This is the first part of a nine part documentary. You can check out the rest of the movie on YouTube.
The technique shown for saving tomato seeds can be applied to all juicy fruits where the seed is encased in a gelatinous substance, from cucumbers to passion fruits. Any seeds that are floating in the liquid after fermentation should be discarded as they are most likely not viable (won't germinate). Make sure you thoroughly dry the seed before storage to prevent rot. Seeds are best stored in a cool dry place.