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.
| Step | Change in total solid fat | Change in polymorph distribution |
|---|---|---|
| Heat to 50°C | Solid fat falls to essentially zero — all six cocoa-butter polymorphs (Forms I–VI) melt, including the desirable Form V, wiping out any prior crystal history. | Melt is a single homogeneous liquid; no polymorph memory remains. |
| Cool to 25°C and hold | Solid fat content rises sharply as the melt undercools and nucleates. | Nucleation favours the least stable, lowest-melting polymorphs (Forms I–IV) because they have the lowest nucleation energy barrier at this degree of undercooling; the crystal population is a disorganized mixture dominated by these unstable forms. |
| Reheat to 32°C | Total solid fat content drops again, but only partially — enough heat is supplied to melt out the unstable Forms I–IV (which have lower melting points) while leaving the more stable, higher-melting seed crystals intact. | The polymorph balance shifts decisively toward Form V — this selective-melting step is exactly what "tempering" means: it removes the unstable seeds so only Form V nuclei survive to template the final crystallization. |
| Pour into a mold and cool to room temperature (10 marks) | Solid fat content rises to its final, high plateau as the whole mass solidifies around the surviving seeds. | Because only Form V seeds remain from the previous step, the bulk of the chocolate crystallizes directly into Form V — the glossy, snapping, stable polymorph — rather than reverting to the unstable forms, which is what gives properly tempered chocolate its gloss, snap and resistance to fat bloom. |
Milk fat is cooled quickly through its crystallization range specifically to control the number of nuclei formed, not to maximize crystal amount: crystallization kinetics are governed by a competition between nucleation rate and crystal growth rate as functions of undercooling. A rapid quench takes the melt quickly through the moderate-undercooling window where growth on a modest number of existing nuclei dominates, and out of the deep undercooling range where nucleation rate explodes and produces vast numbers of tiny crystals; a slow cool, by contrast, lingers in the high-nucleation-rate zone and produces a fine, numerous crystal population. Fewer nuclei, each fed by more of the available high-melting triglyceride mass, grow into fewer, larger individual crystals. Large crystals are then easier to separate by centrifugation for two physical reasons captured in Stokes' settling behaviour: the centrifugal separating force on a particle scales with its volume (radius cubed) while the opposing viscous drag scales only with radius, so a larger crystal experiences a much higher net separating force per unit drag and settles (or is thrown to the bowl wall) far faster than a small one; and large crystals are also mechanically easier to retain on a separator's crystal-collecting surfaces without being re-entrained into the liquid stream, whereas a fine crystal population behaves almost like a colloidal suspension and resists clean mechanical separation.
