The reason "microwave popcorn" in a bag typically does not contain actual butter is because butter is expensive and might go rancid. Until now, manufacturers have added some fat or oil, salt, a preservative, and synthetic diacetyl, the latter being the factory-made version of what gives butter its "buttery" taste. In recent years, there have been serious adverse health impacts reported for those workers most exposed to diacetyl while processing microwave popcorn and other buttery flavored foods, opening up the question of whether consumer exposure to diacetyl vapor emanating from a freshly-opened, hot bag presents an unacceptable exposure. (This would be affected by consumer proximity to the bag, a preference to inhale the smells when opening, and frequency of consumption, of course.)ConAgra, the agri-products giant, and Pop Weaver brand are taking diacetyl out of their microwave popcorn, having been shown, by USEPA, the results of a government sponsored study of consumer diacetyl exposure. The entire issue has been laid out in great detail in The Pump Handle blog. We suggest you read the entire entry, here.
A controversy has emerged about why the food processing industry was shown the results of USEPA's exposure study prior to the report being published in a peer reviewed journal. Assuming that the EPA-measured exposures were considered adverse by Agency toxicologists, this "private showing" could be considered prudent, as it offered proactive members of the food industry an opportunity to reformulate as soon as possible. The remainder of the industry, what we might term the "wait until the lawyers say we have to" crowd, won't act until well after the peer reviewed article appears anyway. We're not sure why the controversy, given that ConAgra is phasing out diacetyl, establishing that information is not being "quashed."
For those who prefer, it is indeed easy to make your own microwave popcorn with butter: look here for TreeHugger "how to" advice. Word of caution: brown lunch bags, which are made of "kraft process" paper, may not be suitable for direct food contact.
That "buttery flavor" is not created by diacetyl only. As with the best cheeses, butter flavor is shaped by what kind of cows produce the milk, what they are fed or graze upon, time of year they are allowed to pasture, and the effect of climate on plants in the pasture. Even the age of the cream affects butter flavor. So please, lets not go off on the 'diacetyl is natural' tangent.
For anyone who loves to prepare food at home, here's an inside butter tip from a writer who grew up in Wisconsin, USA.
The strongest flavored natural butters can be described as "grassy". In the US butter grading is done rather subjectively on the combined qualities of texture, sweetness, flavor, color, etc. The grading may range from the near-tasteless, white "AA" to the sometimes grassy-tasting, relatively yellow and inexpensive "D."
The lower grades, of course, are cheaper and likely to be strong flavored, meaning that a little goes a long way, for less money. These "buttery-est" grades are commonly bought up on contract by bakeries and food processing companies. You'll not find low graded, US-made butter in a US market. However, if you live near a real creamery and can ask the manager to find you some strong tasting butter you might be in luck. A little of that high test stuff is fantastic on popcorn or in baked goods.
Note: a low butter grade is not a guarantee of strong taste; and, quality changes through the year, as mentioned earlier. (The grading system is relativistic and based on a combination of qualities.) So, you should tell the dairyman what you want it for - strongly flavored, for cooking.
If anyone knows of a better way to get strong tasting, inexpensive, locally produced butters, please leave a comment.