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) Carbon dioxide — beneficial as a deliberate modified-atmosphere packaging (MAP) component and carbonation gas: it is bacteriostatic/fungistatic against many aerobic spoilage organisms and dissolves to lower surface pH. Detrimental as a spoilage by-product: $CO_2$ produced by unwanted microbial growth causes can/pack swelling ("gassy" spoilage, blown containers).
(ii) Organic acids (lactic, acetic) — beneficial as deliberate fermentation products that preserve food by lowering pH below the range pathogens/spoilage organisms tolerate (yogurt, sauerkraut, pickles) while contributing desirable tang. Detrimental when they appear as an unwanted spoilage signature (souring of milk or juice by contaminating lactic/acetic-acid bacteria), giving off-flavours in products not meant to be fermented.
(iii) Alcohols (ethanol) — beneficial as the intended product of yeast fermentation in beer, wine and spirits, contributing flavour and some antimicrobial preservation. Detrimental as an indicator of unwanted yeast spoilage/fermentation in products that should not ferment, producing a "boozy" or yeasty off-flavour in juice or canned goods.
(iv) Hydrogen peroxide — beneficial as a natural antimicrobial produced by some lactic acid bacteria (part of the lactoperoxidase system in raw milk), contributing to biopreservation. Detrimental in that it causes oxidative off-flavours/bleaching of pigments, and at higher residual concentrations (e.g. as a sanitiser residue) is itself toxic and must be controlled.
(v) Volatiles (sulphur compounds, ammonia) — beneficial as characteristic desirable flavour notes in some aged/fermented foods (sulfurous notes in aged cheese, allium flavours). Detrimental as the classic warning odours of microbial protein putrefaction — ammoniacal/fishy odour in spoiled fish or meat, rotten-egg $H_2S$ from spoiled eggs — almost always a strong rejection signal for consumers.
(vi) Polysaccharides — beneficial as deliberately produced microbial exopolysaccharides (EPS) used as food texturisers/thickeners (xanthan gum, dextran) and for desirable viscosity in some fermented dairy products. Detrimental as the cause of "ropiness"/ sliminess spoilage in bread, milk or meat from unwanted EPS-producing spoilage bacteria, and as the matrix of unwanted biofilms on processing equipment.
(b) Intrinsic factors (properties of the food itself): pH/acidity, and water activity ($a_w$) — both directly set which organisms can grow at all. Extrinsic factors (properties of the storage environment): storage temperature, and the surrounding gas atmosphere/relative humidity — both are controlled by the processor/ distributor rather than being fixed properties of the food.
