When margarine was invented in 1870 by the Frenchman Hippolyte Meg–Mouries, it was made by churning ox fat with cream. Today margarine is made from several blends of fats and oils, liquids (water/and or milk), emulsifiers, preservatives, artificial flavors and colors, vitamins, and salt. Each one of these ingredients can be kosher or non-kosher. In Europe, in recent years, three of the six most common blends of fats included whale oil or – animal fat (which, of course is not kosher), and the other three blends often contain some animal fat. (Source: Magnus Pyke, Food Science-and-Technology. Pyke’s table – 20 reads as follows: 

Six Blends of Fats for Margarine Manufacture

                                    Percent        Percent 

Coconut oil 50-60 Palm kernel oil                 50 

Hydrogenated Hydrogenated palm oil          20 

Vegetable fat                                          20-25 

Oil*                             20-25 Oil              30

Hydrogenated Hydrogenated groundnut oil 70 whale oil                   25 

Coconut oil 10 Coconut oil                          50 

Oil 20 Oil                                                  25

Hydrogenated Beef fat                                  25

whale oil                                                          20 

Palm oil                15 Coconut oil                  35 

Coconut oil        20 Oil                                 40 

Palm kernel oil                                                  20 

Oil                                                                  20 

*This can include clarified animal oils from which the solid stearins have been separated of vegetable oil. 

Since 1979, almost all American margarines have been made from vegetable oils, but beef-fat margarines are still sold. The difference between hard, soft, and liquid margarine depends on the ratio of fats (solids) to oils (liquids). Maintaining the fats and liquids in solution, rather than in layers, requires emulsifiers, which might be of either animal or vegetable origin. Fats and oils, which by law must constitute 80 percent of margarine, must be meticulously checked to verify their origin and to make sure that the equipment on which they were processed was not used to process animal fats and oils. Absolute stringency is required by Jewish law. 

The preservatives, colors, and flavors also require supervision, as explained elsewhere. 

The consumer must also be aware that most margarines contain milk or other dairy products. In general, even kosher margarine cannot be assumed to be pareve unless this is specifically stated on the label.