Food Chemistry and Nutrition
What is tested
Food chemistry and nutrition questions ask you to relate composition to behaviour: why foods brown, go rancid, set into gels or lose vitamins. You must know proximate principles, water activity, macronutrient and micronutrient functions, and the main chemical reactions in foods.
Core concepts
- Proximate composition: moisture, crude protein (Kjeldahl nitrogen × 6.25; dairy uses 6.38), crude fat (Soxhlet), crude fibre, ash and nitrogen-free extract.
- Water activity (aw) is the availability of water for microbial growth and reactions, not the moisture percentage. Most bacteria need aw above 0.90; xerophilic moulds can grow near 0.70.
- Enzymatic browning is caused by polyphenol oxidase acting on phenols in the presence of oxygen; blanching or ascorbic acid and citric acid treatment controls it.
- Non-enzymatic browning includes the Maillard reaction between reducing sugars and amino groups, and caramelisation of sugars on heating.
- Lipid rancidity is hydrolytic (lipase or moisture releases free fatty acids) or oxidative (double bonds react with oxygen, giving peroxides and off-flavours).
- Proteins denature on heating or pH change, which affects gelation, foaming and water holding.
- Emulsions are oil-in-water or water-in-oil systems stabilised by emulsifiers; gels are three-dimensional networks.
Worked example
A 100 g snack contains 10 g protein, 20 g fat and 60 g carbohydrate. Energy = (10×4) + (20×9) + (60×4) = 40 + 180 + 240 = 460 kcal. Protein energy share = 40 ÷ 460 × 100 = 8.7%. The reasoning uses Atwater factors - 4 kcal/g for protein and carbohydrate and 9 kcal/g for fat - and expresses the nutrient as a percentage of total energy.
Traps
Do not use moisture percentage as a substitute for water activity. Do not call all browning Maillard browning. Do not forget that fat is the most energy-dense macronutrient. In dairy, use the 6.38 factor only where the syllabus specifies it.