When saving tomato seeds for next year's garden, you have to soak them (along with all the gel and bits of tomato pulp) in water for several days until mold grows on the top. This indicates that fermentation has taken place. Fermentation is necessary, and it is a sign that the gelatinous coating around the seeds has dissolved. However, if you soak your seeds for too long, they'll start sprouting in the jar and will be completely worthless. Here's how to know when it's time to stop soaking your seeds.
- White or green mold begins to form on the surface of the water. It doesn't need to cover the water -- a couple of spots of mold is all you need to see.
- Once you see mold, it's time to remove and rinse your seeds as I described in my tutorial on saving tomato seeds.
- This should only take three to five days. If you don't see mold after four to five days, go ahead and rinse and dry your seeds anyway. Some of the gel coating may still be clinging to the seeds. If this is the case, I recommend rinsing your seeds in a fine mesh strainer and gently rubbing the seeds against the mesh as you rinse under running water --- this will help dislodge the rest of the gel.
It's not an exact science, and it's a good idea to keep in mind that it doesn't need to be a "perfect" job -- if there's still a little gel clinging to the seeds when you start drying them, it's not going to hurt anything or inhibit germination. The important thing is to just go ahead and save your tomato seeds -- you'll get a feel for the whole process the more you do it.