You can say "ripe / mature" if the banana is still edible.
If it has gotten so black as to be inedible and mushy, you can say "rotten / spoiled."
A. You should separate bananas when you buy them.
B. Why?
A. Bananas release a lot of chemicals that are responsible for ripening, so if you leave them in the bunch, they can become too ripe or spoil very quickly.
バナナをまだ食べれそうな場合"ripe / mature" と言います。
黒くなってしまって食べれそうもない場合は "rotten / spoiled."です。
（英文）
A. You should separate bananas when you buy them.
B. Why?
A. Bananas release a lot of chemicals that are responsible for ripening, so if you leave them in the bunch, they can become too ripe or spoil very quickly.
（訳）
A. バナナを買うときは別々にした方がいい。
B. なぜ？
A. バナナは熟しているときにいろんな化学的なものを出している。一緒に置いていたら悪くなったり早く熟してしまう。
The bananas are dark, which means that they are now over-ripe.
Ripened bananas are bananas that have turned black in color. These bananas are mostly used to make banana bread. You can call them over-ripe and ripened.
「黒くなったまだ食べられるバナナ」ということですね。
動画などググってみました。
brown bananas は「茶色のバナナ」という意味です。
「熟したバナナ」を表します。
overripe は「熟しすぎた」という意味です。
overripe bananas で「熟しすぎたバナナ」となります。
例）
Brown bananas should never be chucked in the trash
〔Food & Wine (blog)-Apr 14, 2017 より〕
Banana fritters are a great way to use overripe bananas
〔Winston-Salem Journal-Oct 24, 2017〕
参考になるといいです
ありがとうございました。
▪ Overripe banana
Sometime you can still eat this banana if the inside is not too brown or darker inside. If it is brown inside it is then overripe and not good for eating. You can use this type of banana for baking or to make smoothies.
Example sentence
a. The banana is overripe so we can only make bread with it or a smoothie.
b. Toy can't eat that banana it is overripe.
▪ Overripe banana
中が茶色あるいは黒くなりすぎてないバナナは食べられることもあります。中が茶色い場合は熟れすぎているので、食べるのには適していません。このようなバナナは焼き菓子やスムージーを作るのに使うことができます。
【例文】
a. The banana is overripe so we can only make bread with it or a smoothie.（そのバナナは熟れすぎているから、パンかスムージーを作ることしかできない）
b. Toy can't eat that banana it is overripe.（そのバナナは熟れすぎているので、Toyには食べられません）
Banana that has become black in color can be described as over-ripe banana. Bananas at this stage are very soft and mushy. One famous recipe for these kind of bananas is banana bread.
There are obviously different stages of ageing or decay for all things - and bananas are no exception. As they mature, bananas go green, then yellow, then black. the optimal time for eating is usually during the yellow period, however, some people also enjoy the added texture and sweetness of bananas as they go into the black stage. When the edible part goes very soft it is said to be 'mushy.' Sometime after this point the banana will become rotten and become inedible when it is said to have 'gone bad,' or 'gone off.'
overripe --> past the usual stage of being ready to eat or use
Example :
I don't like overripe bananas.
The overripe banana is too soft to eat.
overripeとは、食べ物などの適した時期を過ぎてしまった状態を指します。
例文：
I don't like overripe bananas.
熟しすぎたバナナが好きじゃない。
The overripe banana is too soft to eat.
熟しすぎたバナナは柔らかすぎて食べられない。
You can call a banana that has become black in colour:
"Blackened bananas"
"Rotten bananas"
A banana that has become black in colour is overripe and is no longer good for eating. Although they are blackened, they can still be used for baking, making smoothies, and especially for making banana bread.
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