To begin the seed-drying process, you'll have to start with fresh tomatoes. As long as you work carefully, you'll be able to harvest hundreds of seeds from a single tomato plant.
First, be sure to pick the choicest tomatoes left in your patch as you prepare to harvest seeds in the fall. For the best results, choose mature, open-pollinated tomatoes that are disease-free. Open-pollinated tomatoes are ideal because they take the previous season's characteristics and carry them into next season's plants; often, they're the tastiest tomatoes in your garden.
After you've chosen some fully ripe tomatoes, wash them thoroughly, and slice them in half, exposing the seeds and gel in the center of the fruit. Cut the tomato seeds away from the gel as much as possible, and place the seeds in a glass or jar.
Fill the glass or jar containing the seeds with 3 to 4 inches of water, and stir several times a day for two to three days. Don't leave the seeds in water for too long, or they'll begin to germinate or darken in color, which is a sign the seeds have gone bad. The fermentation process will begin after just a couple of days, and the remaining gel attached to the tomato seeds will separate and float to the surface. Add more water, and watch as the viable seeds sink to the bottom and the unusable seeds float to the top of the mixture. You can simply throw away these bad seeds.
Once you've saved the good tomato seeds, spread them on an absorbent surface like wax paper, coffee filters or paper plates. We don't recommend paper towels because the seeds will stick to the paper. Metal, plastic and ceramic plates aren't conducive for drying seeds successfully, either. After placing your seeds on a porous surface, wait for up to 48 hours until they are dry. Test the dryness of your seeds before storing them for the winter. Seeds that bend are not through drying; however, dried seeds will be very hard and tough to bite or smash. Making sure your seeds are dry is a very important step because damp seeds will foster mildew.
Move your dry seeds to a location away from direct sunlight, such as on top of your refrigerator. Check on their progress often for one to two weeks, waiting for the seeds to dry even more thoroughly. When drying tomato seeds, be cautious of the temperature, and never attempt to dry seeds in areas with high humidity. Conditions with high temperatures may cause wet seeds to sprout, which will ruin them. Attempting to speed up the process by drying seeds in an oven or other heating device should also be avoided; they'll go bad and will be of no use for the next growing season.