Why your cookies and candy can sit out for days (but milk can’t)
Why can cookies and candy sit out while milk goes bad? It’s all about water activity: sugar and baking remove or bind free water so microbes can't grow. Read quick tips to store treats and know when to refrigerate.
Why can you leave cookies and candy on the counter for days without them going slimy or smelling bad, while milk turns sour in a couple of hours? In short: it’s mostly about water.
What’s really going on
Microbes — bacteria and molds — need three big things to grow: food, time, and water. Cookies and many candies have plenty of food (sugar, fat, carbs), but they usually don’t have enough available water for microbes to grow. That’s the key.
How sugar and dryness help
- Low water activity: Baked goods and hard candies have low “water activity” (not the same as how wet they look). Microbes need free water to metabolize and divide. Remove that water and they can’t thrive. (See: water activity on Wikipedia.)
- Osmosis: High concentrations of sugar (or salt) pull water out of microbial cells. It’s like dehydrating them — they can’t function and they die or stop growing.
- Baking removes moisture: Cookies are cooked at high heat, which drives off a lot of the water. Fat and sugar then help keep them stable at room temp.
- FAT TOM factors: Food, Acidity, Time, Temperature, Oxygen, Moisture — remove moisture and you remove a major growth factor.
So are cookies totally safe forever?
Not exactly. If cookies get damp (high humidity, steam, or if they’re packed with fruit/cream/frosting), water activity goes up and microbes can grow. Mold loves soft, moist baked goods. Also, uncovered treats attract bugs and they’ll go stale faster.
Practical tips
- Keep dry cookies and hard candies in an airtight container to avoid humidity and pests.
- Refrigerate or discard items with dairy, custard, fresh fruit, or cream fillings — they spoil faster.
- If you see mold or smell off odors, toss the item — don’t try to cut mold off most soft baked goods.
Bottom line: Sugar and dryness are the secret preservatives. They starve microbes of the water they need, which is why cookies and many candies can lounge on the counter while milk needs a fridge.
