Plastic Food Storage Bags

From Season 6: American Classics

A stroll down the plastic-bag aisle reveals an array of brightly colored boxes beckoning shoppers with breathless claims. And if that wasn't dizzying enough, manufacturers have divided their product lines into bags for the freezer and bags for general storage. But how meaningful are these fancy-sounding features and designations?

Storage bags have three core functions: keeping moisture in and keeping air and odor out. The first line of defense is the plastic film itself. So we sent the bags to a lab that uses a specialized machine to measure how fast moisture travels through a material—called the water vapor transmission rate, or porosity. The lower (or slower) the rate, the greater the protection.

The lab results arrived, and we made three important discoveries. First, porosity did vary significantly. The plastic film in the old-fashioned baggie had let through nearly six times more moisture than the next least porous bag. Second, forget all those other fancy-sounding features. A plastic bag's relative moisture protection springs from one thing and one thing alone: the thickness of the film. (In fact, all the bags were made from the exact same material—polyethylene.) The thicker the plastic, the better its moisture protection. Most bags ranged between 45 and 72 micrometers thick, which correlated inversely with porosity.

Finally, the results suggested that the freezer bag/storage bag divide is little more than marketing. In general, freezer bags are thicker, but only 5 micrometers separated our thickest storage bag from our thinnest freezer bag—a suspiciously arbitrary difference.

If the plastic film were the only route for moisture, air, and odor, testing would be done; the winner would simply be the thickest bag. But moisture can also escape past the seal. To find out which bags were airtight, we placed five water-filled bags of each type in an airtight container with a half-dozen desiccant packs (those moisture-absorbing packets you find in boxes of new shoes), which we weighed before closing the containers. A week later, we reweighed the packets to see how much moisture they had absorbed. In general, bags with zipper seals proved more airtight than those with slider seals. The problem? As the mechanism slides down the length of the bag's opening, the seal it creates along the way is essentially the same as the regular zipper seal. But the part of the opening directly beneath where the slider tab comes to rest is sealed only by its contact with the tab—and most weren't designed well enough to fit precisely.

It was time to test how bag thickness and seal effectiveness worked in tandem in real applications. First, to evaluate odor protection, we placed bread-packed bags in airtight containers, along with freshly cut onions and garlic. The results were clear: the bags with airtight seals were the odor-protection champs-regardless of film thickness. To test seal security, we divided 15 gallons of spaghetti sauce among 20 bags (two samples each), then sealed the bags to find which was strong enough to survive an accidental plummet from the counter to the floor.

The results of all our tests? Four bags emerged with recommended status.