I only save seed from vine (plant) ripened fruit. I notice my tomato seeds are almost double the size of commercially sold seed. Ii don't know about the other qualities of a seed, but I can guess.

So far, I have been able to save seeds from purely vine-ripened fruits. I will try to continue that practice. I'm sure I will have more than enough ripe fruits, as the season progresses.

Martha

You will know whether or not the seed is viable when you ferment it. Immature seed will float.

Thank you to everyone. I will continue to try to let fruits ripen completely on the plant, but, if necessary, will save counter-ripened seed that passes the "float test."

Martha

I've saved seeds of many tomato varieties over many years and it's quite easy. In fact, of the hundreds I've saved, I can't think of any that failed to germinate. I always harvest the seeds from the mature fruit, and save it if the fruit tastes good. All I do is dump the seeds in a glass of water, let them sit for several days so that a scum develops on top, and then I dump them in a sieve, rinse with cold water, and bang the upside-down sieve on a piece of waxed paper. When the seeds are dry, they are easy to remove from the waxed paper. Then I put them in a labeled envelope and store in a cool, dry place for next year. The purpose of putting the seeds in water for a few days is that it helps dissolve an anti-germination coating that the seeds naturally have.

The last thing I will point out, however, is that harvested seeds will not necessarily be true to the mother plant unless you take precautions. This typically means hand pollinating some of the tomato flowers on a plant from tomato flowers on the same plant with a paint brush, before the flowers are 'contaminated" by pollen coming from other species, and then bagging some of the flowers. Here is an article that explains this in more detail: Prevent tomato cross-pollination.