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.
Antibiotics: beneficial as a natural competitive-exclusion tool (some starter and probiotic organisms produce antibiotic compounds that suppress spoilage/pathogenic competitors in fermented foods) and as a therapeutic residue-free processing aid in some regulated contexts; detrimental when they persist as unintended residues from animal production, contributing to antimicrobial-resistance selection pressure and, for sensitized consumers, allergic reactions. Bacteriocins (e.g. nisin): beneficial as narrow-spectrum, naturally produced antimicrobial peptides used to inhibit spoilage and pathogenic Gram-positive organisms (notably Listeria) in cheese and processed meats without the regulatory burden of a synthetic preservative; largely non-detrimental to the consumer, though resistance can develop in target organisms with repeated exposure. Toxins: essentially only detrimental — preformed bacterial exotoxins (e.g. Staphylococcus aureus enterotoxin, Clostridium botulinum neurotoxin) and endotoxins cause foodborne illness and are not inactivated by normal cooking once formed, making toxin prevention (not just pathogen kill) essential. Amylases: beneficial in baking (improving crumb texture, extending shelf life by retarding starch retrogradation, providing fermentable sugars for yeast) and in brewing/starch processing; detrimental when uncontrolled microbial amylase activity in a raw ingredient causes unwanted starch breakdown, viscosity loss or off-texture before processing. Lipase: beneficial in cheese ripening (developing characteristic flavour through controlled fatty-acid liberation, e.g. blue and Italian-type cheeses) but detrimental as the principal cause of hydrolytic (lipolytic) rancidity in raw milk and fats when uncontrolled. Pectinase: beneficial in fruit-juice/wine processing (improving juice yield and clarification by breaking down cell-wall pectin) but detrimental as a major cause of fruit and vegetable tissue softening and spoilage when produced by contaminating moulds and soft-rot bacteria.
| Food | Risk | Rationale |
|---|---|---|
| Bean sprouts | High | Seeds are frequently contaminated before germination, and the warm, humid sprouting process itself is an ideal growth environment; sprouts are typically eaten raw with no kill step. |
| Lettuce | High | Raw, minimally processed leafy produce grown close to soil/irrigation water is repeatedly implicated in Salmonella outbreaks; no kill step before consumption. |
| Eggs | High | Classic vector for Salmonella Enteritidis, which can be deposited inside the egg via transovarian infection before the calcified outer covering forms, independent of contamination on the outside of the egg. |
| Poultry | High | Salmonella's primary intestinal reservoir; carcass cross-contamination during slaughter/processing is common. |
| Bread | Low | Baking temperature far exceeds Salmonella's thermal tolerance and the finished product's low water activity does not support growth. |
| French fries | Low | Frying is a lethal thermal step, and post-fry moisture content/oil content is unfavourable for survival or growth. |
| Yogurt | Low | Fermentation drives pH well below the range Salmonella can tolerate, and the starter culture's competitive exclusion further suppresses survival. |
| Orange juice | Low† | Commercial juice is pasteurized; †an unpasteurized ("fresh-squeezed"/raw) product reverts to high risk, since unpasteurized citrus juice has caused documented outbreaks. |
