The Oleomargarine Act of 1886:

In 1867, a French food research chemist, Hippolyte Mege-Mouries, discovers a process for the manufacture of artificial butter. The dairy industry, fearful that oleomargarine might be substituted for butter without the consumers’ knowledge and wishing to protect its market share, appeal to Congress for help. The result is passage of the Oleomargarine Act of August 2, 1886. Among other things, the Act directs the Secretary of the Treasury to appoint an analytical chemist and a microscopist, both of whom are subsequently authorized to examine samples of butter for adulteration with oleomargarine for appropriate taxation.

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Is it Butter? .... or is it Parkay?

Promise Margarine commercial.

Beam Me Up, Scotty!

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I'm only going by memory, but I grew up in the 60s/70s in Wisconsin, the dairy state. Two of my uncles were dairy farmers and I spent a lot of time working for them as their sons were older and were off to other pursuits.
I recall that yellow oleo margarin was becoming a big topic. Wisconsin at some time passed a law that it could not be colored yellow like butter. It was white. If you wanted yellow oleomargarine in Wisconsin at that time you had to smuggle it across the Illinois border.
I looked this up today and the law was passed in 1895. There were fines to be paid if caught smuggling yellow oleomargarine into Wisconsin. The Illinois border was called "the yellow curtain" by The American Soybean association. The law lasted until 1966. I recall my Dad back in the 60's getting upset about the law being dropped. He called oleomargarine "yellow Crisco." It was not allowed in our house.
Update: I crossed the "yellow curtain" in 1982 to work at a farm tire factory. Down here they call the Wisconsin border the
"cheese curtain". Some months ago I asked my wife to start buying whipped butter in those convenient tubs like margarine. My cholestrol level is where its supposed to be, even at my weight. And carrying milk in pails to the milkhouse did me no harm either, but I have no neck. ;D

Wow, Cheese Heads and Oleo Smugglers .... guess I'm happy to just be a Troll.

It all sounds a bit corny (and soy beany, too). But I swear, its all true.