Glass food containers: the lid problem

Many glass food containers are only half plastic-free: the clip-on lid is often polypropylene and the seal is usually silicone. That matters most when the lid is heated or sits against food, so the fix is to keep food away from the lid and never microwave with it on.

You don't need to throw away a good set of glass containers because of the lids. A few habits remove most of the contact, and when lids wear out you can replace them with better options.

Why is the lid the weak point?

The glass box does the hard work: you can heat in it, store acidic food in it and put it in the oven if it's oven-safe. The lid has a different job. It has to clip on, flex and seal, which is why makers usually turn to plastic for the lid and silicone for the seal.

Under our definition of plastic we judge a product part by part. A glass dish with a plastic lid has plastic in a food-contact part as soon as food touches the lid, and silicone counts as plastic too, though we always name it separately. Before you buy, check what the lid, the seal and any vent are made of. The listing should say.

Does a plastic lid matter if it barely touches the food?

Contact and heat are what drive release from plastic. The clearest evidence comes from containers rather than lids, but the same material is involved:

• Heat is the big one. Hussain et al. (2023) found that microwaving polypropylene containers released far more micro- and nanoplastic particles into food simulants than other uses. Some containers released millions of microplastics and billions of nanoplastics from a single square centimetre in three minutes.
• Time matters too. The same study found that storing liquids in the containers for over six months, in the fridge or at room temperature, also released particles, though less than microwaving.

A lid that sits above cold food in the fridge is a much smaller contact than a tub being microwaved. A lid that is heated, or that soup and sauces slosh against for days, is a bigger one. The aim is to keep your lid in the first group. For more on heating, see can you microwave plastic containers?

How can you keep food away from the lid?

• Take the lid off before microwaving. Cover the dish with an upturned plate or a second glass dish instead.
• Leave a gap at the top. Don't fill containers to the brim, so liquid doesn't sit against the lid.
• Let hot food cool before you put the lid on. You'll put less heat into the plastic, and less condensation will collect on the lid and drip back.
• Store containers upright. Stacking them on their sides puts sauces in contact with the seal.
• Replace lids that are cracked, warped or cloudy. Worn plastic is harder to clean and no longer seals properly.

What are the alternatives to a plastic lid?

Glass lids. Some sets come with a glass lid and a silicone ring. That cuts most of the plastic contact, but the seal is still silicone.

Stainless steel lids. Some containers use a steel lid with a seal, which is also often silicone. Steel is durable and won't crack, but you can't see through it.

Wooden or bamboo lids. These look plastic-free but often rely on a silicone ring to hold them in place. They also need gentle hand-washing.

A plate, saucer or upturned bowl. For short-term fridge storage this is the simplest plastic-free lid there is. It won't seal, so it's not for carrying food or for raw meat.

Glass jars. Good for dry goods and freezer portions, as in freezer bags → jars. Check what the seal on the lid is made of.

What about silicone seals?

Silicone is more heat-stable than most plastics, and plenty of people are comfortable with it. It isn't inert, though. Fromme et al. (2019) baked cakes in new silicone moulds and found siloxanes in the cakes and in the air. Air levels fell substantially once a mould had been used more than once. The authors advise cleaning silicone before first use and sticking to the maker's temperature limits.

A baking mould in a hot oven is very different from a seal on a fridge container, and we haven't found studies that measure release from storage seals specifically. The practical advice still carries over: wash new seals before use, don't heat them beyond what they're rated for, and keep food from pressing against them.

When do you actually need a sealed lid?

Sometimes a proper seal is the safe choice. The Food Standards Agency advises that containers used for raw foods should have a sealable lid to avoid leaks and the spread of bacteria, and that you keep separate, clearly labelled containers for raw meat, fresh produce and ready-to-eat food. Food safety comes first. A sealed lid on raw chicken at the bottom of the fridge is a better trade than a loose plate.

Wash containers and lids thoroughly in warm soapy water, or in the dishwasher, and dry them before reuse.

The short version

• Keep the glass, and treat the lid as the part to manage.
• Never microwave with the plastic lid on.
• Leave headspace and let food cool before lidding.
• Use a plate for quick fridge storage, and a sealed lid for raw meat and anything you carry.
• When lids wear out, look for glass or steel replacements and check the seal material.

For why glass is worth it in the first place, see the swap plastic tubs → glass, and for wraps, cling film → lids. More ideas are on the food storage hub and in our plastic-free food storage guide.