Answer: You will need to harvest and dry the seeds (they need a period of dormancy) before they will sprout. Here's how:
The tomato seeds are enclosed in a gel like sack that contains growth inhibitors, preventing the seeds from sprouting inside the tomato. The best way to remove this gel covering is to ferment the seeds. Scrape the seeds from the tomatoes and put them in a bowl or glass jar. Add up to a cup of water to help separate the seeds from the pulp. Then set the bowl or jar of tomato seeds and pulp in a warm, out of the way spot. You will need to allow 2-4 days for the fermentation to take place. As it does so, the mixture is going to begin to smell awful, so store where you won't pass by frequently.
I use glass jars. The extra space at the top of the jar controls some of the odor and the clear sides let you keep tabs on what is happening. Covering the top of the jar with cheesecloth or paper towel will keep fruit flies out and also diminish the spread of the unpleasant odor.
What you eventually want to see is a layer of mold on top of your seeds & pulp. The process is done when bubbles start rising from the mixture or when the entire bowl is covered with mold. Don't leave the seeds fermenting past this stage or they may begin to germinate.
Finally you can remove and dispose of the mold covering. Lifting it before rinsing the seeds will make rinsing easier, but it's not necessary. You can add some water to the jar or bowl and stir or shake vigorously. The good seeds will settle to the bottom, allowing you to drain off the excess water. Strain the seed mixture into a colander and rinse the seeds well under running water. Try to remove any remaining pulp bits. Spread the seeds onto either a paper plate or glass dish, to dry. Don't use paper or paper towels or the seeds to stick and be difficult to remove. Set them in a warm, dry spot and allow to dry completely. Shake them on the plate daily to make sure they don't clump and that they dry evenly. Don't try to speed the process by using heat.
Once the seeds are thoroughly dry, you can store them in an air tight container, in a cool, dry place. Remember to label and date your seeds! You can sow your seeds next spring.
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