22-Agric-A7 Chemistry and Microbiology of Foods · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A7 Chemistry and Microbiology of Foods, National Exams December 2013 — a three-hour closed-book exam (approved Casio/Sharp calculator permitted; one aid sheet, both sides). The paper is in two sections: Section I (Food Chemistry, Questions 1–7) and Section II (Food Microbiology, Questions 8–14); candidates answer any four questions from each section for a 100-mark paper (each question worth 12.5 marks). All fourteen 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. (Maillard/enzymatic browning, water activity and sorption isotherms, lipid oxidation and rancidity, sucrose glass transition, protein denaturation at interfaces, myoglobin chemistry); R.P. Singh and D.R. Heldman, Introduction to Food Engineering, 5th ed. (reaction kinetics in food processing, thermal process lethality); J. Jay, M. Loessner and D. Golden, Modern Food Microbiology, 7th ed. (microbial growth curve, intrinsic/extrinsic factors, Listeria monocytogenes, food preservation hurdles, irradiation, spoilage patterns); C. Mortimore and C. Wallace, HACCP: A Practical Approach, 3rd ed. (CCP identification/monitoring/verification for milk pasteurization).
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) A saturated salt solution — a salt held at a fixed temperature with excess solid salt still undissolved in contact with the liquid — fixes the headspace equilibrium relative humidity (ERH) at one specific, reproducible value determined only by the salt's identity and the temperature, independent of how much water or salt is actually present. This is because, as long as both solid salt and its saturated solution coexist, the solution's water activity (and hence the vapour pressure it exerts) is pinned at the salt's saturation value; any water absorbed from or released to the chamber air simply dissolves a little more salt or precipitates a little more out, without changing the solution's composition or its equilibrium vapour pressure. An unsaturated solution, by contrast, has an aw that drifts as it gains or loses moisture. Published tables give the %RH produced by many common salts at various temperatures (for example, saturated NaCl gives about 75% RH at 25°C, saturated MgCl$_2$ about 33% RH), so a processor can select a salt to buffer a storage chamber at essentially any desired, self-correcting RH set point for stability testing or controlled storage.
(b) The water activity of a food is measured by sealing a sample in a small closed chamber and allowing the headspace air to come to moisture equilibrium with the food at constant temperature, then measuring the resulting equilibrium relative humidity (ERH) of that headspace; by definition $a_w = \text{ERH}/100$. In practice this is done with (i) a chilled-mirror dew-point hygrometer, which cools a mirror in the headspace until condensation first appears and reads the dew-point temperature, from which the equilibrium vapour pressure (and hence $a_w$) is calculated; (ii) an electric (resistive or capacitive) humidity sensor sealed in the equilibrium chamber with the sample, which reads %RH directly once equilibrium is reached; or (iii) the isopiestic (equilibrium desiccator) method, in which small samples are stored over a series of saturated salt solutions of known $a_w$ (part (a)) until each sample's moisture content stops changing, giving a full moisture sorption isotherm from which the food's own $a_w$ at its current moisture content is read.
