2.2. Apple juice preparation and treatment
Red Delicious apples (Malus pumila Miller) and onion (Allium cepa L.) were obtained from local markets (Gyeongsan, South Korea). In order to prepare fresh apple juice, the pulp of apples was homogenized using a stomacher (Bagmixer 400, Interscience, France) at speed 9 for 12 min, followed by centrifugation at 10,000g for 20 min at 4 C. Supernatant was collected as an apple juice sample. Onions were washed and cut into cubes with a size of 0.5 cm3. Moisture level of onion was removed by a freezedryer (FD-1, Eyela, Tokyo, Japan). Freeze-dried onion was pulverized using a mortar. Powdered onions were kept at 20 C for further use.