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) Maillard reaction — a non-enzymatic reaction between the carbonyl group of a reducing sugar and a free amino group of an amino acid or protein, proceeding through an Amadori rearrangement to brown melanoidin pigments and a wide range of flavour volatiles; it requires heat and proceeds fastest at intermediate water activity. Example: the brown crust that forms on baked bread or a seared steak.
(a)(ii) Enzymatic browning — oxidation of phenolic compounds, catalysed by the enzyme polyphenol oxidase (PPO) in the presence of molecular oxygen, forming quinones that polymerise into brown pigments; it requires an active enzyme, a phenolic substrate, and oxygen, and typically occurs when plant tissue is cut or bruised. Example: the browning of a cut apple or potato surface exposed to air.
(a)(iii) Foam — a colloidal dispersion of gas bubbles within a continuous liquid or semi-solid phase, kinetically stabilised by a surface-active film (protein or surfactant) adsorbed at the gas–liquid interface that lowers interfacial tension and resists bubble coalescence and liquid drainage. Example: whipped egg-white meringue, or the head on a poured beer.
(b) In the sealed, oxygen-free vacuum pack, the meat's myoglobin pigment exists as deoxymyoglobin — the iron in the haem group is in the ferrous ($\text{Fe}^{2+}$) state with no ligand bound to its sixth coordination site — which appears a dull purplish-red. As soon as the package is opened and the cut surface is exposed to atmospheric oxygen, myoglobin reversibly binds $O_2$ at that site to form oxymyoglobin (a $\text{Fe}^{2+}$–$O_2$ complex), a bright cherry-red pigment; this rapid colour change, called "bloom," is a simple reversible oxygenation, not an oxidation of the iron itself. If the cut surface stays exposed to air for an extended period, myoglobin is instead slowly and irreversibly oxidised to metmyoglobin ($\text{Fe}^{3+}$), a brown pigment, which is why meat displayed too long eventually turns brown at the surface rather than staying bright red.
