22-Agric-A6 Physical Properties of Biological Materials and Food Products · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A6 Physical Properties of Biological Materials and Food Products, National Exams May 2017 — a three-hour closed-book exam (approved calculator permitted; one aid sheet, both sides). Nine questions are set and candidates answer any five, each worth 20 marks, for a 100-mark paper. All nine are worked here so the set is a complete study resource.
Reference texts. M.A. Rao, S.S.H. Rizvi, A.K. Datta and J. Ahmed, Engineering Properties of Foods, 4th ed. (rheology of fluid and semisolid foods, particle size, surface/interfacial properties); N.N. Mohsenin, Physical Properties of Plant and Animal Materials, 2nd ed. (thermal properties, calorimetry, texture and rheological testing); R.P. Singh and D.R. Heldman, Introduction to Food Engineering, 5th ed. (thermal-property measurement, freezing-point depression, particle size); J.F. Steffe, Rheological Methods in Food Process Engineering, 2nd ed. (viscometry, viscoelasticity, the Kelvin-Voigt/Maxwell models, time-dependent flow behaviour); R.L. Earle, Unit Operations in Food Processing, 2nd ed. (particle-size averages, specific surface from sieve/count data).
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) Squeezing a tomato. A shopper presses a thumb gently into the tomato's side and reads the small, elastic give-and-recovery of the skin and flesh as an indicator of ripeness and firmness. (i) Small strain — the shopper deliberately avoids bruising the fruit, so the applied deformation stays in the linear-elastic, non-damaging range. (ii) Slow rate — a gentle, quasi-static press, not a sudden poke. (iii) Lab simulation: a low-strain (a few percent), low-speed compression test with a flat or curved probe on a texture analyzer, reporting the initial slope of the force–deformation curve as a firmness/elastic-modulus proxy, well below the fruit's failure strain.
(b) Thumping a melon. A consumer taps or thumps the melon's surface with a knuckle and listens to the resulting sound. (i) Large, but very brief and localized, strain at the impact point — the tap itself is a rapid, small-amplitude compressive impulse rather than a static press. (ii) Fast (impact) rate — this is fundamentally a dynamic, not a quasi-static, test. (iii) Lab simulation: an instrumented impact/acoustic test — a small calibrated hammer or pendulum strikes the fruit and a microphone or accelerometer records the resonant frequency/damping of the resulting sound, which shifts with the melon's internal firmness and, indirectly, its ripeness-related internal structure (a duller, more damped thump for an over-ripe or internally broken-down melon).
(c) Judging a milkshake's consistency. A consumer draws the milkshake through a straw (or watches how it pours/coats a cup) and judges its "thickness" from the effort/speed of that flow. (i) Not applicable as a solid-mechanics strain — this is a continuous SHEAR flow rather than a bounded strain, occurring over a wide, MEDIUM-TO-HIGH range of shear rates as the fluid accelerates through the straw's narrow bore. (ii) Medium-to-fast shear rate — drawing through a straw or a pour imposes shear rates roughly in the 1–100 s\(^{-1}\) range, well above a simple gravity-only trickle. (iii) Lab simulation: a rotational or capillary viscometer sweep across that shear-rate range, reporting apparent viscosity (or the power-law K, n) at the shear rate that best matches straw-drinking, since a single-point viscosity at the wrong shear rate can rank two milkshakes in the opposite order a consumer would.
(d) Chewing an apple. A consumer bites through and chews a piece of apple, sensing crispness (an initial fracture event with an audible snap) followed by juiciness and texture breakdown as chewing continues. (i) Large strain — mastication carries the tissue well past its fracture point, repeatedly. (ii) Fast rate — the jaw closes over a fraction of a second per bite, a dynamic impact-like loading similar in speed to (b) but sustained over repeated cycles. (iii) Lab simulation: an Instron/texture-analyzer single- or double-bite compression test at a high crosshead speed with a wedge or incisor-shaped probe, reporting the initial fracture (peak) force/stress as a crispness index and the shape of the post-fracture curve (a jagged, multi-peak trace for a crisp, cellular apple tissue) as a texture fingerprint, sometimes paired with an acoustic sensor to capture the audible snap directly.
(e) Biting a potato chip or cheese puff. A consumer bites down on the thin, brittle, aerated structure and senses a sharp, low-force "crunch" and shatter. (i) Large strain — the structure is driven well past its (very low) fracture strain almost immediately. (ii) Fast rate — a rapid bite, and the material's own brittle failure is itself a very fast (near-instantaneous) event once initiated. (iii) Lab simulation: a three-point bend or flat-probe compression test at high crosshead speed on a texture analyzer, capturing the characteristic jagged, multi-peak, low-force force–deformation trace (each peak a local cell-wall fracture) together with an acoustic-emission microphone, since for a brittle cellular snack the AUDIBLE crunch pattern (number and timing of sound peaks) is often a more sensitive quality indicator than the peak force alone.
