Quercetin is a plant flavonol from the flavonoid group of polyphenols. It is found in many fruits, vegetables, leaves, seeds, and grains; capers, red onions, and kale are common foods containing appreciable amounts of it. It has a bitter flavor and is used as an ingredient in dietary supplements, beverages, and foods.
Peel of 3 lemons
Peel of 2 grapefruits
Option 1: Chop up all the peel into a pan of water, bring to the boil and simmer for 2 hours.
Option 2: Chop up all the peel and freeze it. When ready, place in a pan of water and allow to steep for 24 hours.
Either freezing or boiling breaks up the cell walls to release the quercin which is then soluble in water.