The science of cooking

Cooking transforms food through heat and chemistry. It makes many foods safer by killing microbes, easier to digest by breaking down tough fibres and starches, and more flavourful by creating new aroma compounds.

Some anthropologists, notably Richard Wrangham, have argued that cooking played a major role in human evolution, since cooked food yields more energy with less chewing and digestion, possibly allowing our ancestors to support larger brains.

The brown crust on bread, seared steak, roasted coffee and toasted nuts comes largely from the Maillard reaction, named after the French chemist Louis-Camille Maillard, who described it in 1912. When amino acids and certain sugars are heated together, they react to form hundreds of new flavour and aroma compounds.

The Maillard reaction proceeds quickly only at temperatures well above the boiling point of water, which is why boiled meat stays grey while grilled meat browns. Patting food dry before searing helps, since surface water must evaporate before browning can begin.

Caramelisation is a different process: sugar alone breaks down under heat. Table sugar starts to caramelise at around 160 degrees Celsius, turning golden and then deep brown while developing nutty, buttery and slightly bitter flavours.

Slowly cooked onions become sweet and brown through a combination of caramelisation and Maillard reactions, as their natural sugars and amino acids react over long, gentle heating.

Proteins are long molecules folded into specific shapes. Heat makes them unfold, a change called denaturation, and then bond together into new networks. That is why egg whites turn from clear liquid to opaque solid, and why meat firms up and releases juice as it cooks. Overcooking tightens those networks too far, making meat dry and eggs rubbery.

Starch granules in rice, pasta and potatoes absorb water and swell when heated, a process called gelatinisation, which softens them and thickens sauces. As cooked starches cool, they partly recrystallise, which is why bread goes stale and cold rice becomes firm.

• Salt enhances flavour, suppresses bitterness and draws moisture out of foods through osmosis.
• Acid, from lemon juice or vinegar, brightens flavours and can firm proteins, as in ceviche.
• Fat carries flavour compounds that dissolve in fat but not water, and gives richness and texture.
• Umami, the savoury taste identified by Kikunae Ikeda in 1908, comes from glutamates in foods like tomatoes, cheese, mushrooms and soy sauce.
• Heat from chilli peppers comes from capsaicin, which triggers pain receptors rather than taste buds.

Baking depends on precise chemistry. When wheat flour is mixed with water and kneaded, proteins form gluten, an elastic network that traps gas. Yeast produces carbon dioxide by fermenting sugars, while baking soda and baking powder release carbon dioxide through chemical reactions with acids, making cakes and breads rise.

Oil and water do not normally mix, but an emulsifier can hold them together. In mayonnaise, lecithin from egg yolks surrounds tiny droplets of oil and keeps them suspended in lemon juice or vinegar. Hollandaise, vinaigrettes and many sauces rely on the same principle.