What happens inside food when heat, water, mixing, fermentation and other cooking processes change its structure.
Dutch-process cocoa is treated with alkali to raise its pH. It is usually darker, less acidic, and smoother-tasting than natural cocoa, but alkalization can substantially reduce flavanols. The acidity difference can also matter in recipes that rely on baking soda.
Fried foods absorb oil because heat drives out water and creates a porous crust. Some oil remains on the surface and can move into that structure as the food leaves the fryer and begins to cool.
Ovens and air fryers are both hot-air cooking methods. With the same food and the same amount of oil, nutrition can be very similar; the difference becomes much larger when air frying is compared with deep frying.
Raw foods can preserve more heat-sensitive nutrients such as vitamin C. Cooking can make carotenoids easier to access, change concentration through water gain or loss, and improve safety. There is no universal winner.
Vitamin C and folate are more vulnerable to heat and water than minerals. Shorter methods such as steaming can limit losses, and keeping cooking liquid in soups or stews retains nutrients that moved into the water.
Boiling lets some water-soluble nutrients move into the cooking liquid; steaming reduces direct water contact; baking and pan-cooking often concentrate food as water evaporates. Added oil can change calories more than the method itself.
Frying usually raises calories per 100 g because food retains oil and loses water. The effect can be large with deep frying and small in a skillet using very little oil; what matters most is how much oil actually stays in the food.
Cooking does not make a vegetable universally more nutritious, but it can improve carotenoid availability. Tomatoes, carrots, and spinach are clear examples, while long cooking can reduce vitamin C and folate. Eating both raw and cooked vegetables captures different advantages.
Cooking can make some carotenoids, including tomato lycopene and carrot beta-carotene, easier to access because heat and processing alter plant structure. The effect is not universal: vitamin C, for example, can decline with heat.
Cooking rice, potatoes or pasta gelatinizes starch; cooling lets some starch chains reorganize into type 3 resistant starch. The increase is real but varies widely with the food, variety, cooking, time, temperature and laboratory method. In one study, white rice rose from 0.64 to 1.65 g of resistant starch per 100 g after 24 hours at 39°F (4°C) and reheating. The potential benefit is not a reason to leave food at room temperature: cool and refrigerate it under normal leftover-safety rules.
Honey crystallizes when some of its glucose, which is less soluble than fructose, comes out of solution and forms crystals. It is a normal physical process and does not by itself mean honey is spoiled or adulterated. Crystallized honey is still usable; if you prefer it liquid, gentle warming in a water bath can dissolve the crystals without boiling it.
Rancid olive oil usually loses much of its fresh aroma and develops stale, old or waxy notes. The International Olive Council defines rancid as the flavor of oils that have undergone intense oxidation. Bitterness and throat pungency, by contrast, can be positive attributes of extra-virgin olive oil rather than signs of spoilage.