Question 4 of 9: Physical Properties Governing the Formulation and Processing of a Fabricated Food Product

Question 4: Physical Properties Governing the Formulation and Processing of a Fabricated Food Product (20 marks)

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Product selected: processed cheese sauce (a fabricated, shelf-stable, pourable emulsified cheese product used as a pasta/nacho topping). It is formulated from natural cheese, emulsifying salts, water, milkfat, stabilizer gum and acidulant, cooked under shear to form a stable oil-in-water emulsion, then hot-filled and cooled.

pH. Held in the 5.6–5.9 range: high enough to avoid the sour, curdled flavour of a low-pH cheese, but low enough (with the emulsifying salts) to help the casein network stay dispersed rather than aggregating into a grainy texture, and low enough to contribute a hurdle against Clostridium botulinum outgrowth in a hot-filled, reduced- oxygen package.

Water activity (a_w). Formulated below ≈0.95 (typically 0.92–0.96 for a spoonable cheese sauce) by controlling added water and salt/sugar solutes; a_w is the single biggest lever on microbial shelf life and is combined with pH as a two-hurdle preservation strategy rather than relying on refrigeration alone.

Surface tension (γ). The emulsifying salts (e.g. sodium citrate/ phosphate) function partly by lowering the fat–water interfacial tension so the milkfat can be sheared into small, kinetically stable droplets during cook; too high an interfacial tension under the available shear leaves large fat droplets that coalesce and "oil off" on storage/reheating.

Moisture diffusivity (D_m). Governs how fast the sauce would dry a skin or lose moisture to a food it is served on (e.g. into pasta or a crisp taco); a low D_m (aided by the stabilizer gum, which raises the serum-phase viscosity and binds free water) keeps the sauce from "weeping" a thin liquid layer during hot-hold service.

Thermal diffusivity (α) and thermal conductivity (k). Both set how fast the batch heats through during the cook-and-melt step and how fast a filled container cools; a low k/α (typical of a high-fat emulsion) means the centre of a large batch lags the jacket temperature, so process time (and the associated risk of scorching at the wall) must be set from the actual measured value, not assumed equal to water's.

Specific heat (c_p). Sets the steam/energy duty of the cook kettle and the refrigeration duty of post-fill cooling; because milkfat has a lower c_p than water, a higher-fat formulation needs proportionally less energy per degree, a real cost lever at plant scale.

Rheological properties (μ, n, K, yield stress). The product is designed as a pseudoplastic fluid with a small yield stress: the yield stress lets the sauce sit without slumping off a nacho chip or pooling away from pasta at rest, while the strongly shear-thinning flow behaviour index lets it flow easily through a pump, filler nozzle, or squeeze package under the higher shear of dispensing. The consistency coefficient K is tuned (via gum level and cook shear history) to hit a target apparent viscosity at the reference shear rate the company's quality-control method specifies, since K alone (without n) does not fully describe a "thick enough" sauce across the full range of use shear rates.

Textural attributes. Beyond flow behaviour, mouthfeel attributes (smoothness/graininess from any un-emulsified fat or protein aggregates, and "stringiness" from excess intact casein structure) are controlled by the same emulsifying-salt/shear/pH system that sets pH and surface tension above, illustrating how these properties are not independent design knobs but a coupled system that must be optimized together, not one at a time.