OK, I'm not one to argue with the top dogs on here but I do know for a fact that oranges are ripe when they are still somewhat green(skin). If you cut one open, you will notice the seeds are pretty much as ripe as they ever will be and the pulp is as sweet and as orange as it's gonna get as well.
Now I know that we're not talking about oranges but I'm sure peppers are almost the same way. I've eaten many green jalapenos and serranos in my time and the seeds look as ready as they will ever be other than drying them out. As a matter of fact, sometimes some seeds start to rot when the pepper starts getting too ripe.
By green jalapenos and serranos, I mean peppers that have reached their full size and won't be getting any bigger and have pretty much reached it's mature stage. If a pepper has reached it's full size, I'm sure the seeds have as well but again, I am no expert and I'm just going by my experience from the peppers that I have eaten probably millions of in my lifetime.
I guess it really depends on the variety because I've also eaten millions of pequin and tepin and these are almost all seed inside and since they're so small, the seeds dry out before the pepper starts to rot which is probably almost never because the flesh is so thin. By the way, these start drying out almost as soon as they turn red. They do turn orange which is when I like to pick them because the birds will probably get them if I try to allow them to get red. I've also noticed that the seeds are good to go when these are orange.
I'll also use the ever popular green bell pepper(the one you find at your local produce store) as an example as far as seeds go because I just saved some not that long ago. Although they are called green bell peppers, they will turn red if left on the plant long enough....but, I decided to test the seeds from one of these store bought bell peppers. As usual when using bell peppers, I just use the flesh so I cut out all that's in the middle, mostly the seed sack or whatever it's called. I dried out the seeds, saved them, then planted them and sure enough, they almost all germinated...I'd say at above 90% rate.
So then, when is a green bell pepper fully ripe? Remember, they do turn red as their final color as do jalapenos and serranos but the seeds from these peppers were already mature when they were still green.
So I guess seeds mature differently on different varieties.
I guess the bottom line(for me anyway) is that we can say there is a difference between a pepper being mature and it being fully ripe. My opinon is, when a pepper is mature, the seeds will be also.
The maturing point for me on peppers that aren't called red or are known for being red, such as purple peppers, yellow peppers, orange peppers, etc...is when they turn fully colored for what they are known for. Examples: Jalapeno, serrano(green), habanero(orange), Yellows this or that(yellow), Black Pearl(black), purple jalapeno(purple), etc...
Again, this is strictly my own opinion and something I know from experience or atleast with the peppers I have experience on.
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