Rice grain types. Rice, a staple food of much of the world's population, consists of seeds of the rice plant (Oryza sativa). The seeds change in appearance depending on how they are processed. There are two layers around the seeds, the aleurone layer (brown), and a dry husk (pericarp, black). These layers, which are removed by milling and polishing, are seen in the rice here, ranging from unmilled rice (black), through partially milled rice (brown), to polished rice (white). Milling and polishing changes the taste and degrades the nutritional value of the rice by removing these two outer layers where most of the vitamins and protein of the original seed are found.