Sources of vegetable oil that can be used for biodiesel include rapeseed (canola), soybean and even waste vegetable oil, such as frying oils or trap grease from restaurants, as well as animal fats such as tallow or lard. Of utmost concern to kashrus organizations and the kosher consumer is how biodiesel glycerin, the byproduct of the oil split, will impact the kosher marketplace.
Glycerin is an extremely versatile natural food substance that has a myriad of food and pharmaceutical applications. Glycerin has a sweet sugary taste with a syrupy consistency and is used in liquid medication, salad dressing and candies. As we have seen, it can be produced from animal and vegetable sources, as well as synthetically. If biodiesel production will skyrocket, and a natural resource of raw material will be spent restaurant oil, vegetable glycerin will have to be even more carefully monitored.
Moreover, of greater kashrus consequence is the potential introduction of naturally produced biodiesel propylene glycol in the marketplace. Propylene glycol has always been assumed to be produced synthetically for commercial use and has been accepted as a kosher ingredient. Propylene glycol has a myriad of uses, as well as endless non-food applications. Although the kashrus status of propylene glycol has never been questioned, propylene glycol can also be produced from glycerin. As biodiesel production grows, thus making the production of propylene glycol from glycerin more economical, this status quo is subject to change. Judging by the billions of pounds of spent french fry oil waiting to be recycled, it is a good bet that in the future we will not be able to make any assumptions regarding the kashrus of propylene glycol.