Food Chemistry and Composition
This domain asks you to explain why foods behave as they do - why plantain darkens when cut, why palm oil turns rancid, why a biscuit browns but a boiled cassava stick does not - and to connect that behaviour to composition tables, processing choices and nutrient retention. Expect recall questions on the nutrient classes, calculation questions on moisture/dry matter and energy, and prediction questions on changes during cooking or storage.
Water: content is not activity
- Moisture content = g water per 100 g food. Water activity (aw) = vapour pressure of the food divided by that of pure water at the same temperature. Two foods with identical moisture can have different aw because solutes bind water.
- aw governs spoilage: most spoilage bacteria need aw > 0.90, most yeasts > 0.88, most moulds > 0.80, xerophilic moulds > 0.65. Staphylococcus aureus can grow and produce toxin down to about aw 0.86. Below aw 0.60 most microbes are inactive - the basis of drying.
Carbohydrates
Starch gelatinises in water at roughly 60–70 °C (irreversible swelling), then retrogrades on cooling, squeezing water out (syneresis) and staling bread. Gelatinisation vs dextrinisation (dry heat breaking starch chains) vs caramelisation (sugars only) vs Maillard browning (reducing sugar + amino group, needs heat and moderate aw; also destroys lysine and forms acrylamide).
Proteins and lipids
Proteins denature with heat, acid or salt, then may gel (egg, milk) or coagulate; the isoelectric point is the pH of minimum solubility. Lipids spoil by hydrolysis (lipase → free fatty acids, soapy off-flavour) and auto-oxidation (double bonds + oxygen → peroxides → rancid aldehydes). Palm oil's red colour is carotenoid (provitamin A), destroyed by over-heating or bleaching.
Vitamins and minerals
Water-soluble vitamins (C, B-complex) leach into cooking water and are destroyed by heat, light and alkali. Milling rice or maize removes the bran and germ, and with them thiamine, niacin and fibre.
Worked example
Fresh cassava flour weighs 100 g at 60 % moisture and must be dried to 12 % moisture. What is the final weight? Dry matter is unchanged: 100 − 60 = 40 g. At 12 % moisture, dry matter is 88 % of the final mass, so final mass = 40 ÷ 0.88 = 45.5 g. Water removed = 100 − 45.5 = 54.5 g. Note the reasoning: always work from the component that does not change.
Traps that cost marks
- Quoting moisture content when the question asks about aw, or assuming a drier food always has a lower aw.
- Calling every brown colour "Maillard"; caramelisation needs no protein.
- Forgetting that Maillard browning lowers protein quality by binding lysine.
- Giving a fat content as "oil" only, or ignoring that deep-frying adds fat to the food's composition.