There are two versions of the Grape-Nuts name’s origin. One is that Post thought that what he referred to as “grape sugar” (glucose) was created when the product was baked. Since the product has a nutty flavor, he is said to have combined the two into “grape nuts.” The second version is that the product resembles grape seeds, or grape “nuts.”[9]
https://en.wikipedia.org/wiki/Grape-Nuts