Preparation of peroxidase and phenolics using discarded sweet potato old stems.

Sweet potato (Ipomoea batatas L.) is the sixth most important food crop in the world. The industry discarded huge amount of sweet potato stems, rich of peroxidases and phenolics. A simple procedure was developed to make peroxidases and phenolics from sweet potato old stems. Dried stem powder was loaded into columns with water and eluted sequentially with water and 50% ethanol. Peroxidases (91%) were extracted in 5.5-fold water extracts and 87% phenolics were extracted in 4.4-fold ethanol extracts. Purified peroxidases powder was yielded at 3.1 g (8.6 unit/mg) per kilogram stems by PEG6000/Na₂SO₄ aqueous two-phase purification from the water extracts (93.2% recovery), followed by ethanol precipitation and vacuum freeze-drying. The purified peroxidase had high activity in transforming tea catechins into theaflavins. Phenolics powder containing 43% phenolics and 27% flavonoids was yielded at 76.9 g per kilogram stems after vacuum-concentrating the ethanol extracts. This method can make valuable functional products using the sweet potato waste.