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In Food Science, Dave McCowan from the University Of Chicago’s Department Of Physics answers our confounding questions about the mysterious world of food.

So what’s the difference between butter and margarine?

Butter is essentially concentrated milk fat. When fresh milk is left to sit, it separates into two liquids: skim milk (low in fat) and cream (high in fat). If this cream is agitated—a process called churning—the protein globules that encapsulate the fat will rupture and the free fat molecules will clump into bigger blobs. When strained, kneaded and shaped, this resulting emulsion becomes butter. A lot of what we love about butter comes not just from the fat flavor, but from the luxurious texture that this churning process provides.

Margarine, on the other hand, is an artificial emulsion of a non-dairy fat with water, skim milk, flavors and coloring. The first margarines used animal fats like beef tallow or lard because they were solid and led to a product that could be molded into sticks like butter. Today, most margarine is made from plant oils which have been chemically processed and turned from liquids into solids. Almost any oil blend can be used, from cottonseed—which was popular in the early 20th century—to canola, corn, sunflower, and olive.

The appeal of using alternate fats to make a cheap butter substitute is clear: It takes an astounding 11 quarts of milk to make just one pound of butter! But the reality is that the rich flavor of butter is hard to emulate. Hundreds of chemical flavor compounds are found naturally in butter and many of these provide for interesting secondary flavors when butter is heated. (And speaking of cooking, margarine doesn’t melt the same way as butter and falls apart when used for baking.)

Taste aside, though, some of the most important differences between butter and margarine are health-related. Not all fats are created equal.

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Plant-based oils are rich in unsaturated fats which are part of a good diet. They are considered essential (as in, we need to consume some to live) and they reduce the levels of “bad” (LDL) cholesterol in our bodies. Their chemical structure, though, means that they are liquid at room temperature (which is why we call it olive oil instead of olive fat).

Animal-based fats, on the other hand—like the milk fat in butter—contain more saturated fats, and tend to be solids. The American Heart Association recommends limiting saturated fat intake, linking it to an increased risk of heart disease, and also warns against eating too much dietary cholesterol, an issue with all animal products. But not all doctors agree. There is serious disagreement within the medical community about what impact saturated fats and cholesterol have on heart health, and the understanding of how saturated fats are metabolized in the body is evolving.

One of margarine’s biggest selling points from the 1960s onward was that, being plant-based, it was much lower in saturated fats and cholesterol. But in the 199os, food scientists realized that the vegetable oils in margarine were not equivalent to their liquid counterparts. The process that firms up oils when making margarine is called partial-hydrogenation, and we know today that this turns healthy unsaturated fat into very unhealthy trans fats. Though only slightly different in molecular shape, trans fats actually increase “bad” cholesterol levels and lower “good” (HDL) levels. The FDA has ruled that no amount of trans fats in your diet is considered safe and called for companies to remove trans fats from processed foods beginning in 2015.

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Today’s margarines are changing by using healthier fully-hydrogenated oils which are trans fat-free, but higher in saturated fat. Others are moving towards lower-fat spreads made with liquid oils, or using naturally semi-solid oils like coconut and palm.

And then there’s Country Crock. The company tried to move to a simpler recipe that emphasized natural ingredients—without trans fat or artificial preservatives—and the response was... not so good. From claims that “You have ruined waffles” to comparisons with actual garbage, customers proved that change is hard.