To start saving your own tomato seeds at home, begin by collecting a handful of the biggest ripest fruit you can find on the plant. Make sure they have not been attacked by bugs, have any signs of disease or other imperfections. The seeds from this fruit will need to be as healthy as possible to give you the best chance at large healthy plants when you go to plant these seeds the following year.
Wash & Remove The Seeds
Next, wash the fruit in warm water to remove any dirt or insects that may be clinging to it then dry the tomatoes and set them aside.
Over a large dish or bowl, pull the tomato in two, by pushing your thumbs into the blossom end ( bottom) of the tomato. Using your fingers scoop out all the tomato guts, juice, seeds, the whole nine yards. I do this with my hands to avoid cutting into the seeds with a knife.
Fermenting The Tomato Seeds
Wrap or seal the dish containing the seeds and set in a dark spot such as the corner of a shelf for 3 or 4 days to ferment. The seeds and juice will start to bubble slightly showing that fermentation is happening. This fermentation is good for the tomato seeds as it helps kill any bad bacteria and can increase the seeds resistance to various diseases.
Rinse & Dry
Once the seeds have fermented for three to four days, remove the covering and in a larger bowl rinse the seeds with a good amount of fresh water. Healthy seeds that will sprout should sink to the bottom. Pour off the pulpy water mix, and rinse the seeds another two times until they look clean and free from any pulpy material.
Spread the seeds out on a sheet of paper towel or a clean cloth to air dry. I find it is best to dry the seeds overnight by placing them close to a radiator, but not on the radiator. You just want the warm air from the radiator to help dry the seeds, don't cook them!
Storing Your Seeds
Once the seeds are completely dried it is best to store them in an airtight container such as a glass mason jar. Little 250ml jars which I labelled with the date that the seeds were stored and the variety of tomato work wonderfully. Keep these saved seeds in a cool and dark location for next year's planting season!
The tomato seeds may last up to six years if they were properly fermented, washed, and stored, though most likely they will be used long before then!