Marlene Parrish - What Einstein Told His Cook 2

Restaurants have deep sinks with overflow pipes, in which running cold water circulates around huge pots of hot stock. At home, you can put your pot in a sink filled with cold water up to the level of the stock inside (any higher than that and the pot will float); stir the stock periodically and change the water as it warms up.

Refrigerate the strained, cooled liquid and remove the solidified fat from its surface.That’s not only for the usual fat-phobic reasons, but because while solidifying, the fat will trap any of the oily flotsam that evaded your skimmer. (You’ll see it clinging to the bottom of the cake of fat.) But fat is flavorful, so don’t be ruthless. In the case of chicken stock, especially, you might want to allow some of the fat to remain.

Whatever you do, don’t put any pot of hot stuff directly into the refrigerator. A large pot of liquid contains a lot of calories of heat that will warm up everything in there, encouraging spoilage. Either cool the whole pot as described above, or divide the contents into several small containers, seal them, and let them cool individually before placing them in the fridge. They will cool much faster than if you try to cool a whole potful, because there is more surface area for the cool air to make contact with.

Now freeze your stock in convenient portions to be used for boosting the flavor of soups, sauces, and such thirsty dishes as risotto.

Sidebar Science: Simmering and quivering

A POTis truly simmering when you can see only an occasional bubble breaking the surface. Bubbles are little pockets of water vapor, created at the bottom of the pot where the temperature is highest. They then rise, but most of them revert to liquid and collapse as they cool down on the way up, never reaching the surface. The only ones we consider “real bubbles” are those that make it all the way.

Several cookbooks attempt to define a simmer by stating specific water temperatures, often widely diverging ones, somewhere below 212°F (100°C). But the exact temperature of a simmering pot will depend on the characteristics of the burner, the pot, and its contents, not to mention the altitude of your kitchen and the weather. (At low barometric pressures, water boils at a lower temperature.) And if you’re shooting for a specific simmering temperature, where are you supposed to measure the temperature of a stock? Near the bottom of the pot, where it’s hottest, or somewhere higher up, where it’s cooler?

So forget about trying to achieve a certain temperature and use the small number of occasional bubbles as your criterion for a proper simmer.

French cooks sometimes make a distinction between the simmering of soups or stews that contain solids, and the simmering of liquids such as water, milk, or thin sauces. In the former case, they use the verb mijoter , which is more or less equivalent to the English verb simmer . But when the entire surface of a pot of liquid is clearly visible to the cook, undisturbed by icebergs of meat and vegetables sticking up, there is a discernible pre-simmer or pre-bubble stage that the French call frémir , meaning to quiver or to tremble.

If you look closely at a pot of heating water as it approaches a simmer and before any bubbles break the surface, you will see the surface quiver—or, as some would have it, smile. The quiver is caused by convection currents, plumes of hot water rising through zones of cooler water, giving up some of their heat to the air when they reach the surface, and then, being a bit cooler than before, falling back down. The slight disturbances of the liquid’s surface as these plumes reverse their direction creates a visible quivering effect.

An egg may be coddled or minimally cooked by frémissement ( frémir- ing it) rather than by mijotement ( mijoter- ing it), because it is completely submerged in the water. The average temperature of the cooking water will be slightly less than at a simmer.

BACTERIA IN SUITS OF ARMOR

Why all the cautions about cooling a soup or stock quickly to prevent the growth of dangerous bacteria? After all, the stuff has just been simmered for more than an hour. Wouldn’t that have sterilized it, as long as I keep it covered while it cools so that new bacteria don’t drop in for dinner?

Unfortunately, no. Not all bacteria are killed at 212°F (100ºC). Some of them can survive by protecting themselves within virtually invulnerable coatings. They’re then called spores.

Most species of bacteria reproduce by binary fission, each organism splitting into two whole new organisms. That’s why they can grow at exponential rates. Once they get started, bacteria can increase their numbers from, say, 5,000 to 10,000 to 20,000 to 40,000, and so on, doubling as often as every ten minutes, until they can reach as many as 10 billion in every milliliter (one-thirtieth of an ounce) of your soup or stock by the time they run out of nutrients.

But when conditions are not conducive to their growth, or are even out-and-out hostile, some species of bacteria (and fungi) can ride it out as spores—dormant and virtually indestructible forms. Protected by tough, horny suits of armor, the spores are capable of surviving such calamitous surroundings as boiling water, nutritional deprivation, dryness, freezing, ultraviolet light, corrosive chemicals, and even heavy-metal rock music. When conditions improve, such as when your stock cools to a comfortable growth temperature, the spores can transform themselves into whole new individuals that will resume reproduction in the normal way.

A common pathogenic genus of spore-forming bacteria found in soil, water, and the intestinal tracts of humans and animals is Clostridium, especially the species C. perfringens, which is a major cause of food poisoning, and the much rarer C. botulinum, which produces botulin toxin, one of the most potent poisons known. Clostridium bacteria don’t need oxygen to live; in fact, they can’t survive in air, so the interior of a pot of stock is a perfect growth environment for them.

To kill spores, temperatures significantly higher than 212°F (100ºC) are needed. That’s why medical and surgical equipment is sterilized in an autoclave, a sort of pressure cooker. Under higher pressures, water boils at higher temperatures. Pressure cookers and autoclaves are closed containers in which the steam pressure from boiling water builds up enough to raise the boiling temperature to about 250°F (141°C), high enough to kill most bacterial spores.

I have traveled in quite a few countries in which my American stomach was unaccustomed, and therefore vulnerable, to the local…shall we say, wee little beasties that can be found in food. I stuck as much as possible to deep-fried foods (which are often the best local snacks anyway), because oil at 350°F (177°C) will kill almost anything.

The inside of a food can is an excellent oxygen-free breeding place for Clostridium spores. That’s why, after being filled and sealed, canned foods are sterilized by being heated in high-pressure steam kettles or cookers at temperatures of 240 to 250°F (116 to 141°C). If the sterilization isn’t complete and live bacteria grow in the can, they produce hydrogen gas, which can cause the can to bulge. So if the end (the weakest part) of a food can bulges or buckles even slightly when you press on it, use the can to practice your shot-put into the nearest landfill.

A BONE-BENDING EXERCISE

I learned at my mother’s knee to add a little acid—lemon juice, vinegar, or wine—to increase the amount of calcium that would come out of the bones when making a stock. Does it work?