22-Agric-A7 Chemistry and Microbiology of Foods · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A7 Chemistry and Microbiology of Foods, National Exams May 2017 — a three-hour closed-book exam (one aid sheet, both sides; approved calculator permitted). The paper is in two sections: Section I (Food Chemistry, Questions 1–6) and Section II (Food Microbiology, Questions 7–12); candidates answer any three questions from each section for a 100-mark paper (each question worth 16.7 marks). All twelve 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. (enzyme kinetics, water activity and sorption isotherms, lipid crystallization/polymorphism, protein gelation, popcorn starch/glass transition); R.P. Singh and D.R. Heldman, Introduction to Food Engineering, 5th ed. (reaction-order kinetics, quality-loss modelling); J.M. Steffe, Rheological Methods in Food Process Engineering, 2nd ed. (creep-recovery of viscoelastic doughs); J. Jay, M. Loessner and D. Golden, Modern Food Microbiology, 7th ed. (bacterial growth curve, intrinsic/ extrinsic factors, Salmonella, quorum sensing, viral/prion foodborne agents, rapid methods, sampling plans); C. Mortimore and C. Wallace, HACCP: A Practical Approach, 3rd ed. (the seven HACCP principles).
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.
Popping is governed by two coupled transitions inside the kernel's hard, moisture-impermeable pericarp (hull). First, the starch granules in the endosperm are in a glassy state at room temperature; heating carries them through the glass transition ($T_g$) into a rubbery, then a gelatinizable state, at which point the starch can absorb and immobilize the kernel's internal moisture as a superheated matrix. Second, and simultaneously, the water trapped inside the sealed kernel is heated well past $100\,{}^{\circ}\text{C}$ without boiling because the pericarp holds it under pressure — a liquid-to-vapour (steam) phase transition that cannot relieve itself until the hull mechanically fails. When internal steam pressure exceeds the pericarp's rupture strength (roughly 175–180 $^{\circ}$C kernel temperature), the hull bursts explosively; the sudden pressure drop flashes the superheated water to steam almost instantaneously, and the gelatinized starch – now a hot, expandable, rubbery foam – is blown outward and inverted by the escaping steam. As it expands into the surrounding cool air it passes back through its own glass transition on cooling, vitrifying into the rigid, crisp, expanded foam structure of a popped kernel. The critical popping temperature is therefore set by the balance between the pericarp's mechanical failure point and the starch's gelatinization/glass-transition temperature: a kernel that is too dry gelatinizes too slowly relative to the pressure buildup and fails to expand fully, while one that is too wet ruptures the hull before enough of the starch has gelatinized to form a well-expanded foam.
Applying a constant force at $t=t_0$ produces an instantaneous elastic jump, then a slower, retarded (viscoelastic) creep as the force is held; removing the force at $t=t_1$ produces an instantaneous elastic recovery followed by a slower retarded recovery that approaches a residual permanent set — the fraction of deformation that never recovers because it represents genuine viscous flow between polymer chains. More inter-protein disulphide crosslinks stiffen the gluten network and restrict that chain slippage, so the modified dough shows a smaller instantaneous jump, less creep during loading, and a smaller (more nearly complete) recovery gap.
The modified dough (more disulphide crosslinks) is preferred. Stamping out an intricately detailed shape requires the cut edges and fine features to hold their form through handling and proofing rather than sagging or slumping under their own weight — exactly the creep resistance a stiffer, more elastic (less viscous-flow-prone) crosslinked network provides. The original, less-crosslinked dough would show more time-dependent creep and a larger permanent set, letting a delicate cut-out slump and blur its detail before baking.
