Phenolic compounds and antioxidant activity of blackberry, black raspberry and blueberry seed meals

Berries are highly valued crops due to their unique flavour, texture, colour and phytochemicals. They are rich in phenolic compounds which have been recognized as having beneficial health effects in humans. Phenolic compounds are present in the free, soluble ester and insoluble-bound forms; these were extracted using four different solvents (methanol–acetone–water (7:7:6, v/v/v), 70% methanol, 100 % water, and 80% acetone). The insoluble-bound phenolics were procured after alkaline hydrolysis and subsequent extraction into diethyl ether-ethyl acetate. Phenolic extracts of each fraction were separately assayed for their antioxidant activity using several methods, namely oxygen radical absorbance capacity (ORAC), the reducing power capacity, as well as iron (II) chelation capacity, among others. There were significant differences in the total content of phenolics, flavonoids, and anthocyanins between blackberry, black raspberry, and blueberry seed meals. The bound phenolics contributed the highest proportion to the total contents of different classes of phenolics. Furthermore, blackberry seed meals had higher total antioxidant activity compared with raspberry and blueberry seed meals in all assays employed. High-performance liquid chromatography (HPLC) coupled with diode-array detection and mass spectrometry was used to identify and quantify the phenolic compounds. Hydroxybenzoic and hydroxycinnamic acids, anthocyanins, flavonols, flavan-3-ols, and proanthocyanidins were identified and quantified in the aforementioned fractions. Extracts were found to contain various levels of phenolic compounds that were specific to each berry seed meal type.