I hope you are refering to after they have been removed from the plant? If so, then put them in a brown paper bag and keep completely closed (NOT PLASTIC). Check them each day for browning and ripeness. Banana's contain Ethylene gas which is a ripening agent found in most all fruits. Ethylene is used in large climate controlled storage units, where prematurely picked fruits are kept. As market demand increases, some not all the fruit is exposed to the gas, which ripens the fruit in a very short time. Making it visually appealling to markt consumers. It may look ripe, but the feel and taste is not the same as vine ripened fruit. This way a steadier supply can be brought to market. Otherwise the mase growers and supplier would be running amuck trying to dump billion of pounds of fruit before it all went bad. Banans's which are just beyond eating, can be frozen and used later, as a buffer in blended smoothies. I put my own grown organic, fresh picked strawberries, in a blender with any frozen bananas add ice and blend on high. It's to die for. While living in Hawaii on the big island a friend turned me on to banana pancakes, using bananas which were beyond putting in my mouth. I have not tasted pancakes that good since I left the islands, over 20 years ago. Ethylene gas is also used by tabacco growers, since the tabacco plant does not ripen all at once, leaves picked with different coloring can be harvested together. During the drying process, tobacco leaves are exposed to Ethylene gas, which accellerate the color change on leaves which are picked to soon. This makes dried tobacco leaves a more uniform color before going to market. Quality of the tobacco, a farmer brings to a tobacco auction, is issued by goverment graders before auction, who examine leaves for maturity and color, then issue a grade on a set amount of tabacco. Color uniformity is a desired quality with the tobacco industry and means a higher auction price for farmers.
If you are looking to ripen banana's still on the plant, just as fruit begins to show you can use organic bone meal, sprinkled on the soil at the plant base working outward. You can also use kelp or greensand both organics high in potassium. I hope this information was useful.
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