Prior to the post about rapid ripening from @JustPeachy, I was going to suggest mass treatment using a 5lb CO2 bottle and placing a whole bunch of persimmons into a 5 gallon keg like those used to homebrew. Usually they are recycled Coca Cola kegs.
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@disc4tw I did consider investing in a larger CO2 tank and making a bigger chamber, but I decided to experiment with this smaller version first because I don’t have enough fruit or the budget to warrant the larger scale version just yet. Refilling a 5lb CO2 tank at Airgas would be significantly cheaper per pound than at the paintball store. If my younger persimmon trees all end up producing as much as my three oldest trees do, then I will need to reconsider that idea (and get a stall at the local farmer’s market to sell them!).
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I’m just speculating. Maybe it’s ethylene that’s helping them those in the bucket ripen in a semi enclosed dark space. Do the ones in the bucket which are untreated still have astringency?
Do you have any not in the bucket, just laid out? Are those post-CO2 treatment as hard as those simply laid out? That’s how I would compare firmness over the course of a few days. The paper is stating that it triggers rapid-onset of post-astringency softening. I assume this occurs for all persimmon, kaki or virginiana, but the speed of onset probably differs due to the exact transcription factors expressed in each cultivar and the environmental conditions of the test.
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The untreated persimmons that start to soften are still quite astringent until they get very soft.
I have left a few untreated persimmons on the counter and not in a container, but I haven’t made careful observations. I prefer to keep my persimmons in some kind of container because sometimes they soften up and start to leak before I notice them.
I’m sure you’re right about ethylene playing the key role in ripening, but I would expect the treated fruit in the enclosed CO2 chamber would have even more exposure to ethylene than the fruit in an open bucket, right? Unless the bananas and apples in my kitchen are producing enough ethylene to affect the open bucket of untreated persimmons down the hallway and in the bedroom 30 ft away.
I’ve noticed on my trees that any fruit that has even a small bird peck will ripen long before the others on the tree. So maybe even small damage or bruising could also accelerate ripening.
Maybe the fruit in the study was much closer to fully ripe when treated with CO2 than the fruit I pick and put in my CO2 chamber. Or like you said, maybe there are differences between cultivars. Most of my treated fruit has been Giombo and Tecumseh. I’ve also treated a few fruit off my younger trees - Smith’s Best/Giboshi, Eureka and Nikita’s Gift. Interestingly, the three hybrid Nikita’s Gift fruit I tried were still too astringent to eat after 3 days of CO2 treatment, while all the kaki fruit were non-astringent. My young Nikita’s Gift tree only had three fruit on it this year, so I can’t try any more and see if a longer treatment period would remove the astringency.
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If I remember correctly, ethylene formation occurs by fruits (including persimmon) on their own, so you end up with accelerated self ripening since it kind of causes a snowball effect once it starts. However, the synthesis of ethylene requires some sufficient concentration of oxygen, which is why the CO2 thing (or rather low O2) is a thing for storage of many fruits to prolong their life.
Incidentally, CO2 also is a thing that helps address astringency in persimmon, which I assume shares some but isn’t identical as the biological pathways for firmness. (Bear in mind, I didn’t read every word of all the relevant papers cause they I am really not THAT bored. I just skimmed through a few of them. At some point, tastes good is tastes good regardless how you get there.)
This could explain why you can use pure ethylene to accelerate softening of persimmon, even though the result is still astringent. Conversely, you can use CO2 to address astringency, where the result is non-astringent fruit that maintains firmness (for how long is a separate issue).
I assume the fruit is bletting when damage occurs like this exposing the flesh.
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Re astringency: I didn’t read exhaustively myself either, but my take-away was that in an anaerobic environment (e.g., CO2) endogenous ethanol is reduced to ethane (acetaldehyde), which binds tannins.
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The more I read and experiment, the less I think know. 
I’ve been working only with the American variety Prok. I haven’t tried CO2, but I have tried alcohol. From what I’ve seen, alcohol didn’t seem to reduce astringency any faster than the mere passage of time – ~2-3 days at room temperature. But after that much time, the fruit was fairly mushy.
Often it’s better to be lucky than smart. In this case, I may have stumbled on on method that reduces astringency in American persimmons without turning the fruit to mush. The 1st piece of good luck was that the thermostat on my refrigerator broke. In that condition, it worked at one speed, which kept the box at ~60 F. The 2nd piece of good luck is that I started storing my still astringent Prok persimmons in the broken fridge – mainly hoping to keep away fruit flies.
At this temperature, it seems that the fruit took a little longer to lose astringency – maybe 3-4 days versus 2-3 at 70 F. But it was also slower to turn to mush. The resulting fruit was the best in flavor and texture that I’ve gotten from the Prok tree so far.
Take this all with a grain of salt. It’s some half-assed observations and some shaky inferences. But is there something useful here? Is it possible that the specific chemical reactions that reduce astringency may be less sensitive to low temperature than the reactions that cause softening.
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Thanks for sharing this experiment. I have a Sodastream fitted with a CO2 bottle, and tried hastily using a gallon ziplock with the air squeezed out and a few persimmons in it, then filled the bag and zipped it shut to leave on counter for 2 or 3 days.
I got very mixed results with store bought Hachiya. I like your more controlled setup.
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I agree that ncdabbler’s setup is perfect!
Using the Sodastream I’ve found that even heavy duty freezer bags leak CO2. I’ve switched to even heavier duty ziplock - the ones for storing tools. I open the corner of the bag each day and top up with CO2. I’m probably using more CO2 than if I had a leak-proof container!
The paintball stores in my area are all gone but the local Airgas sells new 5 lb CO2 tanks for $60. Online there are 4 lb refurbished tanks for $37.
Getting close to making the switch!
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Maybe the fridge is inducing bletting since it’s a cold environment. Otherwise chemical reactions should slow down at lower temperatures.
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Thanks. Maybe, but it’s not very cold – Note, it was a broken fridge with an internal temp ~60 F. And what is bletting (ripening and softening) at temperatures well above freezing if not a bunch of chemical reactions?
Anyway, don’t really care what the mechanisms may be so long as it works. Do astringent persimmons generally ripen well – lose astringency but retain flavor and texture – in cool temperatures?
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I’m sure others can jump in and comment about Kakis… But since this thread is about American persimmon, I’ll just tell you my experience with them.
The D. virginiana at my location do ripen on the tree for the most part and when they do it’s during this month of October when our temps generally drop to 40-50s. I have yet to pick them rock hard to try ripening on the counter. Sometimes I pick them slightly hard but still slightly soft, and sitting on the counter for a week or in the fridge, they seem to ripen just fine. I haven’t bothered to keep a log book comparing room temp to fridge though. I can pick a whole tray of them and whenever they all ripen to a gooey mushy state, we pretty much polish off an entire case. There isn’t much to experiment with since we consume them pretty fast, as they are quite small (relative to kaki).
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OK, thanks. Makes sense. My season must be slightly ahead of yours and/or Prok is earlier than your varieties. Mine ripen soft on the tree starting in late Sept. By now, the harvest is 90-95% done. I’ve been harvesting some drops as well as mostly fairly soft fruits from the tree. Roughly 50% of the drops are still somewhat astringent but need only ~1 day at room temp. Roughly 80% of the fruits picked soft on the tree are still astringent and need 2-3 days at room temp. I haven’t yet picked less ripe fruit that needs a week, except from my Asian IKKJ.
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I was also thinking about the gelatin in reducing tannins in wines might be useful to reduce tannins in a puree. Did you try it yet?
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Yeah, I made a couple attempts with store-bought gelatin, assuming that the gelatin would bind the tannin and render it incapable of interacting with saliva. I didn’t see any impact. But I was making up the protocol – dosage, heating, mixing. I assume that some more intelligent approach might have better results.
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A final note in this thread . . . As described above, sodium and potassium bicarbonates turned out to be too salty for use removing astringency. Whether the astringency is diminished at all seems academic if the resulting paste is inedible. So recently I tried calcium carbonate on paste prepared from some very ripe Prok persimmons that had been refrigerated for a couple weeks. Before treatment, the paste was still slightly astringent, enough to make it unappealing.
The results were disappointing. Astringency was not eliminated. I had used a fairly heavy dose, so I feel pretty confident saying that calcium carbonate doesn’t work. Moreover, the paste ended up tasting chalky. So chalk tastes chalky – what a surprise!
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I know you’ve moved on from Prok, but here’s some food for thought
https://www.sciencedirect.com/science/article/pii/S2405844022020047#bbib33
Tsurunaga et al 2022.pdf (1.5 MB)
The authors removed astringency and reduced separation of the pulp by adding common, inexpensive polysaccharides like pectin, Xanthan gum, or sodium alginate. All the polysaccharides they tested worked on both removing astringency and preventing separation. Some were better at one or the other, suggesting a blend may be the ticket. Most interestingly, the astringency did not return when the pulp was heated! They don’t go into a lot of detail on the mixing step, but it sounds like they simply added 1% polysaccharides by weight to firm ripe, still astringent Saijo fruit and chucked it in a blender. I’ve filed this away for something to try whenever my trees start producing.
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Thanks. It sounds like a great suggestion.
Honestly, I had to review this thread to remember what I had tried before. I had forgotten about the carbonates. But I did remember trying protein (i.e, gelatin) because I subsequently saw a reference to advice from China never to eat PCA persimmons with protein because the proteins and tannins will bind together like cement. That was consistent with my own experience – gelatin plus Prok paste produced a mixture resembling grout.
On to the carbs! Though I have top-worked the Prok tree, I’ve left some buds where Prok could grow. So the tree now has some Prok fruit along with fruit from the grafts of other American varieties. So I’ll be able to experiment with this suggestion. Stay tuned!
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A question, maybe intended for any chemists in the crowd: I read the linked article. The evidence shows that two classes of pectins bind tannins and thereby can substantially remove the astringency from persimmons.
In a separate segment of my life, I sometimes make hard cider. I know that apple juice contains pectin; and good cider juice contains tannins. The tannins are responsible for desirable astringency and bitterness.
Why doesn’t the pectin in apple juice bind the desirable tannins?
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A good question, and one that I was wondering as well. I do know that there are many kinds of tannins, but I don’t know how different they are in terms of reactivity to pectin. I also know that all fruit contains at least some pectin (you can make jam from almost anything without added pectin!) and that it breaks down as part of the following process. The breakdown is sped up if you grind or crush the fruit. Store bought pectin doesn’t seem to do this, or maybe the concentration is high enough to exhaust the enzymes?
All of that is just a rambling way to say “I dunno”
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