Food Science and Food Chemistry
Food Science and Food Chemistry
This domain tests how composition and chemical reactions control quality and shelf life. Expect water activity, pH, browning, protein denaturation, lipid oxidation and proximate analysis.
Core rules
- Water: microbial growth depends on water activity (aw), not moisture percentage. Most bacteria need aw >0.90; xerophilic moulds can grow near 0.65. Drying and sugar or salt addition lower aw.
- Carbohydrates: Maillard browning needs a reducing sugar, amino groups and heat; caramelization needs heat without protein. Starch gelatinizes with heat and water and retrogrades on cooling.
- Proteins: heat, acid and salts denature proteins; gelation forms structures such as curd and tofu. At the isoelectric point, protein solubility is minimum.
- Lipids: hydrolysis and autoxidation cause rancidity; antioxidants and low oxygen slow it.
- Analysis: moisture by oven drying, ash by muffle furnace, fat by Soxhlet, protein by Kjeldahl (N × 6.25 for many foods), carbohydrate by difference.
Worked example
A jam has 68% soluble solids, pH 3.4 and aw 0.78. Why is it stable at room temperature? Reasoning: aw 0.78 is below the growth limit of most bacteria and many yeasts; pH 3.4 inhibits pathogens; high sugar creates osmotic stress. It is not sterile, so opened jars can mould. Hot filling and an intact vacuum seal protect the surface.
Traps
Do not equate moisture percentage with aw; not all browning is Maillard; pH changes preservative strength; the 6.25 protein factor is not universal.