04-BS-13 · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-BS-13, Biology. Three-hour, closed-book exam (one double-sided aid sheet permitted, approved Casio/Sharp calculator allowed). Format: Part I offers 6 questions (any 3 constitute a complete answer, 20 marks each) and Part II offers 4 questions (any 2 constitute a complete answer, 20 marks each) — a full paper is 5 questions. All 10 are solved below for completeness. Most questions require an essay-format answer; Q1–Q4 and Q7 are calculation questions.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts (2nd ed., Prentice Hall) — elemental/electron balances, yield coefficients, fermenter energy balances, growth kinetics; Madigan et al., Brock Biology of Microorganisms (15th ed., Pearson) — bacterial/viral morphology, physiology and growth control; Toledo, Fundamentals of Food Process Engineering (3rd ed., Springer) — plant/animal tissue structure.
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.
The baseline curve (Fig. 1) is a standard sigmoidal growth curve for Listeria monocytogenes at 15°C, $a_w=0.99$, pH 6.8: a modest lag, a log (exponential) phase, and a stationary plateau. Each condition below is a resettable perturbation on this same lag–log–stationary shape, since Listeria is a psychrotrophic, facultatively anaerobic organism able to grow (slowly) even near refrigeration temperature, at $a_w$ down to about 0.90–0.92, and in the presence or absence of oxygen.
(a) 37°C, $a_w=0.99$, pH 6.8. 37°C is close to Listeria's optimum growth temperature (range roughly 30–37°C, versus its much wider tolerated range of −0.4 to 45°C). The curve keeps the same lag–log–stationary shape but is compressed in time: a shorter lag phase, a steeper (faster) exponential phase (higher $\mu_{\max}$), and an earlier onset of stationary phase, typically reaching a comparable or slightly higher maximum population density than the 15°C baseline.
(b) 37°C, $a_w=0.42$. An $a_w$ of 0.42 is far below the ~0.90–0.92 minimum required for Listeria growth (0.42 is characteristic of a low-moisture/dry food, not a growth-supporting medium). Regardless of the favourable temperature, the curve shows essentially no growth: the population stays flat at (or slowly declines from) the inoculum level — water activity, not temperature, is the dominant limiting factor here, and a sufficiently low $a_w$ overrides an otherwise optimal temperature.
(c) 15°C, $a_w=0.99$, pH 6.8, MAP (20% CO₂/80% N₂). Listeria is facultatively anaerobic, so it is not fully suppressed by the absence of O2 the way a strict aerobe would be — but elevated CO2 has a genuine bacteriostatic effect (it dissolves into the cell membrane/cytoplasm, lowering intracellular pH and directly inhibiting key enzymes). The curve keeps the same sigmoidal shape as the 15°C baseline but with a longer lag phase and a somewhat reduced growth rate during log phase; it is not flattened to no-growth the way (b) is, since MAP inhibits rather than prevents growth for a facultative organism like Listeria.
