It's America's most popular fruit, and we've all been there: waiting on a greenish banana to reach its sweet, yellow zenith. 

But for one of the nation's largest banana wholesalers, ripening is an exact science.

"We ripen about 35,000 boxes a week," said Mark Levin, CEO of the family run, 108-year-old M. Levin & Co.

"It's a lot of bananas."

Millions of bananas pass through the M. Levin ripening rooms at the Philadelphia Wholesale Produce Market.

They flow off ships from Central and South America, most of which dock in Wilmington, Del., Baltimore, Md., or Camden, N.J.

They ripen in climate-controlled rooms, stacked three stories high — all with the help of ethylene gas, a natural ripening agent. After anywhere from three to seven days, your banana is ready for its produce-aisle debut.

"It'll come in green as grass and when we're ready to send it out it'll be half-color — half yellow, half green," said Levin.

Bananas from M. Levin & Co. travel as far west as Ohio, as far south as Virginia and as far north as Toronto. But it all starts in the ripening rooms of Philadelphia.

"You're paying close attention all the time," said David Levin, a co-owner who has been ripening bananas for 37 years. "I may expect them to ripen one way and they are very stubborn and don't ripen that way. So, yes, I have to wait for them."