Cooking science, oriented

Pans, ovens, broilers and fryers deliver heat to the outside of food in three ways. Frying oil and oven air carry it by convection. A broiler or glowing coals send infrared radiation, which reaches only a few millimeters in. A pan passes heat by direct contact where metal meets meat, and many methods mix all three. Whatever happens at the surface, conduction moves the heat on through a solid food, so a thick piece heats from its outside toward its middle.

Water sets a ceiling. At sea level it boils at 212 °F (100 °C), and any part of a food that still holds water stays at or below that point. A layer gets hotter only once it dries out, and a bigger flame just drives the water off sooner. Frying oil beats the cap because it carries heat at temperatures water cannot reach. The Food Safety and Inspection Service (FSIS) gives 350 to 375 °F (177 to 191 °C) as the oil range for chicken. Surface water leaves as vapor, and a crust builds.

Suppose you sear steaks in a skillet. The cold meat draws heat from the pan, and for a given load, how far the pan cools depends on its heat capacity, specific heat times mass. Twice as much of the same metal drops only half as far. Conductivity, how readily heat crosses the metal, is a separate property, and copper and aluminum both outdo stainless steel on it. Raw beef is roughly three-quarters water, and the United States Department of Agriculture (USDA) notes that wet meat browns poorly. So blot the surface dry first: a wet patch is stuck at the boiling point while its water lasts.

The trap is crowding. More cold pieces pull out more heat, so the pan's temperature sinks further, and every piece adds its own water. USDA warns that overloading a fryer basket risks food that is undercooked and cooked unevenly. In an air fryer, tightly packed food can also block the moving air that crisps it. Cooking in smaller batches keeps that drop shallow.

Rule: ask how heat reaches the surface, how far conduction must carry it, and whether water is still holding that surface at the boiling point.

In the Maillard reaction, reducing sugars combine with the amino groups of proteins and amino acids, with no enzyme involved. Its end products, melanoidins, supply the brown, and compounds formed on the way, such as pyrazines, bring roasted, nutty notes. Caramelization is heat breaking sugar down by itself, so plain sugar in a pan can only caramelize. A food holding both sugar and protein can run both reactions at the same time. Cut fruit browns by a third, enzyme-driven route that uses oxygen.

Four levers set the Maillard pace. Heat: it can crawl along even below freezing, yet in flour products a rise of 18 °F (10 °C) can speed it three- to fivefold. Water: it typically runs fastest in food that is moist but not wet, since a little water lets the reactants meet while more dilutes them. Acidity: a higher pH generally speeds browning, while acid holds it back by tying up amino groups. Sugar: only reducing sugars, such as glucose, fructose, lactose and maltose, take part directly.

Consider beef for a stew. Meat that only simmers in liquid is held at the boiling point or below and takes on little color. An early-1900s cookbook already has the meat fried in a little fat before it braises. The meat supplies both partners itself: its juice carries proteins, amino acids and peptides, and it holds ribose, a reducing sugar. When liquid then goes into the browned pan, it carries that color into the sauce.

The trap is adding more table sugar for more color. Sucrose is not a reducing sugar, and under mild heat it does little for the Maillard reaction. A reducing sugar such as glucose or fructose speeds it, and lactose is the pick when color matters more than sweetness. Milk browns readily because it carries both partners, lactose and lysine-rich proteins. Push past your target, though, and the same reactions keep building melanoidins toward black and bitter.

Rule: to change how brown food gets, change what the reaction needs, such as surface water, heat, pH or a reducing sugar, and stop at the shade you want.

Heat unfolds a protein's folded shape, a change called denaturation. The unfolded proteins then cling together in a network, which is how runny egg white becomes a gel and how meat firms. Each protein has its own range. Egg white starts losing fluidity near 140 °F (60 °C), with ovotransferrin among the first proteins to set. Ovalbumin, about 54% of the white, holds on until about 176 °F (80 °C). In Baltic herring, the muscle protein myosin started denaturing at around 104 °F (40 °C).

Meat adds two systems. Its fibers shrink as they warm, and in beef their lengthwise shrinkage keeps pace with juice loss. In one beef test, pieces were held 30 minutes after reaching temperature. They lost roughly 7 to 11% of their weight at 122 °F (50 °C), but 29 to 32% at 158 °F (70 °C). Collagen generally shrinks at roughly 144 to 154 °F (62 to 68 °C), dissolving into soft gelatin only with time. Consider eye of round against tenderloin. One beef study found more collagen in the eye of round than in the tenderloin. Long cooking dissolves such collagen, and FSIS sends chuck and round to braising and other moist heat. The lean tenderloin got no more tender with extra heat while its juice loss kept rising, and loin and rib can go on the grill.

Safety is judged by internal temperature, not by color. FSIS sets 145 °F (63 °C) for steaks, chops and roasts of beef, pork, veal and lamb, followed by a rest of at least 3 minutes. It sets 160 °F (71 °C) for ground meat and 165 °F (74 °C) for poultry, read in the thickest part. The trap is counting on carryover to get there. In one lab study, a steamer was switched off once a perch's core reached 167 °F (75 °C). With the door left shut, the core climbed on its own to 176 °F (80 °C). Carryover can finish a quality target like that, but FSIS wants the safe minimum reached before the food leaves the heat.

Rule: name your target first, a safe minimum reached before the food leaves the heat, or a texture set by which proteins unfold and how long collagen gets to dissolve.

Water's boiling point follows the air pressure above it. It is 212 °F (100 °C) at sea level, 208 °F (98 °C) at 2,000 feet and near 198 °F (92 °C) at 7,500 feet. Once a pot boils, more heat only turns more water into steam, so a rolling boil is no hotter than a gentle one. A pressure cooker raises the pressure, and the boiling point rises with it.

Dissolved salt and sugar move water. By osmosis, water crosses cell walls toward the more concentrated side, which lets a strong brine draw water out of vegetables while some of the salt moves in. Salt enters meat by diffusion, fastest at the start and quicker in warmer brine, though FSIS still has poultry brined in the refrigerator. Dissolved sugar lowers an ice cream mix's freezing point, so less water turns to ice, and dissolved salt or sugar lowers water activity, which slows spoilage.

Consider green beans bound for the freezer. Freezing only slows the enzymes that would keep degrading them, so a quick blanch in boiling water inactivates them first. FSIS then has you cool the beans fast. Keep the heat short: in work one review cites, beans blanched 60 seconds still held 94.4% of their chlorophyll, against 75.5% after 300 seconds. Heat and acid both encourage hydrogen to replace the magnesium in chlorophyll, which fades bright green to olive brown.

The trap is answering altitude with more heat. At 7,500 feet a simmering stew tops out near 198 °F (92 °C) however hard the burner runs, so it cooks cooler and slower. FSIS advises adding time instead. Simmered or braised meat may take up to a quarter longer at 5,000 feet. An oven roast can keep its sea-level time, since altitude does not change oven temperature.

Rule: in water, change the time or the pressure rather than the flame, and remember that what you dissolve decides which way water moves.

Starch sits in tiny granules built from two glucose chains, straight amylose and heavily branched amylopectin. Raw, it adds no useful texture. Heated in a large excess of water, the granules swell, release amylose and lose their ordered structure, a change called gelatinization. As they fill more of the liquid's volume, the liquid thickens. Starches differ in when this happens: in one comparison, waxy rice starch began thickening near 153 °F (67 °C) and cassava near 165 °F (74 °C). Once cooled and left to stand, the chains repack into ordered crystals, a change that firms stale bread and makes a starch gel weep.

Consider a white sauce. Fine starch dropped straight into boiling water forms lumps, so a slurry stirs it into a little cold liquid first. A roux, flour cooked in fat, is the other start. Fannie Farmer poured the hot milk onto her cooked butter and flour in three parts, stirring, then beating it smooth. A smaller measure of cornstarch can do the work of a bigger measure of flour, and wheat flour holds only about 85 percent starch. Farmer also noted that browning weakens flour's thickening, so her brown sauce took a little extra.

The trap is expecting a sour filling to thicken like a plain one. Acid cuts starch chains as the mixture cooks, so the sour batch can end up runnier than an acid-free one. Other gels have their own terms. High-methoxyl pectin, the jam kind, sets above about 55 percent sugar in a strongly acidic mix near pH 3. Gelatin melts just below body temperature and sets again as it cools. Agar disperses only in water hotter than 194 °F (90 °C). Once set at 1.5 percent, an agar gel holds firm until about 185 °F (85 °C). Xanthan gum thickens hot or cold liquid with no cooking, yet does not gel by itself.

Rule: match each thickener to its conditions, starch to heat and water, high-methoxyl pectin to sugar and acid, gelatin to a cool set, before adding more of it.

Oil and water will not mix, so an emulsion breaks one into small droplets dispersed through the other. Left alone, it tends to separate as droplets rise, sink or clump. An emulsifier slows that because each molecule has a part drawn to water and a part drawn to oil. The molecules collect at the boundary and wrap each droplet in a film. Smaller droplets tend to make a steadier emulsion. Mayonnaise is oil droplets in a watery phase even at 65 to 80 percent fat, while butter is the reverse, water in milk fat.

Consider mayonnaise. You beat oil into the yolk and acid slowly, starting with single drops, so the yolk can absorb it. Pouring everything together at once can flip it to a water-in-oil emulsion. It classically breaks when oil goes in too quickly, too cold or in excess. To rescue it, beat the broken sauce bit by bit into a fresh yolk. For a small batch, Escoffier let half a coffee spoon of mustard stand in for that yolk, since mustard's mucilage acts as an emulsifier. The yolk goes in raw, and USDA advises against eating raw yolk. Its safe route is to cook the eggs, still liquid, to 160 °F (71 °C) first, or to use pasteurized eggs.

A foam holds only for a time: its walls drain, big bubbles grow by emptying small ones, and thin films burst. Whipping loosens egg-white proteins so they spread into films around the air. The trap is adding all the sugar at the start. Sugar steadies a meringue's shape, but tipped in at the outset it can suppress foaming, so add it a little at a time after the whites start foaming. Fat undermines the foam too: in one study, whites carrying 0.5 percent yolk foamed less, and the foam lasted less well.

Rule: an emulsion or foam lasts only while the film around each droplet or bubble holds, so build it gradually and keep out whatever attacks that film.

There are at least five basic tastes: sweet, sour, bitter, salty and umami, the savory taste of free glutamate. Cells for every one sit all over the tongue, so the old map with sweet up front and bitter at the back is discredited. Taste comes from compounds that do not evaporate, aroma from volatile ones your nose detects. As you chew, aromas rise through the back of your throat to your nose and arrive with the taste, so you perceive one flavor. Smell carries a large share of it, so food seems flat when a cold blocks your nose.

Tastes interact rather than add up. In simple mixtures, salt suppresses bitterness at every level but lifts sweetness only at low levels. When salt and sour are both low, each boosts the other. Glutamate mixed with inosinate gives about eight times the savory response of glutamate alone, the logic behind Japanese dashi, where kombu brings glutamate and dried bonito brings inosinate. Release matters too. Most aroma compounds dissolve more easily in fat than in water, so fat holds aroma back and lets it out slowly. Warmth, meanwhile, can push more aroma into the air.

Suppose you want a tomato soup to taste right with less salt. In one tomato soup study, citric acid made the soup taste saltier, and liking rose with acid to a peak near 0.15 to 0.3 percent, then dropped sharply. The favorites there combined moderate amounts of salt and acid. In one study, a low-salt soup with citric acid also did as well on liking as a high-salt one, or better, so acid can carry part of the salt's work. Since liking falls fast past its peak, add acid a little at a time.

The trap is tasting a sauce once, still thin and over the steaming pot, and calling it seasoned. In one study, a thickener lowered perceived sweetness, sourness and saltiness, so taste again after thickening. Repeated sniffs of one aroma can dull it, too, so after hovering over a pot, your nose may have adapted. With drinks, temperature counts as well. In one study, tasters tied hot green tea to sweet, roasted notes and cold green tea to sour ones, so judge a drink as it will be served.

Rule: taste food as it will be eaten, at its final warmth and thickness and with a rested nose, then adjust in small steps.

The trap is believing that searing seals in juices. In a study of pork patties, those seared and then cooked sous vide, sealed in a precisely heated water bath, lost more weight than unseared ones. The longer the sear, the bigger the loss. Searing's real effect is a drier surface that browns faster, and diners generally prefer a browned one. Flames mislead too. USDA's retention figures cover a baked or simmered dish with alcohol stirred in. About 40% of the alcohol remains at 15 minutes, about a quarter at an hour and about 5% at 2½ hours. Flaming leaves about 75%.

Color fails as a doneness test. Ground beef can brown before it reaches a temperature that kills bacteria, FSIS warns. By its count, one hamburger in four may look done and still not be safely cooked. A patty at 160 °F (71 °C) all the way through is safe whatever its color. Consider how to read that patty. A dial instant-read thermometer averages over roughly the last 2 to 3 inches of its stem, so slide the probe into a thin burger from the side, to the middle. In thicker food, read the thickest part, away from bone, fat and gristle.

A reading is only as good as the tool. In ice water a thermometer should show 32 °F (0 °C). Boiling water reads 212 °F (100 °C) only at sea level, and for most thermometers ice water is the test to prefer. If the reading is off, adjust it or allow for the gap. An oven can stray from its dial setting, hot or cold, and an oven thermometer hung from a center rack shows by how much. Weighing beats cups for accuracy, too: a cup of all-purpose flour weighs about 113 grams, a cup of granulated sugar about 198.

Rule: when a kitchen belief rests on something you can see, such as a crust, a color or a flame, settle it with something that measures.