When saving seeds from this year’s harvest for next year’s, you’ll find best results with heirloom tomatoes rather than hybrids (hybrids are typically the ones you purchase in grocery stores and regular greenhouse seedlings).
Once you make your initial purchase of heirlooms, you’ll be able to do this year after year. Farmers markets are a great place to find them.
Here’s how you do it: Cut tomatoes in half then squeeze out the pulp into a clear glass container (wide mouth jar or small bowl for example). Add a couple tablespoons of water (double the amount if there isn’t much juice) then cover jar or container with a piece of cheesecloth (a coffee filter will work well too), secure in place with an elastic band.
Leave the container to sit at room temperature for about 4 or 5 days until you notice a layer of white scum/mould form on the top (this process is fermenting). Skim off the white scum then pour into a large bowl, fill with cool water and let sit for a minute or two. Pour off the water, keeping the seeds that are resting on the bottom and allowing the ones floating on top to pour off (the ones floating on top are not viable).
Pour the saved batch into a fine sieve or strainer and rinse under cool running water until they are clean and no pulp or gel remains (stir and shake the sieve while doing this to help speed up the process). Tap the sieve a few times to remove excess water then pour them onto a paper plate or coffee filter. Arrange them so that they are in a single layer.
Allow to dry for about a week to 10 days, stirring the batch every day gently with your finger to help promote an even drying process.
Once they’re fully dry, store them in a glass jar (sealed), envelope or packets and keep in a cool, dark location until ready to use.
A few tips: Don’t mix different varieties…make sure to save only one variety of tomatoes per container (if saving more than one variety, make sure to label the container). Wash all tools and hands well before moving on to the next variety. Select healthy and prized specimens that are really ripe for the best results, this will give you a better chance of preserving quality seeds for next season’s crop. Removing all the gel/coating before drying is necessary because this is what prevents them from germinating. They can last for quite awhile when properly stored and can still produce impressive yields up to 5 years after being first dried.