A better method of producing worm castings involves utilizing buckets or large boxes. To make castings on a smaller scale, buckets are often utilized, while larger scale operations use rectangular boxes. For this method of producing castings, real earthworms are often utilized instead of red wigglers.
Regardless of the container, the contents are filled with the bedding for the worms (the dirt) and the food source. Since worms will eat a lot of different things, this food source can vary from one operation to the next. In some cases, the food source will even vary from one batch to the next because of availability issues.
After a period of time, the worms will consume the organic matter and most of the bedding and produce large amounts of castings. At that point, the producer then takes the product and packages it up. This results in a mixture of materials that is not pure earthworm castings, although it does have them in the product. The downside of this approach is that you don't know if you're getting 80%, 50% or less in castings. The rest could just be dirt, rocks, leftover food for the worms, and worm cocoons.