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)(i) The most likely mechanism is non-enzymatic (Maillard) browning, not enzymatic browning. Steam-cooking the peeled potatoes is a blanching-equivalent heat step that denatures and inactivates polyphenol oxidase (PPO), the enzyme responsible for enzymatic browning, well before the mash ever reaches the heated drying drums. What survives the process are reducing sugars (glucose and fructose released by starch breakdown during cooking) and free amino acids/proteins native to the potato, and it is precisely these two ingredients — a carbonyl group and a free amine — that react through the Maillard pathway once the mash is spread thin on a hot metal drum at drying temperature. The characteristic golden-brown colour, appearing "as soon as they are produced," is consistent with a fast, heat-driven, non-enzymatic reaction occurring during drum drying rather than a slower oxidative browning developing over time in air.
(a)(ii) Running the dryer under nitrogen is not likely to be effective. The Maillard reaction is a condensation reaction between a reducing sugar and an amino group; it does not consume molecular oxygen and proceeds equally well in an inert atmosphere. Excluding oxygen would help only if the discoloration were oxidative in origin — either enzymatic browning (which needs both an active PPO enzyme and $O_2$) or non-enzymatic lipid/pigment oxidation. Since the enzyme has already been inactivated by the steam-cook step and the reaction driving the colour is Maillard browning, the processor's proposed fix addresses the wrong mechanism; she would do better to reduce drum surface temperature or residence time (both of which control Maillard reaction rate directly), lower the reducing-sugar load pre-drying, or adjust pH, rather than change the drying atmosphere.
(b) Guinness is dispensed through a nitrogen widget/restrictor plate rather than being carbonated with $CO_2$ alone, so its bubbles are almost entirely nitrogen and are nucleated as very small bubbles by the fine mesh of the dispense system; a light beer's bubbles are larger $CO_2$ bubbles nucleating on the glass surface. Since the two drinks are stated to have similar bulk viscosity, viscosity cannot explain the difference — the governing factor is bubble size. By Stokes' law, the terminal (buoyant) rise velocity of a small bubble scales with the square of its radius, $v \propto r^2$, for the same fluid viscosity and density difference. The much smaller nitrogen bubbles in Guinness therefore rise dramatically more slowly than the larger carbon-dioxide bubbles in a light beer, even though the two liquids feel about equally viscous to the tongue.
