Food Chemistry and Composition
This domain tests the chemistry behind observable food changes: water activity, browning reactions, lipid oxidation, protein behavior, starch and pectin functionality, and enzymatic reactions. Expect questions that ask you to name the mechanism behind a defect or a desirable attribute.
Core rules
- Water activity (aw) is the availability of water for reactions and microbial growth, not moisture content. A dry cracker and fresh bread can differ greatly in aw at similar moisture percentages.
- Maillard browning requires a reducing sugar and an amino group, usually from protein, and is favored by heat and intermediate aw. Caramelization involves sugars alone at high temperature. Enzymatic browning in cut produce is polyphenol oxidase acting on phenolics in the presence of oxygen.
- Lipid oxidation is a free-radical chain reaction producing peroxides and then aldehydes and ketones. Peroxide value measures early oxidation; anisidine value measures secondary products. Antioxidants slow propagation but cannot reverse existing damage.
- Starch gelatinizes with heat and water, then retrogrades on cooling, causing staling and syneresis. Pectin plus acid and sugar forms gels; pectin methylesterase and calcium affect firmness.
- Proteins denature with heat, acid, or shear, changing solubility and water binding. Enzymes such as lipase, protease, and pectinase cause rancidity, softening, and cloud loss.
Worked example
A milk-and-sugar custard browns during baking, while a sugar syrup browns when heated alone. The custard browns by Maillard because milk protein supplies amino groups plus lactose; the syrup browns by caramelization because no amine is present. If a cut apple browns at room temperature, the mechanism is enzymatic, and blanching or ascorbic acid controls it.
Traps that cost marks
Treating moisture content and water activity as interchangeable. Claiming Maillard browning occurs in pure sugar systems. Assuming an antioxidant can restore already oxidized oil.