22-Agric-A7 Chemistry and Microbiology of Foods · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A7 Chemistry and Microbiology of Foods, National Exams December 2013 — a three-hour closed-book exam (approved Casio/Sharp calculator permitted; one aid sheet, both sides). The paper is in two sections: Section I (Food Chemistry, Questions 1–7) and Section II (Food Microbiology, Questions 8–14); candidates answer any four questions from each section for a 100-mark paper (each question worth 12.5 marks). All fourteen questions are worked here so the set is a complete study resource.
Reference texts. S. Damodaran, K.L. Parkin and O.R. Fennema (eds.), Fennema's Food Chemistry, 5th ed. (Maillard/enzymatic browning, water activity and sorption isotherms, lipid oxidation and rancidity, sucrose glass transition, protein denaturation at interfaces, myoglobin chemistry); R.P. Singh and D.R. Heldman, Introduction to Food Engineering, 5th ed. (reaction kinetics in food processing, thermal process lethality); J. Jay, M. Loessner and D. Golden, Modern Food Microbiology, 7th ed. (microbial growth curve, intrinsic/extrinsic factors, Listeria monocytogenes, food preservation hurdles, irradiation, spoilage patterns); C. Mortimore and C. Wallace, HACCP: A Practical Approach, 3rd ed. (CCP identification/monitoring/verification for milk pasteurization).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(a) The sucrose in freshly spun cotton candy is in the amorphous (glassy) state, not crystalline. Spinning the melt through narrow nozzles cools it so quickly that the sugar solidifies before its molecules can organise into a crystal lattice, freezing it instead into a disordered, metastable glass. When cotton candy is left in a moist environment, the hygroscopic amorphous sugar absorbs atmospheric water vapour (driven by the gradient between the food's low initial $a_w$ and the humid surrounding air). Because water is an excellent plasticiser for sugar glasses, even a small amount of absorbed moisture lowers the glass transition temperature $T_g$ — and it can drop $T_g$ below the ambient storage temperature within minutes. Once $T_g$ falls below room temperature the material converts from a rigid glass to a soft, sticky rubber (the fine web structure collapses into a wet, sticky mass), and, being thermodynamically metastable, the plasticised amorphous sugar can also begin to recrystallise over time, which is why cotton candy left out quickly turns into a sticky, collapsed clump rather than staying fluffy.
(b) The driving force for the moisture migration is the difference in water activity (equivalently, water chemical potential) between the moist raisins and the comparatively dry wheat flakes: water always migrates from the higher-$a_w$ component toward the lower-$a_w$ component until the two reach a common equilibrium $a_w$, exactly as in Question 3. Adding glycerol to the raisins works thermodynamically: glycerol is a humectant that hydrogen-bonds strongly with water, depressing the raisin's own water activity so that it sits much closer to the flakes' $a_w$ — this shrinks the driving force for migration itself, rather than blocking the pathway. Coating the raisins with oil works kinetically instead: the hydrophobic oil film does not change the raisin's internal water activity at all, but it forms a physical barrier around the raisin surface that greatly increases the resistance to water vapour diffusion, slowing the rate at which moisture can cross to the cereal even though the underlying thermodynamic driving force to do so is unchanged.
