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) Hydrogenation is the catalytic addition of hydrogen gas across the carbon–carbon double bonds of unsaturated fatty acids in an oil (typically using a nickel catalyst under heat and pressure), converting some or all of those double bonds to single bonds. This raises the degree of saturation of the fat, which raises its melting point and oxidative stability and converts a liquid oil into a more solid, plastic fat suitable for margarine or shortening; partial hydrogenation also isomerises some remaining double bonds from the natural cis to the trans configuration.
(a)(ii) Interesterification rearranges the fatty acids already present on the glycerol backbones of a fat or fat blend — randomising, or under enzymatic (lipase) catalysis directing, which fatty acid sits at which position of each triglyceride — without hydrogenating any double bonds and without changing the overall fatty-acid composition of the blend. Because a triglyceride's melting behaviour depends strongly on which fatty acids share the same glycerol molecule, this redistribution changes the fat's crystallisation and plasticity (softening point, solid-fat content profile) even though its degree of unsaturation is untouched; it is widely used as a trans-fat-free alternative to partial hydrogenation for producing plastic fats.
(b) A fat is not a single pure compound but a mixture of many different triglycerides, each with its own individual melting point, and, more fundamentally, crystallisation (freezing) is a nucleation-limited process while melting is not. As a liquid fat is cooled, it can be carried well below the temperature at which the solid phase is thermodynamically stable before a stable crystal nucleus actually forms and growth begins — this supercooling delay means the fat does not solidify until some temperature measurably below its true thermodynamic freezing point. Melting has no equivalent kinetic barrier: as soon as the temperature reaches a crystal's melting point, that crystal simply melts with no need to "nucleate" the liquid phase. Fats are also polymorphic (they can crystallise into several different crystal forms — $\alpha$, $\beta'$, $\beta$ — of different stability and melting point); on cooling, the least stable, lowest-melting polymorph typically nucleates first (Ostwald's rule of stages), further lowering the observed freezing point relative to the melting point of the more stable form that eventually develops on standing.
