Saving tomatoes seeds is easy. In the past I have just scooped out seed and let them dry and had no major problems. However the seeds stick together and can easily mold. So this method below involves fermenting and guarantees that your seeds will be clean and disease free.
I like to start saving seeds right after I pick the tomatoes and I’m about to cook with them. Cut the tomato in half and simply scoop out the seeds into a cup labeled with the tomato variety.
Fill the seed cup half way full of water and wrap plastic over the top. I reused a plastic bag from plastic plates or cups. Poke holes in the plastic for air flow and sit the cup on a windowsill.
Stir the seeds a couple times over the next 3 days. You will see a layer of scum on top of the water and your seed will be on the bottom. Scrape the scum off and strain your seeds out. I used my plastic wrap to strain the seeds then let them air dry on top.
Once they are completely dried put them in a paper packet and don’t forget to label with the tomato variety and date (we’ve made a few mistakes not labeling correctly!).