Many people rely on plastic containers to store leftovers in the refrigerator or freezer with the hope that the tightly-fitting lids will keep the food fresh. But are these containers really airtight?
Many plastic containers are considered airtight. That means their lids fit with a tight seal that prevents air from getting in or out of the container. These types of containers keep food fresh for longer periods of time and are generally leakproof.
There are several different thicknesses and grades of plastic. Some containers made of thin plastic are designed to be used several times and then recycled or discarded. Others are made of a thicker, more durable plastic, such as acrylic, and are designed to be long-lasting.
But it’s not the type of plastic that determines whether the container is airtight—it’s the lid. The firmer the seal of the lid, the less likely air can leak through.
Plastic containers have different kinds of lids, including:
- Press on, like Mainstays food storage set
- Screw on, like Ziploc Twist N Loc
- Push button lid, like Oxo POP containers
- Latching, like Rubbermaid Brilliance
The type of lid does not necessarily indicate air-tightness. Instead, it’s the tightness of the seal, which varies with the quality of the container. Higher-quality lids with gasket-like liners usually result in a sounder seal.
How Can You Tell If A Plastic Container Is Airtight?
The lids of plastic containers differ, with some fitting more tightly than others. But even with a snugly-fitting lid, how do you know if it’s providing an airtight seal?
A good way to determine whether a plastic container is airtight is to test whether it is leakproof. Fill the container with water and put on the lid as firmly as you can. Turn the container upside down over your kitchen sink. If the water doesn’t drip out, it’s both leakproof and considered to be airtight.
That said, molecules of air are obviously smaller than water droplets. To test for true airtightness, you would need a laboratory and scientific testing equipment. For your purposes of keeping food fresh, the leakproof test is sufficient.
There are some other home tests to perform, if you are truly interested.
Cook’s Illustrated website performed several tests that you can replicate to check for airtightness, including the leakproof test. Another involved using Indicating Drierite, moisture-absorbing beads used in packing and passive dehumidifiers such as Damp-Rite. Indicating Drierite is blue but turns pink after it absorbs moisture.
After placing a few crystals of Indicating Drierite into the plastic containers, the testers dipped them into water for a couple of minutes. If the crystals turned pink, it meant that the containers were not leakproof or airtight.
If that sounds like a lot of trouble, you may like the third test Cook’s Illustrated used. They just put a little tuna in the plastic containers and put them in the fridge overnight. The next day, they dumped out the tuna, washed the containers, and sniffed them to see if they could still smell fish.
Chances are, you will find out by trial and error which containers you like the best. However, there are some plastic food storage containers that do advertise that they are leakproof.
Rubbermaid Brilliance is advertised as both leakproof and airtight. These containers are made from a BPA-free plastic that is clear as glass. They have latching lids with a rubber lining that provides the seal.
The OXO Good Grips containers are of a similar design and material. These lids have a silicone seal.
Popit! plastic food storage claims to be 100% airtight. They also have latching lids.
You may conclude that latching lids have tighter seals. This is because they are lined to create a seal with the rim of the container. The press-and-seal type lids that comes with typical (and less expensive) plastic containers don’t have the extra lining.
Do Airtight Containers Work?
Does it make a difference whether you get an airtight plastic container or a simpler design? Will it really help keep your food from spoiling?
Placing your food in an airtight container will keep it fresher longer. Although all food will eventually go bad, keeping the air out prevents oxidation, which accelerates bacterial growth and causes chemical reactions that cause food to deteriorate or go rancid.
When air comes into contact with food, whether it’s raw or cooked, the food may first lose color. Think about a peeled and sliced apple; leave it out for just a few minutes, and the surface of the apple will start to turn brown. That’s the result of oxidation.
The air molecules also react with the food molecules in other ways that affect the flavor and nutritional value. Food containing fat or oil can turn rancid when exposed to air. Air causes the fat to break down and form acids, which in turn causes foul odors.
Storing foods uncovered or without a loose-fitting lid will allow the oxidation process to happen quickly. Therefore, the food will stay fresher longer in an airtight container.
However, even in an airtight container, you cannot expect food to last forever. Some dry ingredients, such as coffee and baking soda, can be stored for up to two years, but most pantry items will need to be discarded after a few months.
Fresh fruits and vegetables won’t last long, even when stored in an airtight container and refrigerated. Hardy vegetables, like carrots and turnips, will stay fresh for a couple of weeks, but most will only be viable for a few days.
As for leftovers, they pose the biggest threat in terms of spoilage because air getting into the storage container is not the only thing you need to worry about. Bacteria have already been allowed to start to grow during the time the food was set out on the table being served. Additionally, there may have been an introduction of enzymes from saliva into the food by someone licking the serving spoon.
Even under optimal conditions, leftovers should be thrown out within 3-4 days after cooking them, or even sooner for some foods, such as sushi. Even if they look and smell okay, there could be harmful bacteria growth. Just remember the old saying: “When in doubt, throw it out.”
Are Ziploc Bags Airtight?
Maybe you’ve noticed that when you close a Ziploc or zipper bag, air sometimes gets trapped inside the bag, making the sides puff out like a balloon. So if the Ziploc zipper seal can hold air in, does that mean it can also keep air out?
The Ziploc brand states that its triple-zipper closure is airtight. Other zipper bags such as Glad Flex’N Seal also claim to be airtight. However, many people have difficulty closing zipper bags properly, so human error can negate the design of the bags.
The zipper closure works much like a clothes zipper: when you slide your fingers from one side to the other on the bag, the edges on either side of the closure interlock together, forming a seal. Under ideal conditions, the seal is airtight.
However, some tests refute the claim. In one study, zipped-up bags were immersed in water for 30- and 60-minute intervals. Most of them leaked at least a little water. If we follow the previous rule of thumb, if the container is not leakproof, it will not be airtight either.
It might be more accurate to call Ziploc bags “air resistant.”
Additionally, many people don’t close the zipper properly or completely, so the bag is neither leakproof nor airtight.
To create an airtight seal, close the bag most of the way, but leave a small opening on one end. But squeezing out the air with your figures is not considered adequate. Instead, put your mouth over the opening and suck out the air; then finish zipping the bag.
If you would prefer not to put your mouth on the bag, you can use a straw.
If you have trouble squeezing the zipper together with your thumb and index finger, lay the bag down on the countertop. Then, using the counter to push against, rub your thumb along the closure. The slider bag is easier for some people to close, especially people who may have arthritis or disabilities. Ziploc also claims its slider bags are airtight or at least air-resistant.