Egg nog seems like one of those things that should be slaying people by the score, just like egg salad sickens people around the 4th of July. But it doesn't and that's curious since sugar plus raw egg is pretty much the perfect environment to breed intestine-destroying bacteria.
Presumably it's the alcohol in the egg nog that keeps the bacteria from growing but questions remain. Namely, at what point does the alcohol in the egg nog murder the bacteria in a contaminated batch of egg nog? Thankfully, the fine people over at NPR's Science Friday got in touch with just the right people to put this to rest: microbiologists.
The microbiologists in question come from Rockefeller University in New York and they actually have their own recipe which has been handed down through generations of researchers. The originator of that recipe is Dr. Rebecca Lancefield, who was instrumental in the classification of streptococcus, bacteria which you may be more familiar with as simply "strep throat." In other words, the woman knew a thing or two about bacteria.
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Now, their experiment is interesting because they actually intentionally infected the egg nog with Salmonella bacteria. By removing and growing samples of the egg nog as time passes they can show exactly when the bacteria is no longer active in the egg nog. An additional bonus of this method is that it should detect even small amounts of bacteria still living in the egg nog.
How alcohol kills bacteria is fairly interesting as well. The alcohol infiltrates the "skin" of the bacteria and makes it easier for fluid to flood into the bacteria. This brings more alcohol into the bacteria which accelerates this process until the bacteria pops like a balloon. Even if there isn't enough alcohol to cause the bacteria to pop, alcohol inside of the bacteria damages the cellular machinery to the point where the bacteria simply dies.
But the take home lesson is this: Dump a lot of booze into your egg nog and let it "mature" for at least three weeks if you want to be sure it's safe to drink.