Food Chemistry: Water, Carbohydrates, Proteins, Lipids, Vitamins and Minerals
Domain 2 covers water activity and the chemistry of carbohydrates, proteins, lipids, vitamins and minerals. Questions often ask you to predict changes during heating, cooling, storage or processing.
Core rules
- Water activity (aw) is the ratio of vapour pressure of food water to pure water, 0–1. It predicts microbial growth better than moisture percentage.
- Reducing sugars (glucose, fructose, maltose, lactose) have a free anomeric carbon and reduce Benedict's reagent. Sucrose is non-reducing because both anomeric carbons are in the glycosidic bond.
- Gelatinization is heat-and-water swelling of starch granules with viscosity rise; retrogradation is re-association on cooling, causing staling.
- Maillard browning needs a reducing sugar plus an amino group and heat; caramelization is sugar breakdown without protein.
- Protein denaturation unfolds secondary and tertiary structure without breaking peptide bonds; at the isoelectric point solubility is minimum.
- Lipid hydrolysis releases free fatty acids (hydrolytic rancidity); oxidation forms peroxides and aldehydes (oxidative rancidity).
Worked example
Sucrose gives a negative Benedict's test. Reason: glucose and fructose are joined through their anomeric carbons, so no free aldehyde or ketone is available. After acid hydrolysis, the glycosidic bond breaks and the resulting invert sugar reduces Benedict's solution. This is why honey and fruit juices test positive while pure sucrose does not.
Traps
- Confusing aw with moisture content; a dry biscuit can have low moisture but still support mould if aw is high.
- Swapping gelatinization (heating) with gelation (network setting on cooling).
- Saying denaturation breaks peptide bonds; it does not.
- Mixing hydrolytic and oxidative rancidity; antioxidants control oxidation, not hydrolysis.