What Cooking Does

TL;DR. Cooking is the most transformative chemical process your food undergoes, and it runs in both directions. It makes starch digestible, kills pathogens, destroys antinutrients, and multiplies the availability of carotenoids. It also creates compounds that were not there before: acrylamide in browned starch, heterocyclic amines and polycyclic aromatic hydrocarbons in charred meat, and oxidation products in overheated oil. The practical rule that covers most of it is short: golden, not brown; brown, not black, and get the smoke and the drips away from the food.

1. The reactions

Maillard reaction. Amino acids react with reducing sugars above roughly 140 °C, producing hundreds of new compounds responsible for the flavour of bread crust, roast meat, coffee, chocolate, and browned onions. It is arguably the most important chemical process in cooking, and it is also the source of acrylamide.

Caramelisation. Sugars breaking down under heat, above about 160 °C. No protein required.

Denaturation. Proteins unfolding: egg white setting, meat firming, milk skinning.

Gelatinisation. Starch granules absorbing water and swelling above about 60 to 70 °C, which is what makes starch digestible at all. This is the reason cooking made human diets possible: raw starch is barely digestible, and cooked starch is a dense energy source. Richard Wrangham's "cooking hypothesis" argues this is what allowed human brains to grow.

Emulsification, oxidation, hydrolysis, and pyrolysis fill in the rest.

2. What cooking gives you

3. Acrylamide

Formed when the amino acid asparagine reacts with reducing sugars in the Maillard reaction, above about 120 °C, in dry heat. Discovered in food in 2002 by Swedish researchers, which was a genuine surprise, since it had been known as an industrial chemical for decades.

Classification: IARC Group 2A, probably carcinogenic to humans, based on animal studies and a plausible genotoxic mechanism. Human epidemiology has been largely unable to demonstrate a dietary link, which may reflect exposure measurement difficulty or may reflect a small real effect. Regulators treat it as a genotoxic carcinogen with no threshold, so guidance is "as low as reasonably achievable."

Where it is:

How to reduce it, all evidence-based:

• Cook to a golden yellow, not brown or dark. The UK FSA's "Go for Gold" campaign is exactly this. Acrylamide rises steeply with colour.
• Do not refrigerate raw potatoes. Cold converts starch to reducing sugars, which then feed the reaction (Chapter 42). Store them cool, dark, and above about 6 °C.
• Soak chipped potatoes in water for 15 to 30 minutes and dry before cooking. Removes surface sugars; studies find substantial reductions.
• Blanch potatoes before roasting or frying.
• Toast bread lightly, and scrape off burnt bits.
• Boil, steam, or poach where you can. No dry heat, no acrylamide.
• Do not overcook, and do not reuse very dark oil.

4. Heterocyclic amines and polycyclic aromatic hydrocarbons

HCAs form from amino acids, creatine, and sugars in muscle meat at high temperatures, particularly above 150 °C on direct heat, and increase steeply with time and temperature. Highest in well-done, charred, pan-fried, and grilled meat.

PAHs, including benzo[a]pyrene, form when fat drips onto flame or a hot surface and the resulting smoke deposits on the food. Highest in barbecued and smoked food.

Both are IARC-classified carcinogens and both are part of the mechanistic story behind the meat and colorectal cancer association (Chapter 55).

How to reduce them, and these are well demonstrated:

• Marinate. Acidic marinades and those containing herbs and spices (rosemary, thyme, garlic, turmeric) reduce HCA formation by 50 to 90 percent in several studies. Beer and wine marinades work too.
• Microwave or par-cook first, then finish briefly on high heat. Cuts HCA substantially.
• Turn frequently. Reduces peak surface temperature.
• Keep meat away from direct flame; raise the grill, use indirect heat, or use foil or a drip tray.
• Trim visible fat so less drips.
• Cut off the charred parts.
• Cook lower and slower: braising, stewing, poaching, and sous vide produce essentially none.
• Serve with vegetables. Plant compounds and fibre appear to reduce the biological impact.

5. Oil and fat degradation

Covered in Chapter 53, and the key points bear repeating:

• Oxidative stability, not smoke point, is what matters, and it is set by how many double bonds the fat has plus its antioxidant content.
• Reused frying oil is the genuine hazard: repeated heating produces aldehydes including acrolein and 4-hydroxynonenal, plus polar compounds and polymers.
• Discard oil when it darkens, foams, smokes early, or smells acrid.
• Never heat flaxseed oil.

Acrolein, the acrid compound in burning fat, is also a respiratory irritant and is a real contributor to indoor air pollution during cooking (Chapter 95).

6. Other things cooking creates or destroys

Advanced glycation end products (AGEs) form when sugars attach to proteins and fats, both in food during dry high-heat cooking and in your body over time. Dietary AGEs are highest in grilled, roasted, and fried animal foods and lowest in steamed and boiled food. Their significance is genuinely uncertain: the mechanism is plausible, human trials are small, and how much dietary AGE is absorbed is contested. It is a reasonable additional argument for moist cooking methods and not a reason for alarm.

Nutrient losses, from Chapter 37: vitamin C and folate are the casualties; minerals and fibre survive and merely leach into the water.

Nitrosamines form in cured meat, particularly with high-heat cooking, which is why bacon fried until crisp is a worse proposition than bacon cooked gently (Chapter 90).

Furan forms in canned and jarred heat-processed foods, particularly coffee and baby food. Volatile, so stirring and letting food stand reduces it.

Trans fats form in tiny quantities during high-temperature oil refining and prolonged frying, far below the levels once produced by partial hydrogenation (Chapter 13).

7. Microwaves, and the persistent myth

Microwave ovens emit non-ionising radiation at 2.45 GHz, which makes water molecules rotate, generating heat. They do not make food radioactive, do not alter its chemical structure beyond what heat does, and do not "destroy nutrients" any more than other heat.

In practice, microwaving is one of the better methods for nutrient retention, because it uses little or no water and short times (Chapter 37). Comparative studies consistently find broccoli, spinach, and green beans retain more vitamin C when microwaved than boiled.

The genuine microwave cautions are practical:

• Uneven heating leaves cold spots where bacteria survive. Stir, rotate, and stand.
• Superheating: water heated in a smooth container can exceed 100 °C without boiling and then erupt violently when disturbed. Put a wooden stirrer in the cup.
• Do not microwave in containers not labelled microwave-safe, particularly takeaway containers and cling film in contact with fatty food (Chapter 93).
• Never microwave a baby's bottle: hot spots scald mouths.
• Whole eggs explode.

8. Food safety temperatures

The other half of what cooking does, and the half that saves the most lives.

Whole cuts of beef and lamb can be eaten rare because contamination is on the surface, which searing sterilises. Minced meat cannot, because grinding distributes surface bacteria throughout. Same principle for burgers versus steak, and it is the single most useful food safety distinction in a kitchen.

The danger zone is 5 to 60 °C. Get cooked food into the fridge within two hours, or one in hot weather. Cool large quantities fast by dividing them.

Do not wash raw chicken. It aerosolises campylobacter across the sink and surrounding surfaces; cooking kills it, splashing spreads it.

Rice: Bacillus cereus spores survive cooking and germinate at room temperature. Cool quickly, refrigerate within an hour, reheat thoroughly once only.

9. Practical guidance, condensed

The five rules that cover most of this chapter:

1. Golden, not brown; brown, not black. Applies to toast, roast potatoes, chips, and meat.
2. Marinate before grilling, and keep meat away from flame and dripping fat.
3. Use moist methods (steam, boil, braise, poach) as your default, and dry high heat as the exception you enjoy.
4. Add fat to orange, red, and dark green vegetables, and use less water and less time.
5. Cook to safe temperatures, cool fast, and reheat properly.

And two that follow from the rest of the book:

• Do not refrigerate raw potatoes.
• Throw out degraded frying oil.

10. The bottom line

• Cooking makes starch and protein digestible, kills pathogens, removes antinutrients, and multiplies carotenoid availability. It is not a compromise; it is what made human diets possible.
• Acrylamide forms in browned starch above 120 °C in dry heat. Cook to golden, soak chipped potatoes, do not refrigerate raw ones, and remember that boiling and steaming produce essentially none.
• HCAs and PAHs form in charred meat and in smoke from dripping fat. Marinating cuts HCA formation by 50 to 90 percent, and par-cooking, frequent turning, and indirect heat all help.
• Reused, darkened frying oil is the real fat hazard in a domestic kitchen.
• Microwaving is one of the better methods for retaining nutrients, not the worst. The genuine cautions are uneven heating, superheating, and containers.
• Whole cuts can be rare; mince cannot. Do not wash raw chicken. Cool rice fast.

👉 Next: heavy metals and contaminants.