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) Ionising radiation (gamma rays, electron beam, or X-ray) inactivates microorganisms primarily by damaging their DNA, through two complementary mechanisms: direct action, in which a radiation photon or electron directly ionises and breaks the DNA backbone itself, and (the dominant pathway in the water-rich environment of a cell) indirect action, in which radiation instead ionises the abundant water surrounding the DNA, generating highly reactive free radicals (hydroxyl radical $\text{OH}^{\bullet}$, hydrated electron $e^-_{aq}$, hydrogen radical $\text{H}^{\bullet}$) that diffuse to and attack the DNA, causing single- and double-strand breaks and base damage. Because a double-strand break that cannot be correctly repaired prevents the chromosome from replicating, the cell (or virus) loses the ability to reproduce even if it otherwise remains metabolically active; larger, more genome-redundant organisms (moulds, insects) are correspondingly harder to inactivate by radiation dose than small, single-copy-genome bacteria, and bacterial spores (with their very low free-water content) are more radiation-resistant than vegetative cells because indirect, water-radical-mediated damage is suppressed.
(b) Advantages of glove use: gloves provide a continuous physical barrier between the handler's skin (and any microorganisms/allergens on it) and the product, they are an easily and visually verifiable compliance measure (a supervisor can see at a glance whether a glove is worn), and they protect the handler from the product (e.g. from chemical or thermal hazards) as well as the reverse. Disadvantages: gloves can create a false sense of security that leads to reduced handwashing frequency/care underneath them, undetected tears or punctures let contamination through without the visual warning bare-hand contamination might give, prolonged wear traps moisture and warmth against the skin which can actually promote bacterial proliferation under the glove if it is not changed often enough, gloves must be changed between tasks (raw-to-ready-to-eat) just as hands must be washed — a step that is often skipped precisely because the glove is assumed to already be "clean" — and some glove materials (latex) introduce an allergen risk to both handlers and, via residue, consumers.
(c) An egg has several independent, layered natural defences protecting the developing embryo: the calcified outer covering itself is a physical calcium-carbonate barrier whose pores are coated by a proteinaceous cuticle that plugs the pores and is itself antimicrobial, sharply restricting bacterial entry; beneath that calcified covering, the inner and outer egg membranes add a further physical filter and antimicrobial layer, enclosing the air cell; the albumen (egg white) contains multiple antimicrobial proteins — lysozyme, which enzymatically lyses bacterial cell walls; ovotransferrin (conalbumin), which chelates free iron and starves iron-dependent bacteria of a required nutrient; and avidin, which irreversibly binds biotin, denying it to microorganisms — and the albumen's naturally high pH ($\approx$9 in a fresh egg) is itself inhibitory to many bacteria; finally, the vitelline membrane physically separates the nutrient-rich yolk from the albumen's antimicrobial environment, so a pathogen must breach every one of these layers in sequence to reach the yolk.
