its normal for apples to oxidise after puree. its still safe to consume. to stop browning, use a drip of lemon juice(but lemon juice is acidic n you might not want to give it to babies) or cook the apples before pureeing it. you can boil or steam the apples with or without skin. its better to keep the skin but make sure you wash it carefully. you can steam or boil it. dont overboil though. for me, i usually boil it with just enough water to cover the apples and then boil it. after the water boil, i switch off the fire n cover it up. let it stays for a while n try to poke with a chopstick. once its soft its ready.