7What heat does to food

Cooking is heat at work. Heat makes food safe by killing harmful bacteria and other microbes, and it makes food easier to digest. It also changes food in ways you can see and feel.

Heat sets proteins, so a runny egg turns firm. It swells starch, so a thin liquid becomes a sauce. Sugars and fats form crystals, and those crystals decide whether fudge is smooth or gritty and whether chocolate snaps or bends. Learn how heat travels and what it does to each part of a food, and you can control the result.

7.1 How heat moves: conduction, convection, radiation

When food warms up, its molecules take in energy. They vibrate faster and bump into each other. That energy spreads through the food and cooks it.

Heat travels in three ways: conduction, convection and radiation. In real cooking they often work together.

Conduction: heat by touch

Conduction moves heat through direct contact. It is the most common kind. A burner heats the bottom of a pan, and the pan then heats the food sitting in it.

A deep fryer also counts as conduction, because the hot oil touches the food.

It is the slowest of the three. But because the food sits right against the hot surface, it cooks from the outside in. A steak in a cast iron pan, for example, gets an even, browned outside and stays moist inside.

Everyday examples of conduction:

• touching a hot piece of metal and burning your hand
• grilling chicken breasts, pork chops or steak
• dropping steamed vegetables into ice water so they keep their color
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Convection: heat that circulates

In convection, the molecules closest to the heat warm up and rise. Cooler ones move in to take their place, get heated in turn, and rise too. This movement carries heat around.

Natural convection happens on its own. Put a pot of water on the stove. The burner heats the pot by conduction, and the pot heats the water. Warm water moves up and away, cooler, heavier water sinks, and a steady current forms that spreads the heat evenly through the pot.

Mechanical convection uses an outside force to move the heat. Stirring a pot is one example. A convection oven is another: a fan blows hot air over and around the food. Moving the heat this way cooks food faster and more evenly.

Convection also works for thawing. Running cold water over frozen food carries heat into it and thaws it faster. Air at room temperature moving around frozen food does the same job.

Radiation: heat with no contact

In radiant cooking, waves of heat and light hit the food and enter it. Nothing touches the food. There are two main kinds: infrared and microwave.

Infrared heat comes from an electric or ceramic element. Its waves travel at the speed of light and heat food quickly, but they are taken in mainly at the surface. Glowing coals, broilers and toaster ovens all cook this way.

Microwaves are short, high-frequency waves. They go into the food and shake its water molecules, and the friction makes heat. After that, the heat moves through a solid food by conduction and through a liquid by convection.

Because microwaves reach several inches into food, they usually cook faster than infrared heat. They work best on small batches.

Warming your hands at a fire, sitting in the sun and reheating a meal in a microwave are all examples of radiant heat.

7.2 Water: boiling points and dispersions

Water carries heat to food on the range top. Two things about it are worth seeing for yourself: what water looks like at different temperatures, and how things mixed into water change it.

Temperature words

Start by checking your thermometer in boiling water. Then heat a pan of water and watch it change. These terms name what you see:

• Lukewarm: 30 to 37°C.
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• Simmering: 85 to 99°C. Look at the surface of the water to learn what a simmer looks like.
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• Boiling: 100°C. A slow boil and a rapid boil (also called a rolling or fast boil) look different on the surface.
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To change between the two scales, use these formulas: °C = (°F − 32) ÷ 1.8, and °F = (°C × 1.8) + 32.

Hotter or cooler than a boiling pan

Some pans hold water below boiling, and some push it above.

• Double boiler. Water in the top pan sits over boiling water in the bottom pan, and the bottom water should not touch the top pan. The top water reaches 190 to 198°F at most. Think about whether the top water would boil if the bottom water touched it, and what jobs suit this gentle heat.
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• Steamer. A thermometer in a cup of water set in the top of a steamer reaches 91°C. Compare how fast food cooks in a steamer and in a saucepan.
• Pressure saucepan. Pressure raises the boiling point: water boils at 109°C at 5 pounds of pressure, 240°F at 10 pounds, and 121°C at 15 pounds. Think about how this changes cooking time compared with a normal saucepan.
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Dispersions

The questions to answer from your results:

• What kind of dispersion does each added substance make, and how does it change the boiling point?
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• Why might two substances in the same kind of dispersion change the boiling point by different amounts?
• What would each one do to the freezing point?
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7.3 Proteins set: coagulation

The change often starts at about 38°C (100°F) and is finished by 71–82°C (160–180°F). In the bakeshop, three groups of protein do this work: egg proteins, dairy and soy proteins, and the proteins in flour.

Eggs

An egg holds many proteins. The white, called the albumen, has about 40 of them. The largest share is ovalbumin, at 54%, then ovotransferrin, at 12%. The yolk is mostly fat (lipids), with some lipoproteins.

Each protein sets at its own temperature. That is how you can cook an egg with a firm white and a soft yolk:

Milk and soy

Cheese is built on casein, the semi-solid part of milk that forms when milk coagulates. To set milk for cheese, cheesemakers use rennet. Rennet is taken from the lining of the stomach in cattle, sheep and goats, and there is also a plant-based kind. The enzyme that does the work is chymosin, also called rennin. It curdles the milk, which then splits into solid curds and liquid whey.

Acid also sets milk:

• lemon juice (citric acid) or vinegar, for fresh ricotta
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• tartaric acid, for mascarpone
• lactic acid, which builds up as milk sours, so sour milk curdles by itself
• bacteria that make acid, added on purpose to make yogurt or crème fraîche (a thick, slightly sour cream)
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Tofu is the soy version. Soybean milk is set with salt, with acid, or with an enzyme.

Flour

Wheat flour carries two main proteins, glutenin and gliadin. Other grains have smaller amounts. Mix them with liquid and they join into gluten, which stretches. In the oven, heat firms the gluten, and it hardens into the solid frame of the bread. That firming is coagulation too.

7.4 Starches swell: gelatinization

Hydrocolloids

A hydrocolloid is anything that forms a gel when it meets water. Hydrocolloids come in two groups:

• Thermo-reversible gels melt when you heat them again and set again as they cool. Gelatin and agar agar behave this way.
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• Thermo-irreversible gels stay set when reheated. Cornstarch and pectin behave this way. Too much heat can still dry the gel out and make it shrink.

A hydrocolloid does not take up water all at once. As it takes water in, it swells, and swelling grains easily stick together in lumps. So it must be spread through the liquid first. The classic way is a slurry: stir the starch into a little of the recipe's liquid, then add that to the pot. A stick blender or a regular blender also works, and so does mixing the powder first with sugar, oil or alcohol.

Starch gelatinization

When starch and water are heated together, the starch grains (granules) swell and soak up the water. This cannot be undone. The liquid turns thick and see-through, and the result is a gel. Sauces, puddings and creams all rely on it for their texture.

You see gelatinization in three places:

• Sauces. Starch goes into a liquid, usually as it heats. It soaks up liquid, swells and thickens the sauce. The starch you pick sets the finish: some stay cloudy, others turn clear.
• Pasta. Pasta is made mostly from semolina, a durum wheat flour rich in starch. In boiling water the starch takes in water and swells, and the pasta goes soft.
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• Baked goods. Starch makes up most of a baked good. It cannot hold up the shape alone, but it gives the structure its bulk. Baked starch is softer than baked protein. The soft inside (crumb) of bread comes mostly from starch, and the more protein structure a bread has, the chewier it is.
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Thickeners and gelling agents compared

Some starches are simple plant extracts, such as cornstarch, tapioca and arrowroot. Others are modified starches or pre-gelatinized starches, made for special jobs.

These amounts are for thickeners and gelling agents used in the bakeshop.

Gelatin, agar and pectin

Gelatin is a protein taken from animal tissue, from the collagen in bones and meat. It dissolves in water and does several jobs:

• it gels
• it thickens
• it emulsifies
• it works as a whipping agent
• it stabilizes
• it gives food a smooth feel in the mouth

You can buy it as powder or as sheets, also called leaves. It firms up whipped cream and mousse, gummy sweets and marshmallows, and panna cotta, and it turns up in low-fat spreads, yogurt and ice cream.

Agar agar comes from red algae. It thickens or gels liquids and keeps foams and emulsions stable. It takes up water in boiling liquid and copes with a wide range of acidity. It starts to set once it cools to about 40°C (100°F), and it does not melt again until 85°C (185°F).

Pectin comes from citrus and from tree fruits such as apples and pears. It is found in jams, jellies, sweets, fruit juices and milk drinks. Cooks mostly use two kinds:

• High-methoxyl pectin needs sugar to thicken. It is the traditional pectin for jams and jellies.
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• Low-methoxyl pectin needs calcium to gel. Modern kitchens like it because it reacts well to calcium and forms a gel that heat does not undo.

7.5 Sugars and fats crystallize

Many foods contain crystals. Butter, margarine, ice cream, sugar and chocolate each have their own kinds. Ice cream alone can hold fat crystals, ice crystals and sometimes crystals of lactose, the sugar in milk.

Crystals decide whether a food spreads, and whether it feels smooth or gritty in your mouth. Sometimes a cook wants them, and sometimes a cook works hard to stop them.

Sugar

Candy comes in two families:

• Crystalline candies keep crystals in the finished sweet. Fudge and fondant are examples.
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• Non-crystalline candies have none. Lollipops, taffy and caramels are examples. Their recipes are built to stop sugar crystals, because crystals would make them grainy.
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To keep table sugar (sucrose) from crystallizing, give it other sugars to get in its way. Fructose and glucose are the usual ones. An acid can also be added to "invert" the sugar, and fat added to some sweets has a similar effect.

When you boil sugar, you almost always want to avoid crystals. These encourage them:

• a dirty pot or dirty tools
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• sugar with bits in it, such as a scoop that went into the flour bin first
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• hard water, which is high in minerals
• stirring too much while the sugar boils
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To head crystals off, add an interferent while the sugar boils: an acid such as lemon juice, vinegar or tartaric acid, or glucose or corn syrup.

Ice cream

Ice cream is a mix of parts: ice and fat crystals, an emulsion, air cells and stabilizers such as locust bean gum. Together they give the ice cream its body. Lots of small ice crystals give it a smooth texture that melts and cools well. If those crystals grow bigger in storage, called recrystallization, the ice cream turns coarse and less pleasant. Sugar crystals in fondant and frosting, and fat crystals in butter, margarine and chocolate, raise the same worry.

Fats and chocolate

In fats such as butter, margarine, shortening and chocolate, crystals shape how the food looks, its texture, how it spreads and how it releases flavor. To control them, you control three things:

• how many crystals there are, and how big
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• how they are spread through the food
• their shape, called the polymorph

Chocolate shows this best. Tempering, also called pre-crystallization, coaxes the cocoa butter to set in one particular crystal pattern. That pattern keeps the chocolate shiny and gives it the right texture for a long time. It matters for moulding and decoration, and for how the chocolate feels when you eat it.

Badly tempered chocolate shows it in one of several ways:

• It never hardens at room temperature.
• It hardens but looks dull and patchy.
• It is spongy inside instead of crisp.
• It gets fat bloom: fat moves to the surface and leaves white streaks and patches.
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7.6 Thick and thin: viscosity

Chocolate that flows

Chocolate is sold in different viscosities, and a confectioner picks the one the job needs. The amount of cocoa butter is the main thing that sets how runny chocolate is. Emulsifiers such as lecithin also thin melted chocolate so it flows smoothly, and since lecithin costs less than cocoa butter, it can bring the price down.

Match the chocolate to the job:

• Thinner chocolate fills molds such as Easter eggs, because it runs easily into the shapes. It also suits a thin, attractive, protective coat on cookies and most cakes, and the dipping machines called enrobers.
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• Thicker chocolate suits ganache and flavoring creams and fillings.
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Viscosity differs from one maker to the next, and a single type of chocolate may come in more than one. Bakers sometimes change it themselves. A little vegetable oil thins chocolate used to coat squares, which makes them easier to cut later.

Chips and chunks are made to stand up to heat, so they keep their shape in cookies. Some fine chips hold as much as 65% chocolate liquor, yet above about 40% they tend to smear as they bake.

Making things thicker

Bakers thicken with two kinds of agent: starches and gums. Nearly all come from plants. Gelatin is the one exception. Starches all come from land plants, while some gums come from the sea.

Bakers thicken things for three reasons: so fillings are easier to work with and bake, to firm products so they can be served neatly, and to give a glossy skin that looks good and keeps the food from drying out.

7.7 The methods at a glance

Every cooking method applies heat in one of three ways: through moist heat, through dry heat, or by combining the two. Get to know how each one works and you will cook with more confidence.

Moist heat

Moist-heat methods cook food in hot liquid or steam. They depend on time and temperature, so learn the range for each.

• Poaching (160–180°F). The food sits fully under liquid that never goes above 180°F. Either start the food in cold liquid and bring it slowly up to about 170°F, or bring the liquid to a boil, add the food and take the pan off the heat straight away. Both protect the food's quality.
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• Simmering (180–205°F). A long, slow method for tougher cuts of meat, which also tend to cost less. At the right temperature you lose less liquid and stay in control, and a lid cuts evaporation further. Simmering draws out the most flavor and makes protein as tender as it can be.
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• Boiling (205–212°F). A true boil is 212°F. For better control, think of 205–210°F as a gentle boil and 210–212°F as a rapid boil.
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• Blanching. Food cooks briefly in boiling water at 212°F, or in hot fat. After water-blanching, food is usually "shocked": plunged into ice water to stop the cooking.
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• Steaming (212°F and above). Steam is one of the hottest cooking mediums, which is why pressure cookers cut cooking time by about two thirds. Steaming is also recommended for keeping vitamins and nutrients, because they are not washed out.
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Blanching does several jobs:

• It speeds up the final cooking.
• It makes cooking more even.
• It brightens color.
• It helps keep vitamins and nutrients.
• It helps food last longer.
• It can make vegetables easier for some people to digest.
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• It can improve flavor.

When potatoes for French fries are blanched in hot oil instead, they are not shocked. The oil draws out water, stops them darkening and makes them crisper.

Combination methods

Stewing starts by searing the food in hot fat. It then simmers fully under a flavored liquid such as stock or broth, low and slow.

Braising (oven at 275–325°F) also starts by searing and browning meat or vegetables in hot fat. Then the food cooks slowly, covered, in a pot or roasting pan with only a little liquid. It suits tougher, cheaper cuts. For a pot roast, the liquid should reach halfway up the meat, and the roast is turned over halfway through. Braising works on the stove, but the even heat of an oven is better; bring the liquid to a simmer before the pot goes in. A common setting is 300°F for three hours, though a different cut, or a bigger or smaller piece, changes this.

Dry heat

The temperatures above are the working ranges for each dry-heat method.

For indoor smoking:

• Soak the wood chips in water for thirty minutes.
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• Drain them, pat them dry and spread them over the bottom of a roasting pan.
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• Set a wire rack over the chips and the food on the rack.
• Cover tightly and seal with foil.
• Heat on the stove until the chips smoke.
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• Keep the heat steady and finish on the stove or in the oven.
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Smoking dries food out, so marinate or brine proteins first.

Grilling and barbecuing are not the same. Barbecue is slow cooking over hot coals or wood embers at low heat for a long time, so it suits fattier, cheaper cuts. Barbecued food may be finished on a grill or under a broiler, and barbecue sauce goes on near the end or comes on the side. Grilling is fast, over high heat.

Broiling needs very little fat or liquid, so it can be a low-fat way to cook. Marinated food does well under it. It also browns food, toasts breadcrumbs and melts cheese on top of a dish, called a gratinée.

Dry heat with fat

These methods differ mainly in how much fat they use.

• Sautéing (350–400°F). Sauté is French for "to jump": the food is tossed in the pan, and the pan's sloped sides help. Use very high heat and a little fat, about 1 to 1½ ounces in an 8- to 10-inch pan. Heat the pan, add the oil, then the food. Do not crowd the pan: pieces of protein should not touch. A crowded pan traps moisture, makes steam and stops browning. If you dust meat with seasoned flour, do it just before cooking, never ahead, or the flour turns wet and gummy.
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• Pan frying (325–400°F). Use a shallow pan with straight sides and a moderate amount of fat, about 360°F. The fat should come halfway up the food. Too much fat lifts the food off the pan, and it is contact with the pan that browns it. Fry one side, flip, and fry the other. Finish thick pieces, or pieces on the bone, in the oven. Fish cakes, chicken pieces and fritters suit this method.
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• Deep frying (350–375°F). The food goes completely under hot fat, best kept at 360–375°F. Keep the temperature steady, or results will vary. Never overload the basket, because that drops the oil's temperature sharply. Choose an oil with a high smoke point, such as vegetable or peanut oil. Afterwards, strain, filter and cool the oil before refrigerating it.
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• Pan searing (400–450°F). Uses the least fat: a light spray of oil on a preheated pan or on the food, or a food straight from its marinade, such as a tuna steak. Sear one side, turn, and finish the other. Move a very thick piece to a low oven to finish.
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Whole butter is a poor frying fat at high heat: the milk solids in it start to burn at about 175°C (350°F), while clarified butter, with those solids removed, handles about 230°C (450°F).

Whole butter can stand in for clarified butter if oil is added, half and half, to raise its smoke point. That advice is also called a myth: mixing oil into butter does not raise the temperature at which the butter burns, and can make it burn even faster. When a recipe calls for clarified butter at high heat, use clarified butter.

With whole butter, put the food in once the butter has melted and stopped foaming.

Microwaving

A microwave oven sends small waves of energy into food. They make the water molecules move fast, which creates friction, and the friction cooks the food. That is why dried foods with no water in them have to be rehydrated before a microwave can cook them.

The microwave is very good at thawing frozen food quickly, so the food spends less time in the "Danger Zone" of temperature before it is cooked. Cook thawed food soon after thawing it. Steamed vegetables, corn in its husk and potatoes all do well. Turn food as it cooks so it heats evenly, use several short bursts instead of one long one, and cover food you reheat to hold in steam and moisture.