…when you’re just going to cook them in the oven anyway? A good question, reader Toni. The answer is: enzymes. Pears will turn brown when they’re baked if they aren’t poached first. That browning is a result of enzymes that activate when the flesh of the pear is cut.
In an uncut pear these browning enzymes (polyphenoloxidase and peroxydase) are kept inside special compartments in the flesh of the pear, away from both oxygen and the compounds these enzymes commonly interact with, molecules called phenols. The trouble comes when those tiny structures are damaged by, say, cutting or bumping the fruit. At that point the enzymes are freed from their little holding cells and start to run wild. With plenty of oxygen at their disposal (which they need in order to do their job) they hop from phenol to phenol breaking each one into pieces. The resulting wreckage is a variety of smaller molecules, some of which are melanins, or brown pigments. The longer cut pear flesh is exposed to oxygen the browner it will get until all the phenols on the cut surface of the flesh have been sliced and diced, and the flesh is a mottled tan.
How do you prevent this from happening? There are several ways, actually, but one of the most popular is heat treatment which denatures (i.e. “wrecks”) the enzymes, preventing them from getting down to business with the phenols. You immerse the cut fruit in a liquid like water or wine (which itself postpones the browning process by denying necessary oxygen), then bring that liquid up to a simmer for about 15 minutes. Done. And along the way, if you’ve added a little lemon zest and vanilla to the poaching liquid, you’ve also infused some extra aromatics, and that’s very a good thing for your dessert.
So the poaching is both functional and, shall we say…decorative, reader Toni. Hope that helps!