Bad milk smells and taste sour. baby wont take bad milk at all. If it has a strong after taste and smell like soap or a fishy smell, your milk has high lvl of lipase. It is okay for baby to take milk w high lipase. If your baby resist the taste, you might wn to mix it with fresh milk. Another way is to scald e milk before freezing. Heat e milk until you see small bubbles at e side, DONT BOIL THE MILK. However scalding will cause e milk to lose some of its good component. It's still healthier then formula though. Hope it helps!