What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (47 page)

Caveat emptor
department: As with everything else, there are high and low qualities of silicone bakeware. Remember that “silicone” isn’t a single chemical material. Dow Corning, for example, sells dozens of different silicone formulations with different properties, for fabricators to use in molding their commercial products. Some may not be as heat-resistant as others, so check the maximum temperature ratings on the labels. They can range from 450°F (232°C) up to 675°F (357°C) for silicone trivet pads.

Recipes are always telling us to roast something at a certain temperature for a certain length of time. But then they tell us to test it near the end to see whether it’s done, and in my experience it almost never is. Shouldn’t the recipe developer be able to give me a more exact cooking time?

....

T
he quick and dirty answer is no; there are just too many uncontrolled variables.

The cruel fact of life is that when a recipe tells you to cook for “
x
hours at
y
degrees,” it’s only a guideline, an educated guess, a ballpark estimate. It’s what worked—most of the time—for the elves who tested the recipe, but there’s no guarantee it will work for you. So, sorry, Virginia, but there is no Santa Claus. (I’ve been wanting to straighten that kid out for years.)

Except perhaps in a food research laboratory, there is no such thing as a standard roast on a standard rack in a standard pan at a standard position in a standard oven at a carefully regulated oven temperature. Each one of these factors can vary, producing different results even if all other things were equal. But as Wolke’s Law of Pervasive Perversity says, “All other things are never equal.”

You can’t just go around saying that a beef or pork roast or a chicken or turkey should be cooked for so many minutes per pound at a certain temperature. Even if Wolke’s Law didn’t apply and you could magically control everything else, the one variable that you have no control over is the most important one: the shape of the roast. Not its weight but its
shape
: how much surface area it presents to the oven’s heat. Heat can enter the meat only through its surface, so the more surface area a roast has for its weight, the faster it will cook.

Here’s an example.

If we had two roasts of the same weight—that is, the same volume—one shaped like a cube and the other shaped like a sphere, the cubic roast would have 24 percent more surface area than the spherical one. That’s just geometry. Work it out yourself if you get your kicks that way. For my part,

I never saw a cubic cow

I never hope to see one.

But I can tell you anyhow,

It’ll roast about 24 percent faster than a spherical one.

Another example: Suppose we cut that cubic roast in half parallel to one face. Its surface area will then be increased by 33 percent. The two halves, then, should cook in roughly 33 percent less time than the whole one.

So again, dear, naïve little Virginia, no Santa Claus, or even a reasonably good fairy, exists who can weigh your irregularly shaped rib roasts or turkeys and tell you exactly how many minutes per pound to cook it, even if Wolke’s Law were repealed.

I want to roast a piece of meat in an oven for 24 hours at 180°F (82°C). Would this use less gas or electric energy than roasting it for 3 hours at 375°F (191°C)? How about 6 hours at 250°F (121°C)?

....

T
his may sound like an odd question, but it was asked of me by the food authority and author Paula Wolfert when she was working on her book
The Slow Mediterranean Kitchen: Recipes for the Passionate Cook
. Her concept was that long, slow cooking can produce tender, juicy, flavorful meats that higher-temperature cooking cannot match. And as usual, she’s right, as the recipes in her book amply demonstrate (although none of them approaches 24 hours of cooking).

It has always been an oversimplification to say that cooking time and cooking temperature are inversely proportional to each other—that the same, or similar, results can be obtained in a short time at a high temperature as for a longer time at a lower temperature. That concept is woefully inadequate, except over a very limited range of times and temperatures, because cooking is not a matter of simply injecting a given number of calories of heat into a food. As the old jazz song would have it, “It ain’t whatcha do, it’s the way hotcha do it.”

At the time of our discussion, the world was in one of its periodic energy crises, and Paula worried that long, low-temperature roasting might use more energy than shorter, higher-temperature roasting. Fascinated, I leapt at the challenge. Rather than taking the experimental approach, spending days in the kitchen after turning off all electrical devices in the house (it’s amazing how many there are, if you count them) except my electric oven and recording the readings on the electric meter, I decided to take the theoretical approach and try to solve the problem mathematically. Here’s what I came up with.

There are two energy-consuming stages in roasting meat: preheating the oven to the roasting temperature and maintaining that temperature during the roasting period.

It will obviously require more energy to preheat the oven to the higher of the two temperatures. (The actual difference in energy usage will depend on the characteristics of the individual oven.) But in either case the preheating time is short compared with the total roasting time, so we can probably neglect that difference. The difference in preheating times does, however, work in favor of less energy consumption by the low-temperature method.

During the roasting period, the oven will be persistently trying to cool down by losing heat to its surroundings. But whenever its temperature falls to a certain level, the oven’s automatic temperature control feeds in gas or electrical energy to replenish the heat that was lost. Thus, over the entire roasting period, the total energy
input
should be equal to the total energy
lost
by cooling. I could then obtain the energy usage under the two roasting conditions by calculating the rates of energy loss by cooling. The average rate of cooling (in calories per hour or Btu’s per hour) times the number of roasting hours should give me the total amount of energy used.

For my calculations I used Isaac Newton’s Law of Cooling (yes,
that
Isaac Newton), which says that the rate of cooling of a hot body is proportional to the difference in temperature between the body and its surroundings. In this case, the “body” is the air inside the oven, and its surroundings are the air in the kitchen. (The intervening oven walls slow the transfer of heat but don’t change the amount of heat that is ultimately transferred.)

Because all the heat-transfer parameters will differ from one case to the next, I can’t calculate absolute amounts of energy loss. But from Newton’s Law, I can calculate the
break-even time
: the number of slow-roasting hours at which the energy usage becomes equal to the energy usage in the fast-roasting method. If we slow-roast any longer than this, we will be using more energy than in the fast method.

Here are the results of my calculations. (Gluttons for mathematical detail may consult “(Warning: calculus ahead)” on p. 413.)

In Paula’s first example, the energy break-even point for slow roasting at 180°F comes out to be about 9 hours. Thus, roasting for 24 hours at 180°F will use substantially more energy than roasting for 3 hours at 375°F. But 24 hours at 180°F is a rather extreme set of slow-roasting conditions anyway.

In Paula’s second example, the energy break-even point for slow roasting at 250°F comes out to be about 5 hours, which is close enough to Paula’s desired 6 hours. So go for it, Paula! The energy police will not break down your door.

What I’ve found, then, is that long, slow roasting need not use more energy than faster, higher-temperature roasting, provided that the slow roasting is not done at too low a temperature. Somewhere between 225 and 250°F (106 and 121°C) is probably the lowest practical limit. But if energy consumption isn’t an issue, by all means pull out the stops and cook your roast at any temperature above about 165°F (74°C) which is hot enough to kill most surface germs. Or do as Paula recommends in
The Slow Mediterranean Kitchen
: Blast or sear the surface of the meat first to take care of any surface germs before you lower the oven to roasting temperature.