A seed obtained from any tomato is very likely to grow. Even a tomato purchased from the grocery store renders viable seeds. However, most commercially grown vegetables are hybrids. Plants grown from their seeds are unpredictable at best, and are typically disappointing. The best tomatoes to use for saving and propagating seeds are heirloom varieties. These produce plants that are true to type, which means they're little clones of the parent plant. To save the seeds of a tomato that you like, you'll need to dry them properly. The simple process involves fermentation, so don't be surprised by the yeasty odor this project will exude.
Cut the tomato in half. Scoop the seeds and gelatinous material into a sterilized container.
Add 1 tbsp. of water to the material. Stir the mixture.
Snap the lid onto the container and poke five or six holes in the top. This will allow excess air to escape as the tomato seed mixture begins to ferment. Fermentation will kill any bacteria that are present as it breaks down the gelatin. Place the container in a warm, dark spot for two to three days. A cabinet, under the kitchen sink or a seldom-used oven are good locations.
Stir the tomato seed mixture twice daily. Seeds will begin to sink to the bottom. Remove the lid from the container after most of the seeds have sunken.
Scoop floating seeds and tomato tissues from the top and discard them. Stir the tomato seed mixture. Add 1 or 2 tbsps. of water if it's too thick to stir easily. Stir again and pour into a kitchen strainer. Rinse with cool running water while stirring until all residue is washed away from the seeds.
Stack three or four filters and spread the seeds in a single layer on top. Arrange the seeds so that they're not touching each other. Set them in a dark, dry spot for one week.
Put the dried seeds into a sealable plastic food storage bag. Label it with the date and tomato variety. Store your tomato seeds in a dark, cool, dry spot until you're ready to plant them.