Food is primarily made up of four basic types of molecules: water, fat, carbohydrates, and protein. As food is heated, the molecules within it begin to speed up, colliding with each other as they go. As molecules gain speed, they also gain the power to break free of the electrical forces uniting their atoms. Some atoms can split off and join up with other atoms to create new molecules. This process is called a chemical reaction. And the chemical reactions initiated by heat affect the flavor and texture of food. Water, fat, carbohydrate, and protein molecules each react to heat in different, yet predictable, ways. If this seems overwhelming, don’t worry— it’s not. The science of heat, luckily, adheres to common sense.
Fats are slow to cool and heat
Like water, fat is both a basic component of food and a cooking medium. But fat and water are enemies: they don’t mix, and they respond very differently to heat. Fats are flexible; indeed, the broad range of temperatures fats can withstand allows us to achieve many different textures—crisp, flaky, tender, creamy, and light—that simply cannot be achieved without the proper relationship between Fat and Heat. Extended, gentle heat will transform, or render, solid animal fats into pure liquid fats such as pork lard or beef tallow. In slow-cooked meats such as Smoked Chicken, rendering fats essentially bastes food from within, and this self-basting also explains the exuberantly moist texture of Slow-Roasted Fish.
Controlled cooking with fats
At moderate temperatures, fat is an ideal gentle cooking medium, perfect for use in a cooking method called confit, which is essentially poaching in fat instead of water. While the water boils and vaporizes at 212°F, fats can climb to staggering temperatures well beyond that point before turning to smoke. As a result, since water and fat don’t mix, foods containing water (which is practically all foods) won’t dissolve in fat; instead, the surfaces of foods exposed to very hot fats can climb to high enough temperatures to develop crisp textures as water evaporates.
It takes a lot of energy to heat or cool a unit of fat by even a few degrees. This is a boon to the deep fryer; you can relax when frying Beer-Battered Fish, knowing you don’t have to act with lightning-quick reflexes when the oil temperature starts to rise or fall. If the fat gets too hot, just turn off the heat or carefully add a little more room-temperature oil. If the pot gets too cold, increase the heat and wait before adding more food.
Carbohydrates provide food with both structure and flavor
When heated, carbohydrates generally absorb water and break down. If fibrous or stringy is a word that comes to mind when you think of a particular fruit or vegetable, it’s rich in cellulose, a type of carbohydrate that isn’t broken down by heat. Cook cellulose-rich produce, such as collard greens, asparagus, or artichokes, until it absorbs enough water to become tender. Leaves have fewer cellulose fibers than stems or stalks, which is why kale and chard stems cook at a different rate than their leaves and ought to be stemmed and cooked separately, or simply staggered into the pot.
Give the starchiest parts of plants, including tubers such as potatoes and seeds such as dried beans, plenty of water and time over a gentle heat to coax out their tenderness. Starches absorb liquid and swell or break down, so firm potatoes become delightfully creamy, impossibly hard chickpeas transform into buttery bites, and rice goes from indigestible to fluffy and tender.
Using the correct amounts of water and heat
Use too little water or undercooked starches and they will be dry and unpleasantly tough in the center. Cakes and bread baked with too little water are dry and crumbly. Undercooked pasta, beans, and rice are unpleasantly tough in the center. But use too much water or heat, or simply overcook starches, and they will be mushy (think limp noodles and soggy cakes and rice). Starches are eager to undergo browning and will burn easily if overcooked or exposed to too much heat. For example, scorched grits at the bottom of an unwatched pot, or bread crumbs blackened after just 90 seconds of neglect in a hot oven.
How temperature affects proteins?
It helps to picture proteins as coiled threads floating around in the water. When exposed to heat, the threads first denature, or unwind, and then clump together more tightly, or coagulate, entrapping pockets of water, to create structure in foods.
Think of how heat transforms a chicken breast from flabby and watery to firm, tender, and moist when perfectly cooked. But apply too much heat and the protein clumps will continue to tighten, squeezing out the pockets of water. With its water expelled, the chicken becomes dry, stringy, and tough.
Effect of heat on scrambled eggs
This phenomenon is also apparent in scrambled eggs. Cook scrambled eggs too long, or at too high a temperature, and they will dry out. Put them on the plate, and you’ll see their poor, over-squeezed proteins continue to wring water out, leaving a puddle behind. The coiled threads in each type of protein are unique, so the range at which different proteins coagulate is vast. Preserve tender cuts of meat with careful, quick cooking, generally over the intense heat of a grill, preheated frying pan, or hot oven. If cooked to an internal temperature beyond 140°F, the proteins within tender red meats will coagulate entirely, expelling water and yielding tough, chewy, overcooked steaks and lamb chops. Chicken and turkey breasts, on the other hand, don’t dry out until temperatures surpass 160°F.
Tougher cuts, rich in sinewy connective tissue, require a slightly more nuanced cooking approach to reveal their tenderness: the investment of gentle heat, time, and water implicit in braising or stewing. Heat metamorphizes collagen, the main structural protein found in animal connective tissue, into gelatin. The tough and chewy proteins that make undercooked short ribs impossible to chew and unpleasant to eat will transform into gelatin with water, time, and further cooking, yielding the rich, tender textures we associate with barbecued brisket, stewed meats, and properly cooked short ribs.
The key to this transformation is gentle heat. In contrast to the carefully applied quick heat required to cook tender cuts, time and sustained low heat is essential to transforming dark meat’s tough, sinewy connective tissue into luxurious gelatin while its lumps of intermuscular fat render and baste the meat from within.