Bananas which are exported are picked green and are put in what are called â€œripening roomsâ€ after they have arrived at their intended destination. These rooms are specially designed to be air-tight with lots of ethylene gas to help the ripening process along.
The retailer that sells the bananas can choose to get ones that have not been exposed to this gas, though they may still arrive at the grocery store green. These bananas tend to ripen slower and as they do they turn a yellowish brown color. This is when it is best to eat them because they are at their most flavorful.
Even when the banana is allowed to reach the yellow stage, there can still be firm flesh inside the fruit.