Weird food science

I love these kinds of Mr. Wizard questions about why food does what it does. Let's pretend someone has asked me the following questions:

Why are some of the potatoes in my bag green? Are they okay to eat? How do you keep honey from crystallizing? My guacamole turns black when I leave it in the refrigerator overnight. Wassup with that?

Well, kids, let me tell you. When raw potatoes are exposed to light, they can turn green. The color itself, which comes from chlorophyll, is benign. Unfortunately, that’s not the end of the story. A compound called “solanine” forms during this greening process, a compound that is both bitter and toxic. Avoid eating any potato skin or flesh with a green tint—just keep peeling until you reach snowy white potato. In order to prevent this greening process, store potatoes in a cool, dark, well-ventilated place, but preferably not in the refrigerator. In the refrigerator potatoes starches can turn to sugar, resulting in overly sweet potatoes and excessive browning during the cooking process. Don’t wash potatoes before storage (this hastens spoilage), and don’t store potatoes near stored fruit. Many fruits emit ethylene as they ripen, which in turn causes potatoes to sprout.

Honey is primarily made of two simple sugars, glucose and fructose. Those containing a greater proportion of glucose are likely to crystallize over time, especially at low temperatures—the glucose spontaneously precipitates out of the supersaturated sugar solution, leaving hard granules throughout. Most supermarket honeys have been filtered and cooked to decrease the likelihood of crystallization, but “raw” honey is especially susceptible. Store your honey in a sunny, warm spot, never the refrigerator or cold basement. If it does crystallize, all is not lost. Gently warm the honey in a double boiler to about 120 degrees F, stirring until the glucose crystals liquefy.

As per guacs, a vibrant green quickly turns to unappetizing brown when avocado is exposed to air. So how to shield your guacamole from oxygen becomes the challenge. The acid in lime juice retards the process—squeeze extra lime juice over finished guacamole to create an oxygen “barrier.” A swirl of olive or avocado oil achieves the same effect, as does plastic wrap nestled directly against the surface of the dip (not merely over the top of the bowl). Some avocado enthusiasts claim that storing the pit with the guacamole prevents browning, but it really only prevents browning in the guacamole it touches, again by shielding it from oxygen. Even after you’ve protected your guacs, browning will occur over time. Before serving, scrape the topmost layer of guacamole into the trash, revealing the still-vibrant dip below.