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) Ranked from fastest-spoiling to most stable: whole fish > raw milk > sirloin steak > green tomato > deli meat (MAP). Whole fish spoils fastest of all: it has a high, near-neutral surface pH ($\approx$6.5–7, no protective acid barrier), high water activity, and its own autolytic enzymes plus cold-adapted, TMAO-reducing spoilage bacteria (Shewanella, Pseudomonas) remain active even at 4°C, giving the classic "fishy" spoilage odour within days. Raw milk is a rich, near-neutral-pH liquid medium that supports rapid growth of psychrotrophic Pseudomonas spp. at refrigeration temperature, spoiling within about a week. Sirloin steak, though similarly exposed to air on a plate, has a lower muscle pH ($\approx$5.5, from post-mortem glycolysis/lactic acid) and an intact surface with less exposed cut area than a whole fish carcass, giving aerobic Pseudomonas spoilage somewhat longer to develop. Green tomato at 15°C is warmer than the refrigerated items, but its intact waxy cuticle is a strong physical barrier against microbial invasion, and being a still-living plant tissue its spoilage is initially dominated by slow physiological ripening/senescence rather than rapid microbial attack. Deli meat under refrigerated Modified Atmosphere Packaging is the most stable: it is a cooked (low initial microbial load), CO$_2$-enriched, low-oxygen package held cold, which suppresses the aerobic spoilage organisms that dominate the other four foods.
(b) Listeria monocytogenes is a Gram-positive, non-spore-forming, facultatively anaerobic rod that is unusually well adapted to food-processing environments: it is psychrotrophic (able to grow slowly even at refrigeration temperatures, and in some strains close to 0°C), tolerant of relatively high salt concentrations ($\ge$10% NaCl in some conditions) and a fairly wide pH range, and readily forms persistent biofilms on plant equipment (drains, conveyors, slicers) that are difficult to fully sanitise. It causes foodborne illness (listeriosis) by first colonising the gut, then invading the intestinal epithelium and using a virulence factor (listeriolysin O) to escape the phagosome, replicate intracellularly, and spread cell-to-cell — allowing it to cross the intestinal, placental and blood–brain barriers, producing severe invasive disease (septicaemia, meningitis, miscarriage/stillbirth in pregnant women) particularly in the immunocompromised, elderly, neonates and pregnant women. L. monocytogenes is especially associated with deli meats, smoked fish and soft cheeses because all three are ready-to-eat products with no further cook step to kill any contamination introduced after processing (post-lethality contamination, e.g. from a slicer biofilm), are stored refrigerated for an extended shelf life (during which a psychrotroph keeps growing while competing organisms do not), and are moist, near-neutral to mildly salty products (brined/smoked fish, surface-ripened soft cheese) — conditions the organism specifically tolerates well.
