Saving tomato seeds takes a little more time, but it’s just as easy. Harvest ripe tomatoes from several different vines of the same variety, cut each across the middle, and gently squeeze the juice and seeds into a bowl. You’ll see that each tomato seed is encased in a gelatinous coating. (This prevents the seed from sprouting inside the tomato). Remove this coating by fermenting it. This mimics the natural rotting of the fruit and has the added bonus of killing any seed-borne tomato diseases that might affect next year’s crop.
To ferment the seeds, add about half as much water as there are tomato seeds and juice in the bowl and stir the mixture twice a day for about three days. Keep a close eye on the mixture—especially if it’s a warm area, as fermentation happens more quickly at high temperatures. As the mixture ferments, its surface will become covered with white or gray mold. Don’t keep the bowl in the kitchen, anywhere it can be tipped over by animals or children, or where you’d be able to smell it—it will get pretty rank.
When bubbles begin to rise to the top of the mass, or when a thick coat of mold has formed, stop the fermentation by adding enough water to double the mixture, and stir vigorously. The clean, good seeds will settle to the bottom of the bowl. Gently pour off mold, debris and any seeds that float (they're hollow). Add more water and repeat the process until only clean seeds remain.
Capture the seeds to be saved by pouring the liquid through a strainer, wipe the strainer bottom with a towel to remove as much moisture as possible, then dump the seeds onto a glass or ceramic plate to dry. Stir twice a day to ensure even drying and to prevent the seeds from clumping together. Warning: Tomato seeds will germinate unless you dry them quickly. To speed drying, you can use a fan, but don’t put the seeds in sunlight or an oven.
Related: 6 Tips For Storing Your Saved Seeds