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.
Biological materials of interest to a process engineer — fruits, vegetables, tubers, stalks, and by extension animal hides and connective tissue — are built from a small number of tissue systems whose arrangement and cell-wall chemistry directly set the mechanical, transport and sensory properties exploited (or fought against) in handling and processing.
Protective tissues form the outer boundary of the organ. In young plant organs this is the epidermis, a single layer of tightly packed cells coated with a cutin/wax cuticle that is nearly impermeable to liquid water and greatly restricts gas exchange except through stomata; in woody stems, roots, and mature tubers it is replaced by periderm (cork/phellem), whose suberin-impregnated cell walls are even more effective moisture and pathogen barriers. The animal analogue is skin/hide (keratinized epidermis over a collagenous dermis) and, in arthropods, the chitinous exoskeleton. Materially, the toughness and continuity of this outer layer governs bruise and puncture resistance, controls the rate of moisture loss (and hence shrinkage and firmness loss) during storage, and — because it is also the primary barrier to microbial ingress — sets the postharvest shelf life; processed hide (leather) derives its tensile strength from the same collagen fibre network.
Conductive (vascular) tissues are xylem, which transports water and dissolved minerals and whose tracheid/vessel cells are reinforced with lignified secondary walls for both conduction and mechanical rigidity, and phloem, whose sieve-tube/companion-cell complexes translocate photosynthate. Because xylem fibres are strongly aligned along the organ's long axis, vascular bundles impose a pronounced anisotropy on the tissue: wood, stalks (celery "strings"), and fibrous root vegetables are far stronger and stiffer along the grain than across it, which dictates the preferred cutting/splitting direction in processing and the fibrous "stringiness" perceived on chewing.
Ground tissue makes up the bulk volume of most fruits, vegetables and storage organs. Parenchyma — thin-walled, high water content, turgor-pressurized — is the dominant contributor to juiciness and crisp/firm texture; collenchyma, with unevenly thickened but non-lignified walls, gives flexible support (petioles, young stems); sclerenchyma (fibres and lignified sclereids, e.g. the "stone cells" of pears) is rigid and, in excess, perceived as grittiness or toughness. The proportion and wall chemistry of these three ground-tissue types is the single largest determinant of the texture profile — firmness, crispness, mealiness — that governs cutting force, bruising susceptibility and consumer acceptance during processing.
