Ethylene gas is the ripening agent that many fruits and vegetables produce naturally. Ethylene causes them to ripen--and then overripen. While refrigeration and humidity slow the effects of ripening, they don't stop the production of ethylene gas. The more the fruit ripens, the more ethylene gas it makes. This has a big effect on how--and when--farmers harvest their fruits and vegetables for market. Most commercial tomatoes are picked before ripening is completed, so the fruit won't spoil before it gets to your market. But picking early also means the tomato spends less time on the vine, where ethylene would help build more of the sugars and acids that create tip-top tomato flavor.