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The same phenomenon is at work when you toast a marshmallow over a campfire: wait and turn, wait and turn... then brown, black and - poof! - it's aflame. The problem is perhaps most acute when cooking shiny-skinned fish on a grill or under a broiler. Once the skin turns from silver to brown, the heat pours into the fillet, and the window of opportunity for perfect doneness slams shut with amazing speed.
Anytime you cook light-colored food with high heat, inattention is a recipe for disaster. But the physics here is pretty simple, and once you understand it you can use several methods to improve your odds of making that perfectly toasted bun, golden half-melted marshmallow, or juicy grilled fillet.
Radiant in when cooking
At high temperatures - about 400 F (200 C) and up - a substantial part of the heat that reaches the food arrives in the form of infrared light waves rather than via hot air or steam. The higher the temperature, the bigger the part that radiant heat plays in cooking. But this form of heat interacts with color in a profound way.
The bottom of a hamburger bun looks white because it reflects most of the visible light that hits it, and the same is true for infrared heat rays. There is a reason that white cars are popular in Phoenix - they stay cooler in the sunshine, which is full of infrared radiation.
A silvery, mirror-like fish skin is even more reflective than a white car. About 90% of the radiant heat striking it simply bounces away. Because only around 10% of the energy sinks in and warms the fish, cooking initially creeps along slowly but steadily.
That changes rapidly, however, as soon as the food gets hot enough to brown. It's like changing from a white shirt to a black shirt on a sunny summer day. As the food darkens, that 10% of energy absorbed rises by leaps and bounds, and the temperature at the surface of the food soars.
So browning accelerates, which increases heat absorption, which boosts the temperature; it's a vicious circle. By the time you can get a spatula under the fillet to flip it over, it may be almost black, reflecting just 10% of the heat and sucking in 90%.
There are at least three ways around this problem. The simplest is to stare, hawk-like, at the food and lower or remove the heat as soon as browning starts. That works fine for marshmallows but is not always practical in the kitchen or backyard braai.
Practical example of shortening cooking time
In some cases, you can darken the color of the food at the start, for example by slathering it with a dark sauce or searing it in a very hot skillet before putting it on the grill. This is a way to make a fish steak cook more like a beef steak, which is fairly dark even when raw and so doesn't experience such a dramatic shift in heat absorption. This method generally shortens the cooking time.
Finally, try piling other ingredients, such as sliced onions or zucchini, between the food and the coals or the broiler element to moderate the intensity of the radiant heat. Cooking times will lengthen - and you may end up having to toss out the sacrificial buffer ingredients if they get charred - but that window of opportunity will stay open longer.
