Why Pears Brown

• Pears brown when they're cut because of three factors. The first is phenolic compounds inside their cell walls, which provide some of the pears' distinctive flavor and aroma. The second is enzymes contained within the pear, and the third is oxygen from the outside air. Ordinarily, these three components are kept separate by the fruit's cell walls. However, once you bite or cut into the pear, the three mix. The enzymes in the pear react with oxygen and the phenols to produce the browned areas, which act as a defense against worms, insects and other natural pests.

Browning in Purees

• Turning pears into a puree creates a worst-case scenario for browning. Instead of rupturing a relatively few cell walls, as you would when peeling or biting into a pear, pureeing disrupts a very large percentage of the cells in your pears. It also mixes them rapidly, encouraging the interaction of phenolic compounds, enzymes and oxygen. In some cases, the puree might brown noticeably before you've even reduced it to your desired consistency. There are two approaches to countering this, depending whether you prefer a cooked or uncooked puree.

Uncooked Puree

• To minimize the browning of an uncooked puree, start by refrigerating your ripened pears. The browning reaction is slower at cold temperatures. Second, don't peel and core your pears until you're ready to start. Dip their cut surfaces into a bowl of water that's been slightly acidified with lemon juice, because acidity also slows the browning effect. As you puree the pears in your food processor or blender, add additional acidity through either a few drops of lemon juice or larger quantities of apple or pear juice. Refrigerate your puree immediately, and portion it for freezing if the whole quantity won't be used within the first several hours.

Cooked Puree

• Although cold and acidity can slow the effect of the browning enzymes, cooking can destroy them completely. To make a cooked puree, peel and core your pears and dip them in lightly acidic water to inhibit browning. Steam the pears for three to five minutes, until they're very tender, then transfer them to your blender or food processor. Process the pears to a smooth puree. It will retain its clean, pale color for several days in the refrigerator. If you'd like a thicker puree you can either simmer it to evaporate any surplus moisture, or drain excess juice through a coffee filter or several layers of cheesecloth. Retain the drained juice for drinking, baking or cooking.