The one thing you should always do before putting leftovers in plastic tubs to avoid hidden chemicals, say food scientists

The pan is still sizzling, the kitchen smells brilliant, your third portion of pasta bake is already earmarked as “tomorrow’s lunch”.

You grab the nearest plastic tub, flip the lid off with your elbow and start spooning in ladlefuls of food that’s still steaming like a sauna. The tub fogs over, the lid bows a little under the heat, you chuck it into the fridge feeling smugly organised.

You’ve probably done this a hundred times without thinking. Most of us have.

What food scientists will quietly tell you, though, is that there’s one small step you should slot in before the leftovers meet the plastic. It takes a few minutes, costs nothing and can drastically cut the amount of hidden chemicals that make their way into your food.

Why your plastic tubs are not as neutral as they look

We tend to think of food containers as just “there”: solid, harmless, a blank stage for whatever we cook. In reality, plastic is a crowded mix of ingredients – base polymers, plasticisers to make it flexible, colourants, stabilisers so it survives the dishwasher, inks on the lid.

Most of these additives don’t stay perfectly locked in place forever. Tiny amounts can migrate into food over time, especially when three factors appear together: heat, fat and acidity. Hot curry, tomato sauces, creamy stews and oily dressings are all ideal solvents for anything loosely held in a plastic wall.

Laboratory tests back this up. Regulatory levels are set very low and most food-contact plastics are approved for normal use, but scientists consistently see higher migration when you add temperatures close to cooking level, prolonged contact and older, scratched containers. The tub might look fine; the chemistry is still happening.

It is not about panic or throwing away every lunch box you own. It is about using plastics as they were intended – and not quietly turning them into makeshift cooking pots.

Food scientists’ rule of thumb: hot food goes in glass or metal, cooled food goes in plastic.

The one step that makes the biggest difference

The habit they would love everyone to adopt is disarmingly basic: let leftovers cool down before they ever touch plastic.

Not “leave them out for hours”. Not “wait until they are cold to the touch”. Just bring them out of the “cooking hot” zone into “comfortably warm” before you portion and lid them up.

In practice, that looks like this:

1. Take the pan off the heat. Stir once or twice to release trapped steam.
   
2. Transfer the food into a shallow dish – a roasting tray, big bowl, glass baking dish or even two plates. The more surface area, the faster it cools.
   
3. Leave it to cool for 20–30 minutes on the counter until it is hot-but-touchable rather than fiercely steaming.
   
4. Then portion into your plastic tubs, lid and refrigerate or freeze.

Within UK food safety guidance, cooked food should go into the fridge within two hours. A short cool-down in a shallow dish helps you hit that window more easily than boxing it up piping hot, where it stays warm for longer in the fridge.

This one step does three things at once:

• Reduces heat-driven chemical migration from plastic into food.
  
• Helps your fridge stay at a safe temperature because it is not fighting a box of near-boiling stew.
  
• Cuts down on condensation on the lid, which bacteria enjoy.

How heat, fat and time quietly change the rules

Temperature is the big trigger. At higher heat, plastic molecules wiggle more, food molecules move faster, and any mobile additives get a helping hand to cross the boundary into that tasty chilli you were planning to eat all week.

Fat is the second player. Oils and creamy sauces are excellent at dissolving hydrophobic compounds – the sort often used in plasticisers and coatings. A tub of cold lettuce is one thing; a pool of still-hot bolognese is quite another.

Time is the slow burn. The longer hot, fatty food sits pressed up against plastic, the more opportunity there is for migration. That is why food scientists get twitchy about three situations in particular:

• Hot leftovers ladled directly into plastic tubs.
  
• Microwaving in any container that is not clearly marked microwave-safe.
  
• Reusing old take-away tubs that are scratched, cloudy or warped.

None of this means a single hot ladle of soup will doom you. It means the everyday habit is worth upgrading, because the chemistry always follows the same pattern – and you are the one in charge of the pattern.

A simple kitchen routine that fits how you actually cook

You do not need lab equipment, special filters or a brand-new set of containers. You need a slightly different order of operations.

Think of it as a mini “reset” for your leftovers:

• After serving dinner, immediately scrape what is left into one or two wide dishes rather than straight into their final tubs.
  
• Set a 20–30 minute timer on your phone. In that time, you can eat, load the dishwasher, wipe the table. The food is cooling without you having to babysit it.
  
• When the timer goes, check the temperature with clean fingers or a spoon. If it is warm rather than scorching, portion into plastic tubs, label and put in the fridge or freezer.
  
• For very large pots of soup or stew, divide across several dishes so it cools evenly; deep volumes hold heat far longer than you think.

Once you have done this a few Sundays in a row after batch cooking, it stops feeling like an extra job and more like the obvious next step. You are not waiting around; you are simply letting physics and time do their work before asking plastic to do its job.

Habit	When	What it changes
Cool in a shallow dish first	Every time you batch cook	Cuts heat-driven chemical migration and speeds safe cooling
Use glass or ceramic for hot food	For oven, grill and boiling-hot leftovers	Keeps plastics in the low-heat zone they were designed for
Retire damaged tubs	When plastic is cloudy, cracked or warped	Reduces leaching from degraded surfaces

Picking the right containers for the right job

Even with a cooling step, some containers are simply better suited to certain tasks.

Food scientists and safety agencies generally recommend:

• Glass, ceramic and stainless steel for anything straight from the oven, hob or grill, and for most reheating. They are inert and shrug off heat.
  
• Plastic tubs for cooled leftovers, packed lunches and freezer storage, as long as they are labelled for food contact.
  
• Microwave-safe only in the microwave. If a plastic tub is not clearly marked as such, transfer food to glass before reheating.

If you like to nerd out on recycling codes on the bottom:

• Codes 1 (PET), 2 (HDPE), 4 (LDPE) and 5 (PP) are commonly used for food containers and are generally considered safer options, especially for cool or room-temperature storage.
  
• Codes 3 (PVC), 6 (PS) and some 7 (other) are best kept away from heat and from fatty, acidic foods unless the manufacturer explicitly states they are food-safe for that use.

Whatever the plastic, warped lids, peeling prints and deep scratches are signs a container has had its day. When the surface is damaged, there is more area for contact and more places for additives to escape. Retiring that old takeaway box is a small, sensible trade-off.

Tiny habits that slash your exposure further

If you want to go beyond the cooling step, a few tweaks have a surprisingly big impact:

• Do not microwave cling film or take-away lids directly against food; use a plate cover or leave a gap so steam can escape.
  
• Avoid pouring boiling soup directly into freezer bags; let it cool first in a pan or jug.
  
• Skip very greasy reheats in plastic when you can; decant pizzas, pastries and roast meats to a plate or tray.
  
• Store oily foods in glass jars – pestos, curry pastes, chilli oils and tomato sauces are all happier in glass.
  
• Hand wash plastic tubs at lower temperatures or on a gentle dishwasher cycle to slow down wear and tear.

None of this needs to be perfect. The aim is to shift dozens of small, repeated exposures into a safer zone with almost no extra effort.

What you gain by cooling before you box

The food on your plate tonight will taste exactly the same whether you let it steam in plastic or cool calmly in a dish first. The difference shows up over the long arc of daily life.

You lower your contact with substances you never chose but have been quietly eating in trace amounts. Your fridge runs a bit more efficiently. Your leftovers cool more safely, with less bacterial risk riding in on pockets of lukewarm stew. And you gain a small, reassuring sense that your kitchen is working with your body rather than making it do extra chemical admin.

The trick is modest: cool first, then plastic. But like weighting a shower curtain so it finally behaves, once you feel the calm of that new routine, it is hard to go back to the old way.

FAQ:

• How long can I safely leave food out to cool before refrigerating it?
  UK food safety advice is that cooked food should go into the fridge within two hours. Using shallow dishes helps it move from very hot to warm quickly, so you can box it up and refrigerate well within that window.
  
• Is it dangerous to ever put hot food in plastic?
  Occasional slips are unlikely to cause dramatic harm, but regularly combining very hot, fatty or acidic food with plastic increases chemical migration. Cooling first is a simple way to bring that risk right down.
  
• Do “BPA-free” tubs solve the problem?
  They remove one specific chemical, but other additives can still migrate under heat. BPA-free is better than nothing, but the cool-first habit and choosing glass or metal for hot food matter more overall.
  
• Can I put warm (not hot) food straight into plastic tubs?
  Yes. Once food is just warm to the touch rather than steaming, the migration rate drops sharply. Aim for that in-between stage: no visible clouds of steam when you stir.
  
• What if I am short on time after cooking?
  Spread food into wider dishes to cool faster, set a 20-minute timer while you eat or tidy up, then portion into tubs. If you are really rushing, cool in glass containers you can pop straight into the fridge, then transfer to plastic later once fully cold.