Inspired by @cdamarjian 's post last year of a CO2 chamber for euthanizing rats to feed to pet snakes, I made this set-up for removing astringency in kaki persimmons using an airtight plastic container and a paintball CO2 tank.
I can refill the CO2 tank at a local paintball store much more cheaply than buying Sodastream CO2 canisters.
I pick the fruit (Giombo in these pictures) when they are just starting to turn orange and still rock hard.
I put a few drops of bromothymol blue in water with a pinch of baking soda to make a basic solution and put the outlet hose into this glass. When the water changes from blue to greenish yellow, then I know the chamber is full of CO2 and stop filling it. I remove the CO2 tank hose and cover both the inlet and outlet with saran wrap to seal the chamber. I leave it for 3 days in a dark place. The fruit comes out firm and non-astringent.
The first time I tried it last year, I tasted the fruit immediately after removing them from the CO2 chamber and they tasted carbonated, like they had been soaked in soda water. They also still had some astringency. But after leaving them overnight, they were completely non-astringent and still firm.
After this treatment, astringent kakis have a texture just like fuyu-type non-astringent kakis, but I think they are sweeter and more flavorful. Most members of my household don’t really like the gooey texture of a ripe astringent kaki, but they love the crisp crunch of the CO2 treated fruit.
I also dry astringent kakis in a dehydrator, and if you slice them when they are just starting to soften up but still astringent, drying will remove the remaining astringency. It won’t work on hard, unripe fruit, though.