Select strong healthy plants with characteristics that you are looking for. If it is an heirloom variety that you are saving, it is best to select the variety that best fits the description of the variety. If you want to select varieties that fruit earliest, then select the earliest fruiting each year.
Clearly label the plants that you have selected. Tomato plants generally do not cross with one another (they are in-breeders) so you can grow and save seed from more than one variety in one polytunnel.
Harvest and processing
Once the fruit is over-ripe for eating it will be ready for harvesting for seed. At this stage take all the ripe fruit off the plants, then slice them along their equator (use a bread knife for this as it is less likely to damage the seed). Then squeeze out the pulp into a container.
This can then be left for 3-4 days to ferment (this breaks down the germination-inhibitor that surrounds the seed). Do not leave for longer than for days as premature germination can occur. You may notice a mould appearing at the top of the pulp – this is fine and all part of the fermenting process.
Once fermentation is completed then poor the pulp/seed into a long necked container – something like a milk bottle works well. Fill it up to ¾ full and shake the bottle vigorously (with a lid on…). This helps to separate the seed from the pulp. As the seed starts to settle back down to the bottom of the bottle you can start to decant the pulp and water. A few small, light seeds may also come out. Decant until most of the water has left the bottle then repeat this process 5 or 6 times, or until the seed looks clean.
Fill up the bottle, now with clean seeds in, one last time and empty into a sieve. Blot the bottom of the sieve with kitchen towel to dry excess moisture and then turn out the seeds ideally onto a non-stick surface such as a plastic chopping board. Using the bottom of the sieve, spread out the seeds evenly to form one layer of seeds. As you are doing this you can have a bit of kitchen towel inside the sieve which will take away some of the moisture without the seeds sticking to the towel.
Label the seeds and put somewhere dry and airy to dry further. Once the seeds have dried they will peel off the chopping board in one piece. They can then be crumbled up into a paper seed packet and dried further using silica gel. To do this put the seed packet inside an airtight container and put the same amount of silica gel in the bottom of the container. Put a humidity reader into the container and close the lid. Leave until the relative humidity is around 50%. Then remove the silica gel and store the seeds in the airtight container somewhere cool and dry, and they should keep for up to 5 years. Don’t forget to clearly label the seed packet with the variety and the date.