Pureed pears are a healthy food for babies and a suitable substitute for oil in baking. Although a pear may be creamy white when you slice and core it, it turns an unappetizing shade of brown soon after pureeing. Although it's not pretty, the color change is nothing to fear.
As with other white-fleshed fruits, such as apples or bananas, the flesh of pears turn brown once exposed to air. Enzymes and naturally occurring iron or copper in the fruit react with the oxygen in the air. This chemical reaction causes pears to oxidize much like a nail will oxidize and get rusty. Unlike a rusty nail, however, the "rust" on your pears is safe to eat.
How to Prevent It
Cooking pears destroys the enzymes that contribute to oxidation, but you must work quickly to minimize the time the raw fruit is exposed to air before it is cooked. You may mix lemon juice into the puree to keep it from browning, but skip this step if the puree is for a baby. The acid in the juice can upset their stomachs.