Hi Carlos!
I do not use any preservatives on my seeds, but I do not live in a tropical climate.
After air-drying in a paper bag or open dish, I do use a desiccant powder in an open container in the airtight box I keep my seeds in. This keeps the humidity extremely low which increases lifespan of dry seeds (of course many nuts and seeds like your Mangaba would be killed by desiccation, but most seeds really like this). The other thing that can be done is keeping them cold, in the refrigerator or a deep freeze if you have one. A regular frost-free freezer in a kitchen refrigerator is not very helpful because it cycles between frozen and not frozen, to get rid of the frost. I would guess that the desiccant would eliminate fungal problems, less likely to stop borers but might help that too. The desiccant powder I bought at a Hardware Store as "Closet-Dry" or similar name, I don't know what it might be called where you live. Might be worth trying. I found I just need to toss out the powder after a year, when it has absorbed moisture it is quite obvious, it gets moist.
