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Food chemistry and functional properties

Denaturation and coagulation by heat, acid and mechanical action (whisking egg whites forms foams); gluten formation (gliadin and glutenin) gives dough elasticity; gelation (gelatine); enzymic browning (polyphenol oxidase) in cut fruit, prevented by acid or blanching.

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  1. PDFFood chemistry and functional properties – Food Science and Nutrition notes (ZIMSEC A Level)47.2 KB

Carbohydrates

  • Sugars: sweetness, caramelisation (sugar heated above about 160 °C), Maillard browning (reducing sugar + amino acid on heating – bread crust, roasted groundnuts), crystallisation, preservation (lowers water activity).
  • Starch: gelatinisation (granules absorb water and swell on heating, thickening sauces and sadza), retrogradation (staling), dextrinisation (toast), modified starches.
  • Non-starch polysaccharides (dietary fibre) and pectin (setting jams).

Proteins

Denaturation and coagulation by heat, acid and mechanical action (whisking egg whites forms foams); gluten formation (gliadin and glutenin) gives dough elasticity; gelation (gelatine); enzymic browning (polyphenol oxidase) in cut fruit, prevented by acid or blanching.

Lipids

Saturated and unsaturated fatty acids; plasticity, shortening (fat coats flour, limiting gluten), aeration (creaming), emulsification (lecithin in egg yolk stabilises mayonnaise), and rancidity (oxidative and hydrolytic), slowed by antioxidants and cool, dark storage.

Water and food

Water activity (aw) controls microbial growth and shelf life; most bacteria need aw above about 0.91.

Additives

Preservatives, antioxidants, emulsifiers, stabilisers, colours and flavours; regulated and listed on labels.

Exam tip: Link each chemical change to a practical result in cooking or food manufacture.

Check yourself

  1. What is gelatinisation?
  2. Name the reaction responsible for the brown crust on bread.
  3. How does lecithin stabilise mayonnaise?
Show answers
  1. Starch granules absorb water and swell when heated, thickening the liquid.
  2. The Maillard reaction (non-enzymic browning).
  3. It has a water-loving and a fat-loving end, holding oil droplets dispersed in the water phase.
Next topicNutrition, energy and metabolism

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