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.
Native soy globulins (glycinin, 11S; β-conglycinin, 7S) carry a net negative surface charge at neutral pH from ionized carboxylate side chains (Asp/Glu, –COO$^-$), which keeps unfolded/partially unfolded protein molecules electrostatically repelled and dispersed after a heat-denaturation step exposes their hydrophobic and reactive groups. Divalent Ca$^{2+}$ ions neutralize and bridge this negative charge, forming ionic "egg-box"-type crosslinks between carboxylate groups on adjacent protein molecules: $$\text{Protein-COO}^-\ \cdots\ \text{Ca}^{2+}\ \cdots\ ^-\text{OOC-Protein}$$ Each Ca$^{2+}$ ion coordinates two (or more) carboxylate groups from different denatured protein strands simultaneously. Because this happens at many sites along many unfolded chains at once, the calcium bridges knit the previously dispersed, repelling protein molecules into a continuous, cross-linked three-dimensional network that traps the surrounding water — a gel — rather than allowing the proteins to simply aggregate into a dense, phase-separated precipitate. Hydrophobic interactions and some disulphide bonding between exposed nonpolar/thiol groups (freed by the same denaturation step) reinforce the network alongside the calcium bridges.
Both outcomes start the same way — denaturation exposes reactive groups and protein– protein interactions begin to dominate over protein–solvent interactions — but the outcome depends on the balance and rate of attractive crosslinking versus aggregation. A gel forms when moderate, well-distributed crosslinking (ionic bridges, limited hydrophobic contacts, controlled disulphide formation) proceeds slowly enough, and at a protein concentration high enough, that a fine, continuous three-dimensional network forms before the protein strands have a chance to collapse into large, dense clusters; the network's mesh then physically entraps the solvent (water) throughout its volume. A precipitate forms instead when attractive forces are strong, fast and largely uncontrolled — for example, driving the pH to the protein's isoelectric point (where net charge and hence electrostatic repulsion is near zero) or adding excess crosslinking ion — so that unfolded molecules aggregate into large, dense, essentially solvent-excluding clumps that settle out of suspension rather than knitting into an open, water-holding mesh. In short: the same denaturation/crosslinking chemistry gives a gel when it proceeds gradually at a moderate, well-tuned driving force and a precipitate when the driving force is too strong or too fast for an ordered network to form first.
Milk rancidity is driven chiefly by two independent mechanisms — lipolytic (hydrolytic) rancidity, where lipase liberates free short-chain fatty acids from the triglyceride, and oxidative rancidity, where unsaturated fatty acids undergo free-radical autoxidation — so effective control must address both. Against lipolysis: minimize mechanical agitation, foaming, freezing/thawing and temperature abuse of raw milk (activities that disrupt the native fat-globule membrane and expose triglyceride to the milk's own lipoprotein lipase), and pasteurize promptly to inactivate the enzyme before significant hydrolysis occurs. Against oxidation: exclude light (particularly UV/blue light, which photosensitizes riboflavin-catalysed oxidation — opaque or UV-blocking packaging helps), minimize dissolved oxygen and trace pro-oxidant metal contamination (copper/iron from equipment), maintain cold-chain temperatures to slow the radical-chain reaction rate, and consider adding approved antioxidants or ensuring adequate natural antioxidant (e.g. vitamin E/tocopherol, carotenoid) status in the herd's feed. Finally, good general sanitation and rapid cooling after milking limit both the microbial lipase/lipoxygenase load and the time available for either pathway to progress before processing.
