On not refrigerating fudge

How come you don't have to refrigerate fudge? It has both milk and butter in it...

You can’t see it, but a sinister grin is slowly spreading across my face. With your innocent fudge query, you have surfaced one of the most crucial concepts in food safety. 

First, let’s examine fudge. Fudge is delicious. It is delicious because it is really sweet and really fatty,  as well as kinda salty. For science reasons, water clings tightly to molecules that are sweet, fatty, and salty. When the water content in a food is chemically-bonded like this,  we say that the food has a low water activity. 

Why does low water activity matter? It matters because bacteria is thirstier than a 23-year-old Goldman Sachs associate at an East Village bar. Bacteria needs water to live and it can’t access water that is chemically bonded to something else—such as sugar, fat, or salt. When there’s no available water in a food, then that food becomes inhospitable to bacteria. Low water activity foods are the equivalent of a lesbian night at that East Village bar: the Goldman associate can try all he wants, but he’s not getting in. Sorry, buddy. Private event. 

If you have a food that’s sweet enough (Nutella) or fatty enough (olive oil) or salty enough (soy sauce) or, in the tastiest scenarios, some combination of all three (fudge), then there’s no water available for the bacteria to drink. No available water = no possible bacteria growth.

One last thing. You might be thinking: “Wait a minute, fudge—at least the way my grandmother makes it— is nice and moist. How can such nice moist fudge have a low water activity?” 

I’ll tell you why. Water activity isn’t the same as moisture content. It is possible for a food to be moist but still have a low water activity. Peanut butter is a perfect example. You don’t have to refrigerate that, either.

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