They could be in the form of glycosides. Many flavonoid compounds exist in nature bound to several molecules of sugar, which make them more soluble in water.

1000s of water soluble organic compounds are there. Eg Carbohydrates, vit B & C, urea. Polyphenols are generally medium polar in nature. Hydroxy phenolic acids such as gallic acid is readily soluble in water. Glycosylation and salt formation improve the water solubility of phenolics.

Of course they must be "really organic compounds" by definition because they are compounds of carbon. As Mr. Arimboor has mentioned, some phenolic acids are readily soluble in water whereas the flavonol quercetin is not.

Their inherent polarity is dictated by the their molecular size, chain length and functional groups etc. For instance hydroxyl groups (OH) will increase polarity and therefore dissolution in polar organic solvents. Benzene rings with their double bonds will have the opposite effect.

if you have any ideas as to which flavonoids you are looking for, or expect to find, you should look at their chemistry and devise an extraction/isolation procedure to suit.

When you are trying to separate flavanoids(organic compounds) its always better to check its solubility in organic solvents(ranging from non-polar to polar) because water is a universal solvent (next comes DMSO) and when we chose these solvents it makes thing complicated in many case(exceptional cases are the for eg. caffeine is water soluble but can be extracted back by chloroform which is a beneficial point when it comes to purification/crystallization of caffenie.I would suggest methanol is the apt solvent for flavanoids for separation the polarity can varied by adding n-butanol to the TLC mobile phase for separation or add few drops of formic acid

At the molecular level, solubility is controlled by intermolecular forces. However a simple, empirical rule “like dissolves like” and it is based on the polarity of the systems i.e. polar molecules dissolve in polar solvents (e.g. water, alcohols) and non-polar molecules in non-polar solvents (e.g. the hydrocarbon hexane). This is why ionic compounds like table salt (sodium chloride) or compounds like sugar, dissolve in water do not dissolve to any great extent in most organic solvents. The polarity of organic molecules is determined by polar bonds1 due to the presence of electronegative atoms (e.g. N, O) in polar functional groups such as amines (-NH2) and alcohols (-OH). Water-soluble organic compounds include organic anions, dicarboxylic acids, oxo acids, dicarbonyls, carbohydrates, amino acids, aliphatic amines, urea, and some miscellaneous multifunctional compounds containing multiple hydroxy, carboxyl, and carbonyl groups (e.g. glyceraldehyde, malic acid, citric acid, lactic acid, and tartaric acid). Lower molecular weight compounds with polar functional groups are the most water soluble. Methanol (CH3OH), ethanol (CH3CH2OH) and isopropanol ((CH3)2CHOH) are a few examples of low molecular weight alcohols that are completely miscible with water. Dimethylamine, HN(CH3)2 and acetic acid, CH3COOH, are two more examples of organic compounds that are soluble in water.

• Ethanol//water solution in cavitational extractor , like rotocav for example , is an industrial application to valorize agricular waste and to obtain polyphenols. I agree with rajan But methanol is really toxic and you can use it only for analitical application. If u want to utilize the product think that above 10 ppm is the limit for food and pharmacy application.