22-Agric-A7 Chemistry and Microbiology of Foods · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A7 Chemistry and Microbiology of Foods, National Exams May 2017 — a three-hour closed-book exam (one aid sheet, both sides; approved calculator permitted). The paper is in two sections: Section I (Food Chemistry, Questions 1–6) and Section II (Food Microbiology, Questions 7–12); candidates answer any three questions from each section for a 100-mark paper (each question worth 16.7 marks). All twelve 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. (enzyme kinetics, water activity and sorption isotherms, lipid crystallization/polymorphism, protein gelation, popcorn starch/glass transition); R.P. Singh and D.R. Heldman, Introduction to Food Engineering, 5th ed. (reaction-order kinetics, quality-loss modelling); J.M. Steffe, Rheological Methods in Food Process Engineering, 2nd ed. (creep-recovery of viscoelastic doughs); J. Jay, M. Loessner and D. Golden, Modern Food Microbiology, 7th ed. (bacterial growth curve, intrinsic/ extrinsic factors, Salmonella, quorum sensing, viral/prion foodborne agents, rapid methods, sampling plans); C. Mortimore and C. Wallace, HACCP: A Practical Approach, 3rd ed. (the seven HACCP principles).
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.
Water activity is the equilibrium relative humidity of the headspace surrounding the food, expressed as a fraction, and it — not total moisture content — is what governs microbial growth, enzymic activity and many chemical deterioration rates. Inside a sealed package, the headspace air is a fixed, finite volume in intimate contact with the product, so $a_w$ can be managed three ways: (1) include a desiccant or humectant sachet co-packaged with the product, which buffers the headspace relative humidity toward a target value by exchanging moisture with the air rather than the food; (2) choose a barrier film with the right water-vapour transmission rate so that, over the product's shelf life, moisture migration through the packaging wall is negligible compared with the product's own moisture-sorption capacity; and (3) formulate the product itself with humectants (e.g. glycerol, sugars, salt) so its own equilibrium $a_w$ sits at the target value, since a sealed package eventually equilibrates to whatever $a_w$ the product's sorption isotherm dictates for its given moisture content. In a storage room, by contrast, the air volume is effectively infinite compared with any one product, so $a_w$ must be controlled by conditioning the room air itself: (1) mechanical dehumidification/ humidification and temperature control of the HVAC system to hold a target relative humidity; (2) air exchange rate management (minimizing uncontrolled infiltration of outside air, which would otherwise drag the room back toward ambient conditions); and (3) product turnover and loading density control, since a densely packed room of high-moisture product can itself raise local humidity around individual items faster than the HVAC system can compensate.
A food's moisture sorption isotherm — equilibrium moisture content (EMC) plotted against $a_w$ at fixed temperature — shifts downward as temperature rises. Physically, water binds to a food's polar and hydrophilic sites (sugars, proteins, salts) through an exothermic process (hydrogen bonding, hydration of ions); the net heat released when water binds is the isosteric heat of sorption. Raising the temperature adds thermal energy that favours the higher-entropy, unbound (desorbed) state, so at a fixed relative humidity (i.e. a fixed $a_w$, since RH and $a_w$ are numerically the same quantity at equilibrium) less water is needed to satisfy the equilibrium condition — the same $a_w$ is reached at a lower moisture content because the binding sites hold water less strongly at the higher temperature. This is the same phenomenon as a rising vapour pressure of pure water with temperature, applied to bound water in a hygroscopic solid.
Adding a soluble humectant (salt, sugar, glycerol, sorbitol) to a food lowers $a_w$ by Raoult's -law-type depression: dissolved solute reduces the mole fraction (and hence the escaping tendency) of water, so $a_w = \gamma_w x_w$ falls below 1 in proportion to the solute's concentration and its own water-binding strength (captured empirically by the Norrish or Money & Born equations for non-ideal food solutions). In practice, a target $a_w$ is reached by formulating the product with a calculated amount of humectant, guided by the solute's known $a_w$-depression curve, then verifying with a water-activity meter. The limitations are significant: (1) the required solute load to reach microbiologically safe $a_w$ (typically $a_w<0.85$ for many pathogens) is often organoleptically unacceptable — oversalted or oversweet products; (2) different solutes depress $a_w$ non-additively and non-linearly, so mixtures must be tested empirically rather than calculated by simple summation; (3) the depression achieved depends on the solute actually dissolving and equilibrating throughout the matrix, which can be slow in solid or highly viscous foods; and (4) many humectants alter texture, osmotic stress on the product's own cells (for fresh-cut items) or chemical stability (e.g. accelerating Maillard browning) as an unwanted side effect of the very ingredient added to control $a_w$.
