Flavonoids are a type of phytochemical (meaning a chemical compound found in plants). You can find them in certain fruits such as apples, grapefruit and grapes.. Scientists have found that they may be very useful because, among other things, they help to prevent cancers. Another important class of phytochemicals called isothiocyanates — found in broccoli, cauliflower and brussel sprouts — may help to prevent tumour growth.