I use Flax Seed, because it is a flower seed, has no cooked food smell in the air or on your clothes because it is not a food product. My Flax Seed Filled Therapy Bags retain their heat for nearly an hour because the oil in the seed has been heated and the shell traps it in, giving off damp moist heat for a longer period of time. For those sensitive to aromas, my handcrafted flax seed products are unscented. If you wish to have a scent, please just ask ... I have Lavendar freshly grown in my garden, but also offer many other essential oils.
Some offer therapy bags made with corn or rice in them. Corn has a 4% and rice a 10 - 15% oil content and has a tentency to develop a bad odor with continued use. These grain products contain mostly water, which cooks out, leaving the heating medium dehydrated, so it doesn't hold the heat as long and can have an unpleasant odor. My Flax Seed Filled Therapy Bags are made to withstand continued usage that far exceeds the life expectancy of corn or rice as a heat medium. I also double and sometimes triple stitch the products so they are sure to last a long time.