Seed Saving - Tomatoes

Generally seeds saved from open-pollinated or heirloom Tomato varieties breed true to type. Which means you get the same variety from the seed as the one you collected from. But although Tomato flowers are usually self-pollinated, occasionally they may be cross pollinated by insects about 2% of the time. If you want to prevent cross pollination completely 100% you should keep the different varieties about 10' apart. Flowers of older varieties, like some Heirloom Tomatoes, are said to often have longer styles then more modern cultivars. Because it protrudes more it is more accessible to bees and other insects so these varieties have a higher chance of getting cross pollinated.

                                         
But what is a typical backyard gardener to do, most of us don't have the garden space to isolate our Tomatoes for 10 feet apart? You could plant either tall barrier plants between the Tomato plants or put another pollen producing crop between the different varieties. Or you just do it as I do, chance it.
After all it is only a small chance of cross pollination and who knows maybe you get a cool, new tomato variety out of the deal.
For seed saving, pick the best looking fruits, from your healthiest looking, best producing plants, when they are fully ripe. It is a good idea to save seeds from several fruits to maintain a larger gene pool. No need to waste a tomato just for saving seeds. When you are ready to process a tomato for eating or cooking, just scratch some of the seed containing pulp into a small glass jar (old jam jars work really great), I usually add just a splash of water and then let it ferment at room temperature.

 Don't put it in an especially warm place. At 80° F the fermentation takes about 2 days, at 70° F about 3 days. If you leave the seeds in the pulp for a longer period you get a better control of bacterial canker, which is a seed borne disease, but you should not let them ferment longer then 5 days or they might start to sprout. Stir the mixture each day and when it's ready, carefully pour most of the liquid with the pulp out, without loosing your seeds. You might have to keep adding water frequently to free the seeds from the pulp. Just add some more water, swirl, pour and do it until the water looks almost clean and there is no more pulp. Now you can strain it all over in a sieve, maybe rinse them some more and drop the seeds into a paper towel. Fold it over and let them dry for 3 to 7 days. After they are dry you can put the seeds in small zip lock bags or jars.

If you are saving seeds of many different cultivars it is advisable to label your jars with the name of the varieties you have. It is easy to get mixed up. Since I ferment several batches, after I am drying the first batch I put the jar with the label with the next batch on top of the corresponding Tomato variety drying in the paper towel.