing process kill it without cooking the egg?
It turns out that there’s also potentially vinegar and lemon juice (vinegar is an effective antibacterial, in general; you can put it in with your clothes), and this is a factor.
The eggs used in commercial mayo are pasteurized, pretty much eliminating the risk of salmonella, says Thomas Schwarz, an independent food safety consultant. He says that the acids bring the pH of commercial mayo to about 4.2 to 4.5, which “isn’t very inviting to microorganisms,” and the emulsification of the product makes the Aw (water activity, which is a measure of free water available for organisms to grow) “quite low.” The addition of salt also makes mayonnaise a less inviting medium for bacterial growth.
So, there you go.
Second question: Tell us more about “water activity”.
It is actually very interesting. This is the quantity that determines whether your food will lose moistness to the environment or absorb moistness from the environment.
…pure distilled water has a water activity of exactly one. As temperature increases, aw typically increases, except in some products with crystalline salt or sugar.
Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7. See also fermentation.
Water migrates from areas of high aw to areas of low aw. For example, if honey (aw ≈ 0.6) is exposed to humid air (aw ≈ 0.7), the honey absorbs water from the air. If salami (aw ≈ 0.87) is exposed to dry air (aw ≈ 0.5), the salami dries out, which could preserve it or spoil it.
Pictures I ran across while looking for the featured image: