Glass peanut butter jars are another example of the way “natural” living and “environmental” living diverge. (Bigstock)

If you’ve been in a supermarket lately, you’ve probably seen shelves full of “natural” peanut butter, which usually contains just peanuts and salt. The name distinguishes it from the peanut butter many of us grew up with, which you might justly call “unnatural” since it contains a host of wild-card ingredients such as hydrogenated rapeseed oil and mono- and diglycerides.

The proliferation of “natural” peanut butter is a welcome change, but there’s one thing that perplexed me about these new natural peanut butters: Why do so many of them come in glass jars rather than plastic ones? Does the glass jar offer a health or environmental benefit over a plastic one? The answer, in short: No.

Glass peanut butter jars are another example of the way “natural” living and “environmental” living diverge. Other examples include industrial pet foods, which are better for the environment than “natural” alternatives, and canned fruits and vegetables, which are, in some cases, easier on the Earth than fresh produce.

As the small-scale Vermont Peanut Butter Co. writes on its Web site in defense of its plastic tubs: “Glass is almost 1 lb heavier, which increases shipping costs and requires more gas and oil to transport. If you practice recycling, the plastic jar is a greener way to go.”

The science supports this position. A 2008 comparison of glass and plastic baby food jars, for example, found that the glass jars produce between a quarter and a third more greenhouse gases than plastic jars. In fact, the plastic jars bested the glass in almost every category: They released fewer carcinogens into the air, sent fewer pollutants into waterways and required less acreage. A year later, an assessment of soft drink containers also found plastic superior to glass. (The study was funded by a plastics manufacturer, but the company that conducted the review is independent and widely respected.) The results were broadly similar to the baby food study, with glass producing approximately four times as much greenhouse gas as plastic.

The problem, as Vermont Peanut Butter points out, is weight. Glass is far heavier than plastic. Two years ago, peanut giant Planters cut packaging weight by 84 percent by switching from glass to plastic jars. That affects most aspects of an environmental assessment. Lighter products require fewer raw materials, which means they take less energy to make and usually produce a smaller carbon footprint. They take less energy to transport to the consumer, and it’s easier to dispose of them when the peanut butter, baby food or soda runs out.

There are always caveats with environmental assessments, which have to make many assumptions and generalizations. Since plastic’s advantage has so much to do with weight, you could tweak the assumptions in favor of glass by changing transportation distances. Missouri-based East Wind Nut Butters, for example, buys its glass from a manufacturer in nearby Arkansas, cutting down the distance the jars travel before being filled. If the product is distributed locally and if customers walk to the grocery store, glass may be environmentally superior. It would also help if buyers reused glass jars rather than having them hauled to a recycling facility or — for shame — a landfill. There are also factors beyond a consumer’s control or knowledge, such as whether the jarmaker has access to renewable energy.

I try to stay focused on the environment and off human health in this column, but I’ll relax the rules in this case because the health aspects are keeping glass on the shelves. First, plastic peanut butter jars are typically made of polyethylene terephthalate (PET), which does not contain bisphenol-A (BPA), a chemical that has raised concerns in other plastic products. Animal studies suggest that PET doesn’t leak any hormonal disruptors into food products, either.

The most valid concern with plastic is the leaching of antimony, a possible carcinogen. Under normal circumstances, this process happens extremely slowly — so slowly that it shouldn’t worry you in the slightest. A 2013 study in Europe found that the rate at which the chemical migrated into food was around a fortieth of the legal maximum in the European Union. But leaching seems to accelerate at extremely high temperatures. If you store a PET bottle of water at 185 degrees, for example, the water will become legally contaminated with antimony in 32 hours. Those are rather extreme storage conditions. If you keep your peanut butter at ordinary temperatures for a reasonable period of time, there is absolutely no reason you should fear plastic jars.

There may be a better option than either plastic or glass jars: tubes. There have been few ecological assessments of food tubes, but a 2007 study on Tom’s of Maine toothpaste containers suggested that tubes can be an environmentally friendly choice. We don’t have enough tube food in the United States, in my opinion. Tomato paste is one of the few products that come tubed, and it’s a huge improvement over cans. I’ll take tubed peanut butter over jarred any day. Hold the diglycerides.