Since this is a car blog, let's define kosher real quick. According to Star K, kosher is use to designate "foods whose ingredients and manufacturing procedures comply with Jewish dietary laws." What are those laws? Well, there's a rabbi for that.
In any case, this story is about glycerin, a byproduct of biodiesel production. The amount of glycerin available in the U.S. market rose dramatically in the last few years as biodiesel production increased. To make biodiesel, you to separate a fat into methyl esters, the part used to make biodiesel once things like methanol are added, and glycerin. The key problem for the rabbis is that a lot of that glycerin found its way into the food chain (glycerin is a sweet and natural food substance that can be used in a lot of food products) and, since you can make biodiesel from either animal or vegetable fats, knowing whether food made from that glycerin meant you needed to know the source of the glycerin.
This is not easy to figure out, especially if the biodiesel was made from waste restaurant fats – one important question: "was the oil used to fry pork?" – or if the glycerin is a blend of many biodiesel production streams. Over time, the rabbis have worked with biodiesel production facilities to track the fat source in order to verify just what is in the glycerin and, thus, can confidently say whether the resulting foods are kosher or not. Whew.
[Source: Star K via Environment Report | Image: Mike Licht, NotionsCapital.com - C.C. License 2.0]