In the last few decades, there has been a sharp worldwide decline in the use of animal oils. Between 1970 and 1975 the use of animal oils decreased by 39 percent, and the use of marine oil decreased 24 percent. In fact, by 1975, more than two-thirds of the world’s food oil needs were met by vegetable oils. 

These figures should not, however, be construed to indicate corresponding increases and decreases in the United States. In actuality, there has been an increase in lard and tallow (beef fat) use in the U.S. The reasons for this are, first, that it is cheaper, due to a more active consumption in this country of animals as food, with a corresponding increase in the use of animal by-products, and, second, the superior shortening qualities of animal fats. 

The biggest problem for the kosher consumer occurs because oil processors produce both animal and vegetable oils on the same equipment. This is particularly problematic in the case of hydrogenated (hardened) oils. Many of today’s vegetable oils are hydrogenated in order to harden them. The oil is first heated to 250 degrees Fahrenheit in a vacuum (usually in the same equipment used for animal oils). A solid catalyst, which may be non-kosher based, is then added, and hydrogen gas is applied under pressure. 

The exception to this is virgin or extra virgin olive oil, which has been processed using a cold press method. In this way, the oil is never heated and does not come in contact with any non-kosher oils. 

The implications for the kosher consumer are clear. Most vegetable oils and products produced from them, such as margarine and shortening, cannot be relied on as kosher without strict rabbinic supervision.