Mayo Underwood ( an heirloom seed expert) wrote this explanation of tomato seed fermentation for you.
Take it away Mayo….
Why is tomato seed fermentation better than drying seeds on a paper towel?
The gel sac around each tomato seed contains certain chemical compounds that inhibit germination until the tomato ripens, and may contain tomato diseases.
In nature, the fruits fall off the plant and slowly rot. As they do, the natural fermentation process destroys the gel sac (including the growth inhibitors and any diseases) and allows the seeds to germinate when the right conditions are present, generally the following season
Saving tomato seeds by fermenting them is the way to mimic Mother Nature’s fermentation process.
Here’s how to do it:
- Select the tomato(es) you like best (one variety at a time) and scoop out the seeds. Put the seeds in a jar or container with about twice as much water as the quantity of seeds in the jar. (e.g. 1 Tbsp seeds with 2–3 Tbsp water)
- Cover the container with its lid or with plastic wrap. (The cover is only so you don’t have to smell the fermentation process — not a delightful experience.)
- Keep the container at room temperature for several days. Put it where you’ll see it and remember to stir or shake the contents twice a day.
- The seeds will ferment faster in a warmer room (80°F — 26°C for example) than in a cooler room (65°F- 18°C).
- You can often spot individual seeds moving as the “good” seeds sink to the bottom and the non-viable seeds and debris rise to the top.
- In a few days, a layer of greyish-white mold forms on top of the water — a good sign.
- When that layer of mold covers the water’s surface, fermentation is done. If you leave your seeds in the water beyond that point, they’ll start to germinate, so be sure to check the jar often.
- When fermentation is done, add more water to the jar- about twice what was in the jar originally.
- Stir the contents very well, then let the container rest for a minute or two.
- Separation will occur again — with the good seeds settling at the bottom and more debris rising to the top.
- Gently pour off the top layer — and add more water.
- Stir and let rest again, then pour off the top layer.
- Repeat this process several times until all the debris is gone and only the good clean seeds remain.
- Pour the clean seeds into a strainer. Let the strainer sit for a few minutes on a dishcloth or paper towel to absorb excess water, then spread the seeds to dry on a NON-porous surface — glass, ceramic, stainless steel, etc.
- They’ll stick to a porous surface. You can also leave the seeds in the strainer to dry.
- Wherever they are (in a place with good air circulation, out of direct sun), stir the seeds around once a day or so. This exposes more seed surfaces to air, for more even drying, and also prevents clumping.
- The drying process takes a few days. The more humidity, the longer it takes NO cheating, please… Putting seeds in a microwave or food dehydrator or oven, even at a very low temperature, saps their fertility. Not good.
Testing For Dryness
- To test for dryness, bend the seed. If it breaks it’s dry; if it only bends, let it dry further.
- If your fingers are too big to hold a seed and bend it, put it on a hard surface and press down on it with a fingernail,
- It if merely dents, it’s not dry enough; if it breaks, it’s dry.
- If you’re not sure, let the seeds dry for an additional day or two.
There’s not too much worry about over-drying seeds.
When the seeds are dry, store them properly.
When properly fermented and stored, tomato seeds can last 5–10 years.
As a bonus, seeds dried in this manner germinate in just a few days, especially in the first year or 2 when they’re very fresh.
They diminish a bit in viability each successive year.
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