When you put food into an airtight anything, it's still in normal air. There's no magic happening, it's not like the food inside is in an airless vacuum. It's in the same kind of air as if it were uncovered, there's just no exchange of air.

Some food is packaged in "modified" air, with more or less oxygen and sometimes CO2 or nitrogen, as appropriate, to keep it safely fresher longer. There's no way to retain freshness of open packages without a vacuum pack and even doing that has limited effect.

Elmer, good point! :)

I think the best thing to get is those machines that seal the bags. I have one from Costco. Tupperware makes a decent "air tight" but it's not without air. If that makes sense. The bags take out most of the air and seals them well. I use them when I buy meat from Costco. Tupperware doesn't work nearly as well.

When you close the lid of those containers, there is air inside at atmospheric pressure. If the lid is truly 'air tight', no new air can get in, however, any microbes present in the top of jar or any within the contents will be in there. If the lid seals, it may keep the contents from drying out, and I suspect that is the main value. It will not protect from anaerobic microbes trapped inside the container since these dudes do not need oxygen to do their deed.

Recall how the old fashioned 'hot pack' canning worked. The lid was loosely put on the jar and the contents cooked and brought to a boil. The boiling jar was taken out of the cooker and the lid tightened when the boiling stopped. In this process, there is very little to no air in the jar for it is filled with steam (water vapor). On cooling, the vapor condenses creating a vacuum in the jar. After stabilizing, the absolute pressure in the jar is the vapor pressure of the liquid at the liquid's temperature. Those snap-on lidded plastic containers can not come close to the seal, killing of microbes, and exclusion of air of the hot pack process.

Timing is critical for the hot pack process; Screw the lid on too soon while it is boiling and you run the risk of exploding the jar. (My Mom did this once.) If you wait too long (while cooling) air can be drawn back into the jar as the steam condenses and the vacuum will be reduced.

Only vacuum sealed bags are air-tight. All the snapping, burping and squeezing does nothing in plastic containers.

Some of our milk comes in bags, three of them to make four leitres, slightly over a U.S. gallon.

Sometimes I put some veggies just after cooking, or other things, into one that's been rinsed and washed, collect the top part of the bag with my hand and suck out as much of the air as I can, then quickly twist it and install one of the plastic retainers from a bread or milk bag. It doesn't get all of the air out, but most of it, and the material is still quite uncontaminated after having just cooled slightly after the cooking and it seems to keep quite well for a day or two, or even a few, in the fridge or for an exte4nded period in the freezer.

ole joyful

I love Tupperware, it is expensive but they are airtight and will last a lifetime.

Tupperware certainly does seal tightly and are quite rugged and I like it for that. It is great for dry foods to keep moisture out in humid weather. It does stop food from drying out like food that is lightly covered. I have a few vintage canisters that I still use but they are not worth the high price as far as I am concerned. I will not be buying any new ones. Any plastic container that seals well will do the exact same job such as Ziploc or Rubbermaid containers.

Sadly, none of them can delay the breakdown of foods as they age.