Making your own eggnog is an achievement. But thinking about it far enough in advance to age it (and then keeping your hands off it while you’re waiting) is probably harder. It’s worth it. Says Stanford University Writing and Rhetoric lecturer Jonathan Hunt, whose family has been making aged eggnog for three generations, “It’s like a green banana versus a just-ripe one.”

In the Hunt family recipe, you’ll notice that just after mixing, you can taste the sugar, cream, and bourbon as distinctly different flavors. But after three weeks in the back of the fridge, the mixture takes on a golden hue and thickens, and the flavors meld into eggnog.

How does it happen? Newly mixed eggnog is a soup of sugars, proteins, enzymes, alcohol, and other organic compounds. When you age it, several things occur: The volatile chemicals in the spirits that are responsible for their flavor and smell react with the sugars and proteins, causing the flavors to blend, says Barbara Ingham, food science professor at the University of Wisconsin-Madison. The alcohol denatures some of the egg and dairy proteins, causing the change in color and mouthfeel, says Sam Beattie, a food scientist at Iowa State University. Also, the proteins in the egg yolk gelatinize in solution, thickening the mixture.

Aging beyond the three weeks called for in our recipe Best Eggnog (Hunt recommends aging up to a year) continues to cause changes, because enzymes in the egg yolks work very slowly, catalyzing reactions between proteins and sugars, says Tom Szalkucki, assistant director of UW’s Center for Dairy Research. The longer it ages, the mellower it becomes.

But doesn’t leaving a dozen uncooked egg yolks and a quart of milk lying around for three weeks to a year breed salmonella? Probably not, says Beattie, thanks to the booze. To kill bacteria living on solid surfaces such as human skin, scientists recommend a 60 to 80 percent alcohol concentration (the amount in hand-sanitizing gel). That’s stronger than most straight vodkas. But to kill bacteria in liquids, it’s not so clear-cut. The FDA doesn’t recommend consuming unpasteurized raw eggs under any circumstances, but Beattie says that in wine, concentrations of alcohol as low as 8 percent are high enough to kill problematic bacteria that may be present. There haven’t been any similar studies done on alcohol in eggs or milk, but our recipe is more than 20 percent alcohol.

Just be sure to keep your jug at or below 40°F for the entire aging period. This means the fridge, not the garage, where daytime sun can cause wild temperature fluctuations even in very cold climates. During the aging process, throw the batch out if it starts to bubble, undergoes a rapid or dramatic color change, or gets a sour or sulphury smell, Ingham says. And don’t use less alcohol than our recipe calls for! If you want virgin nog, you’ll have to use pasteurized eggs, milk, and cream, and make sure your mixing bowl and storage container are very clean.

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