04-BS-13 · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 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 are calculation questions.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts (2nd ed., Prentice Hall) — elemental/electron balances, yield coefficients, fermenter energy balances; Madigan et al., Brock Biology of Microorganisms (15th ed., Pearson) — bacterial/viral/fungal morphology and physiology; Toledo, Fundamentals of Food Process Engineering (3rd ed., Springer) — plant/animal tissue structure, rheology, water activity.
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.
Water movement. Water crosses the semi-permeable cell membrane by osmosis, driven by a water-potential gradient $\Psi=\Psi_{\text{solute}}+\Psi_{\text{pressure}}+\Psi_{\text{matric}}$: water moves from regions of higher (less negative) water potential to lower, i.e. toward higher solute concentration, until the developing turgor (pressure) potential exactly balances the osmotic gradient at equilibrium. Loss of external water potential (drying, high-solute environments such as sugar/salt cures) reverses the gradient and draws water out of the cell (plasmolysis), while a low-solute external environment drives water in, up to the wall's mechanical limit.
Retention — free vs. bound water. Free water is unbound bulk water: it behaves like pure water (freezes near 0°C, is fully available as a solvent and for microbial growth, and contributes fully to vapour pressure). Bound water is water tightly associated with polar sites on proteins, starches and cell-wall polysaccharides via hydrogen bonding; it does not freeze at normal storage temperatures, has restricted molecular mobility, and is largely unavailable for microbial growth or as a reaction solvent.
Molecular adsorption mechanisms. The transition from bound to free water as moisture content rises is described by multilayer (BET) adsorption theory: the first monolayer binds strongly to specific polar sites, subsequent layers bind progressively more weakly and behave increasingly like bulk water, and adsorption/desorption sorption isotherms of biological materials typically show pronounced hysteresis.
Water activity. Water activity $a_w=p/p_0$ (vapour pressure of water in the material over that of pure water at the same temperature, $=\text{ERH}/100$) is the thermodynamically meaningful measure of "available" water. Most bacteria require $a_w>0.90$, most yeasts $>0.88$, most moulds $>0.80$, while osmophilic/xerophilic organisms can grow down to roughly $0.60$–$0.65$; reaction rates such as non-enzymatic (Maillard) browning and lipid oxidation both show characteristic, non-monotonic dependence on $a_w$ (oxidation is often fastest at very low $a_w$).
Influence on material properties. Water activity and its distribution between free and bound states govern microbial and chemical stability (the basis for drying, curing, and intermediate-moisture-food preservation strategies), and — through the plasticizing effect of water on amorphous biopolymers — control the glass transition temperature $T_g$: moisture uptake above a critical level lowers $T_g$ below ambient, converting a glassy, crisp solid (crackers, dried fruit) into a rubbery one, the physical basis of staling and loss of crispness.
