I used to treat my fridge like a catch-all cold box. After a big Costco run, I’d cram milk, cheese, eggs, produce, and leftovers wherever they fit — cartons in the door, stray apples on the protein shelf, and a tub of yogurt tucked behind the ketchup.

That is, until my 2% milk started turning sour days before the best-by date. I finally asked Bryan Quoc Le, a food scientist, food industry consultant and the author of the book “150 Food Science Questions Answered,” what was going on.

“The door is the warmest part of the fridge because it is the furthest from the fans in the back that blow the cold air into the unit. It warms up the most whenever the door is opened,” he said.

Turns out, the fridge door isn’t just a convenient spot for your go-to items — it’s actually the most temperature-fluctuating zone in the whole appliance. Storing dairy, eggs, leftovers, or anything else sensitive to heat can cut their shelf life short or even make you sick.

My toxic trait? I treat my fridge like a revolving door. Sometimes I swing it open just to stare inside like it’s a window to another world. Other times, I leave it ajar while I cook, ready to grab my milk and cheese without delay.

Even if you’re better behaved than I am, every time you open your fridge, a gust of warm air rushes in. And while the main fridge compartment stays cold and consistent — ideally at 40°F (4°C) or below — the door doesn’t stand a chance of maintaining that low temperature.

Darin Detwiler, a professor of food policy at Northeastern University, explained, “The fridge door is the warmest and most unstable zone because it’s the most exposed to ambient air every time the door opens. Cold air escapes quickly, and warm air rushes in — causing frequent temperature fluctuations, often several degrees higher than the main shelves. While the interior ideally stays below 40°F (4°C), the door can hover several degrees higher and may even cross into the danger zone (above 40°F), especially in hot kitchens or during repeated openings.”

The “danger zone” is a term you’ll hear a lot when talking to food safety experts — and for good reason. It refers to the temperature range where bacteria multiply most rapidly, typically between 40°F and 140°F. According to Le, in a warm kitchen, the fridge door’s temperature can fluctuate anywhere from 5 to 10 degrees higher than the rest of the fridge, landing it squarely in that risky range. That means anything especially perishable stored there is on borrowed time.

Despite those molded compartments that seem tailor-made for eggs, milk and dairy, experts agree: those are the last things that belong in the fridge door.

Le explained, “These foods are highly perishable due to some microbes already being present in them, so an increase of even a few degrees can cause bacteria and fungi to multiply rapidly, which causes spoilage. The risks associated with these can range from wasting food that is visibly bad, to causing food poisoning from Staphylococcus aureus, among other bacteria and fungi.”

Other fridge-door no-gos, according to Detwiler, include:

• Raw meats like chicken, fish or beef
• Yogurt and soft cheeses
• Mayonnaise and dairy-based dressings (yes, Ranch counts)
• Medications that require stable temps, like insulin or certain probiotics

Detwiler explained, “These foods are highly perishable or temperature-sensitive. The danger is that repeated exposure to warmer temperatures allows pathogens like Listeria or Salmonella to multiply. Even small rises in temperature can accelerate spoilage and reduce shelf life, increasing the risk of foodborne illness, especially for children, seniors or immunocompromised individuals.”

Even in the most high-tech, double-doored dream fridges — or the cramped, vintage apartment models — every inch of real estate matters. So what’s actually safe to store in the door? Good news: condiments, jams, pickles and anything loaded with preservatives are fair game.

“A good guideline to use is that if the food can be kept at room temperature, or if it has a large amount of preservatives, then it can be safely kept in the fridge door,” Le said.

So go ahead — fill those door shelves with mustard, soy sauce, jelly and hot sauce. Just don’t banish your milk there and expect it to last.

As Detwiler aptly notes, “Proper fridge organization isn’t just about neatness — it’s about protecting your health.”