E44 Viscosity - Sauces

Topic

The viscosity of a sauce determines how it flows, how it coats the dish it is served on, how it adheres to the accompanying food, and how much of it drips from the serving spoon; most sauces are non-Newtonian fluids, meaning their apparent viscosity changes—rather than remaining constant—based on the rate of deformation to which the sauce is subjected. Consequently, as the applied rate of deformation changes, the sauce's apparent viscosity changes along with it.

A pseudoplastic fluid—such as ketchup, tomato purée, or a starch-thickened sauce—becomes less viscous as the rate of deformation applied to it increases; this explains why shaking a bottle of ketchup (thereby increasing the applied rate of deformation) makes it easier to pour. Conversely, a dilatant fluid becomes more viscous as the rate of deformation increases, although this behavior is rare among culinary sauces. Thus, a higher rate of deformation applied to a pseudoplastic fluid results in lower apparent viscosity.

The Bingham model introduces a specific "yield stress" for sauces like mayonnaise, yogurt, or ketchup: the sauce does not move at all until the applied stress exceeds this yield stress. This is why the sauce remains static on the plate where it is served but flows as soon as pressure is applied via a spoon or the food itself. The Herschel-Bulkley model is even more general than the Bingham model, as it combines this yield stress with the previously described pseudoplastic or dilatant fluid behaviors. A higher yield stress means the sauce offers greater resistance before it begins to flow across the plate. A higher temperature drastically reduces the sauce's apparent viscosity—following an Arrhenius relationship—meaning it flows much more easily when hot than when cold; this temperature dependence determines both the optimal serving temperature and the mouthfeel experienced during consumption. Consequently, the model—characterized by the sauce's apparent viscosity and yield stress—results in a very low Reynolds number; while Reynolds numbers are typically high for low-viscosity fluids, this low value indicates that the flow remains laminar—whether over a spoon or inside the diner's mouth—thereby establishing the characteristic parabolic velocity profile of laminar flow and the resulting sensation of creaminess. A higher apparent viscosity further lowers the Reynolds number, thereby intensifying the sensation of creaminess.

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