It’s a few days before Yom Tov and the green esrog you bought last week is still quite green! This is what you can do to quicken the ripening process to make it yellower.
Take the esrog and place is in a bag or box with fragrant yellow apples, seal the bag/box and leave it in for 24 hours. After that period of time, you should see that the esrog has begun to turn yellow!
Keeping it longer than 24 hours can cause the stem to fall off, so keep on eye on the esrog. If the apples are a few inches away from the stem, it should avoid this problem.
Why it works: Apples produce a gas called ethylene when ripening, and other fruits that are stored in close proximity to apples will ripen more quickly because of the ethylene. Other fruits, such as avocados, bananas, and pears are also rich in ethylene.