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22-Agric-A7 Chemistry and Microbiology of Foods · May 2017

Question 10 of 12: HACCP Principles and Quorum Sensing

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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).

Section I — Food Chemistry

Question 10: HACCP Principles and Quorum Sensing (16.7 marks)

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) The seven HACCP principles, in order

  1. Conduct a hazard analysis — identify biological, chemical and physical hazards reasonably likely to occur at each process step and assess their significance.
  2. Determine the Critical Control Points (CCPs) — the specific steps at which control can be applied and is essential to prevent, eliminate or reduce an identified hazard to an acceptable level.
  3. Establish critical limits for each CCP — the measurable maximum/minimum value (time, temperature, pH, etc.) that separates safe from unsafe at that point.
  4. Establish a monitoring system for each CCP — the procedure and frequency for observing whether the CCP remains within its critical limit.
  5. Establish corrective actions to be taken when monitoring shows a CCP has deviated from its critical limit.
  6. Establish verification procedures to confirm the HACCP system as a whole is working as intended (e.g. calibration checks, record review, periodic validation).
  7. Establish documentation and record-keeping covering the hazard analysis, CCPs, critical limits, monitoring, corrective actions and verification.

(b) Quorum sensing in Gram-positive and Gram-negative bacteria

Quorum sensing is a population-density-dependent gene-regulation mechanism: individual cells continuously secrete a small signalling molecule (an autoinducer) at a low, constant per-cell rate. At low cell density the autoinducer diffuses away and stays below the threshold concentration needed to trigger a response, so density-dependent genes (biofilm formation, virulence-factor secretion, bioluminescence, sporulation) remain off. As the population grows, autoinducer accumulates in the local environment in direct proportion to cell number; once its concentration crosses a threshold, it binds a dedicated receptor and switches on coordinated, population-wide gene expression — behaviour that is only useful once enough cells are present to carry it out collectively.

Low cell density Receptor below threshold: genes OFF High cell density Autoinducer bound: genes ON (group behaviour) Gram-negative: AHL diffuses freely; binds cytoplasmic LuxR-type receptor Gram-positive: secreted oligopeptide (AIP); detected by membrane 2-component kinase
General quorum-sensing circuit: autoinducer accumulates with cell density and switches gene expression on past a threshold. Gram-negative organisms typically use a freely diffusible acyl-homoserine lactone (AHL) sensed by a cytoplasmic LuxR-type receptor; Gram-positive organisms typically use a secreted oligopeptide (autoinducing peptide) sensed by a membrane-bound two-component histidine-kinase receptor, since peptides cannot freely cross the membrane.