A kitchen is a lab
Cooking changes ingredients in ways that cannot be undone, which is the sign of a chemical change rather than a physical one. Raw egg white does not return to liquid when it cools, and toast does not turn back into bread. Most of these changes involve a few families of molecules: proteins, carbohydrates, fats and water. Understanding what each does makes recipes less mysterious.
Browning: two different reactions
Two separate processes cause foods to brown, and they are often confused.
The Maillard reaction occurs between amino acids (from proteins) and certain sugars. It begins at moderate heat and speeds up as the surface becomes hotter and drier. It produces hundreds of flavor and aroma compounds, as well as brown pigments. It is responsible for the crust on bread, the surface of seared steak, roasted coffee and toasted nuts. It is commonly said to be fastest above roughly 140 degrees Celsius, but it can happen more slowly at lower temperatures.
Caramelization is the breakdown of sugars themselves when heated, without needing protein. Table sugar starts to caramelize at roughly 160 degrees Celsius or higher, turning from clear to golden to deep brown, with flavors that shift from sweet toward bitter if it goes too far.
Water is the enemy of both. Because liquid water holds a surface near 100 degrees Celsius, wet food cannot brown well until the surface dries, which is why patting meat dry and using a hot pan helps. A small amount of baking soda raises the alkalinity and can speed browning, an application of the same ideas as the pH scale.
Rising: gas trapped in a structure
Baked goods rise when gas expands inside a sticky or stretchy structure.
- Baking soda is sodium bicarbonate, a base. When it meets an acid in the batter, such as buttermilk, yogurt, lemon juice or brown sugar, it releases carbon dioxide gas.
- Baking powder contains baking soda plus dry acids, so it only needs moisture and heat to start working.
- Yeast is a living organism that consumes sugars and releases carbon dioxide and ethanol in fermentation. The alcohol mostly bakes off, and the gas makes bread rise slowly. Yeast, like all living things, relies on energy-releasing chemistry similar to the processes behind plant photosynthesis in reverse, burning sugar with or without oxygen.
- Steam and trapped air also lift pastries and whipped batters, as when water turns to steam in puff pastry.
Heat then sets the structure, so the bubbles stay in place after the cake cools.
Setting: proteins and starches
Proteins are long chains folded into specific shapes. Heat, acid or vigorous whisking unfolds them, a process called denaturation, and the unfolded chains then bond to one another and form a network. This is why an egg turns from clear and runny to white and firm, and why meat becomes firmer as it cooks. Cooking proteins too hard squeezes water out of the network, which is a common reason for rubbery eggs and dry meat.
Wheat flour contains two proteins that combine with water to form gluten, an elastic network. Kneading develops it, which gives bread its chew. Cake recipes aim for less gluten, and pie crusts use fat to keep it short and tender.
Starches behave differently. When heated in water, starch granules absorb liquid and swell, a change known as gelatinization, which thickens sauces, puddings and gravies. When baked bread or cooked rice cools, the starch can partly recrystallize, one reason stale bread feels firm.
Fats, emulsions and flavors
Fats carry flavors and make food feel rich. Oil and water do not mix, but emulsifiers such as lecithin in egg yolk keep them together in mayonnaise or vinaigrette. This is the same kind of two-sided molecule behavior seen in soap.
Many aromas are volatile molecules that reach your nose as food heats up. They are one of the main ways flavor reaches the human senses, which is why food tastes bland when you have a cold.
Safety and digestion
Cooking also affects safety. Heat kills many harmful microbes, and food safety guidance gives recommended temperatures for meats and eggs, which vary by food and region. Cooking can make some starches and proteins easier to digest, although digestion itself, in the digestive system, is a separate sequence of chemical changes. Browning at very high temperatures can create some compounds that are best not eaten in large amounts, so heavily charred food is better kept occasional. This is general information, not dietary or medical advice.
Using the chemistry
Knowing the principles lets you troubleshoot. A pale loaf may need a hotter oven or a little sugar for browning. A dense cake may have old baking powder or overmixed batter. A broken sauce may need an emulsifier or gentler heat. Recipes are tested chemical procedures, and small changes in time, temperature and proportion can change the result, so it helps to change one thing at a time.