Yes, there is a major difference between the calcium content of hulled versus unhulled sesame seeds. When the hulls remain on the seeds, one tablespoon of sesame seeds will contains about 88 milligrams of calcium. When the hulls are removed, this same tablespoon will contain about 5-10 milligrams (about 90-95% less). Tahini—a spreadable paste made from ground sesame seeds—is usually made from hulled seeds (seeds with the hulls removed, called kernels), and so it will usually contain this lower amount of calcium. The term "sesame butter" can sometimes refer to tahini made from sesame seed kernels, or it can also be used to mean a seed paste made from whole sesame seeds, hull included.
Although the hulls provide approximately 80 milligrams of calcium per tablespoon of seeds, the calcium found in the hulls appears in large part to be found in the form of calcium oxalate. This form of calcium is different than the form found in the kernels, and it is a much less absorbable form of calcium. So even though a person would be likely to get much more calcium from sesame seeds or sesame seed butter (tahini) that contained the hulls, there is a question about how much more calcium would actually be usable. And there would also, of course, be a question about the place of hull-containing sesame seeds in an oxalate-restricted diet.
I believe that sesame seeds, tahini, and sesame butter are healthy foods to include in a Healthiest Way of Eating regardless of their hull status (hulled or unhulled). From a practical standpoint, you're not likely to find tahini in a neighborhood grocery that has been made from unhulled seeds. But you may be able to find a product labeled "sesame butter," especially in a food store specializing in Middle Eastern foods. If this sesame butter has a much darker appearance than the tahini you're accustomed to seeing, and if the butter has a more bitter taste, it is very likely to have been made from whole, unhulled sesame seeds.
As for purchasing unhulled sesame butter online, I've been discouraged from the calls I've made to large online vendors. Most product labels don't seem to specify the hull status. Your best bet here is to make some calls to stores in your local area to find someone familiar with these issues, and to special order an unhulled product if you decide to go in that direction.
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