Every tomato seed is enclosed in a small jelly-like envelope containing chemical substances which keep the seed dormant. This envelope prevents the seed from germinating in the warm, liquid interior of the fruit. A mature tomato retains a surprising amount of heat during the hottest Summer days. If left to nature, the fruits fall from the plant, rot, and the fermentation process destroys the jelly-like envelope.
This fermentation process must be reproduced artificially if you want to produce your own tomato seeds. This is very easy to do. Simply cut the fruit in two, remove the seeds and the juice, and place them in a jar. Water can be added, as in certain conditions it can help the fermentation process. This liquid is left for a few days until a white musty skin appears on the surface. The main agent of fermentation is called Oospora lactis, which helps destroy bacterial diseases.
The time needed for fermentation varies according to the surrounding temperature. Be careful during the hot Summer days, as fermentation can take place in less than 48 hours. In this case, if you wait too long you may lose the seeds. Freed of their jelly-like protection, they will happily start to germinate in very favorable hot and liquid conditions.
Therefore, when the fermentation process is complete, the seeds are cleaned by putting them in a fine strainer and running water over them, stirring vigorously while doing so. Waste and decomposed immature seeds will disappear through the strainer, leaving behind only the good seeds. They are then laid out to dry on a fine mesh (for example, a supple plastic mosquito net). Dry, ventilated conditions are necessary.
It is strongly recommended NOT to dry them on paper (they will stick to it, making it impossible to remove them), nor in the oven (even at low heat), nor in full sunlight. The key element of drying is not heat, but ventilation. This rule holds for all types of seeds.
During a hot humid period, it is strongly recommended to use a ventilator. You should also separate the little piles of seeds delicately by hand during the drying process. The dried seeds should then be placed preferably in glass jars or in small paper sachets, protected from humidity.
Tomato seeds have an average germinating lifetime of 4 years. This said, they can last up to 10 years or more.