TOMATOES ARE CHALLENGING — Some seeds require a little more effort. Tomato seeds must be fermented in water for a few days to remove the gelatinous coating around them and to stimulate the natural rotting of the fruit. Fermentation also kills many seed-borne diseases. Scoop the seeds from an overripe (but not rotten) tomato and place them in a bowl or jar of water. Let it sit at room temperature for two to three days. Stir occasionally to prevent mold from forming. Once the seeds have settled to the bottom, rinse them thoroughly and spread out to air dry on paper or a fine screen. Throw away any seeds that float since they will not germinate.
https://www.almanac.com/news/gardening-news/seed-saving
See how to save your own tomato and vegetable seeds in this video.
