Ethylene is typically not applied to apples, as ripening is not often required in order to achieve great eating quality. However, an improvement on early-season apples may help in converting some of the starch in apples to sugar.
Before attempting this, be sure to ripen a small batch of your apple type and variety. Different types of apples respond differently to apples. Also, this is a seldom-used process and, unless you have experience with ethylene application on apples, a test of ethylene application on a small amount of apples is wise. This way you can ensure that the results, firmness, and shelf life meet your requirements.
For more information on apple ripening, visit the web site of the University of California Postharvest Technology Center: Produce Fact Sheets
Should you need further information, contact the Postharvest Technology Center.