The best time to harvest tomatoes for seed collection is when they are at their fullest and most mature state, right before they start to soften and bruise. Since tomatoes take approximately 70 to 85 days to develop completely, you’re looking at harvesting in late August to early September, if they were started in early March. Once you’ve selected your tomatoes, start deseeding by cutting the fruit evenly in half and scooping them out. There is a gel-like membrane encasing the seeds, which prevents germination, and the tomato seeds will need to go through fermentation to remove these casings.
To ferment the seeds, allow them to soak in the juice from the harvested tomato for two to four days. Water can be added if the liquid volume is low, but adding water can slow down the process. Store the container in a cool and dry place, covering the top with a breathable material like a paper towel or cheesecloth. You can tell that fermentation is complete when a layer of mold has formed across the top, with tiny bubbles appearing, and the seeds have sunken to the bottom. Keep a close eye on this; seeds can be harmed if they sit in fermentation for too long. Once ready, seal the container and shake up the mixture. With everything loosened up, strain it and rinse the seeds thoroughly with water. The seeds will need to be left alone to dry, spread out in a cool place for two to three weeks.