Custard's Surprising Scientific Classification
At first glance, custard appears to be a simple liquid. It pours easily from a jug, but can also thicken considerably when stirred or heated. This variable behavior is the hallmark of a non-Newtonian fluid—a substance that does not follow Isaac Newton's law of viscosity, which posits a constant viscosity for all fluids. There are several types of non-Newtonian fluids, and custard can exhibit properties of more than one, depending on its state and recipe.
The Rheology of Cooked Custard: A Thixotropic and Shear-Thinning Fluid
Cooked custard, the kind typically served warm or as a dessert filling, is a fascinating weak gel. Its fluidity is not constant and can be classified as both shear-thinning and thixotropic. Shear-thinning fluids decrease in viscosity when a shearing force is applied. For cooked custard, this means that the more you stir or agitate it, the thinner and more pourable it becomes. However, this effect is not permanent. If left undisturbed, the custard will return to its original, thicker state, though some recipes note it may not recover its full lost viscosity over time.
The proteins in eggs, and sometimes starches, are responsible for this behavior. When heated, these proteins coagulate, creating a gel structure. This gel structure is what gives cooked custard its body and resistance to flow. When you stir it, you temporarily break down this protein network, allowing the custard to flow more freely. Once the stirring stops, the network begins to reform, and the custard thickens again.
The Other Side of Custard: Dilatant and Shear-Thickening
The science becomes even more interesting when examining uncooked custard powder (often cornflour or cornstarch) mixed with water. This is the mixture famously used for demonstrations of "walking on custard". In this state, the suspension is a dilatant, or shear-thickening, fluid, which is the opposite of cooked custard. When stress is applied to a dilatant fluid, its apparent viscosity increases, and it temporarily behaves like a solid.
- Low Force: If you gently dip your hand into a cornstarch-and-water mixture, it will feel like a liquid.
- High Force: If you punch the surface or run across it quickly, the particles of the cornstarch lock together under the pressure, resisting the force and acting like a solid.
This is because the water acts as a lubricant for the cornstarch particles. Under low pressure, the particles can slide past each other easily. But under high pressure, the water is squeezed out, causing the particles to grind together and jam, increasing the fluid's resistance.
The Importance of Ingredients
The specific ingredients used in a custard recipe play a critical role in its fluid properties. A traditional crème anglaise, which relies solely on egg yolks for thickening, behaves differently than a crème pâtissière, which incorporates starch. The inclusion of starch provides extra stabilization and alters the texture and rheology significantly.
A Comparison of Custard Fluids
| Property | Cooked Custard (Egg-based) | Uncooked Starch Suspension | Newtonian Fluid (Water) |
|---|---|---|---|
| Fluid Type | Non-Newtonian | Non-Newtonian | Newtonian |
| Viscosity | Decreases with force (shear-thinning) | Increases with force (shear-thickening) | Constant |
| Time-Dependence | Thixotropic (viscosity decreases with duration of force) | Rheopectic (viscosity increases with duration of force) | Not applicable |
| Mechanism | Temporary breakdown of protein gel structure | Particles jamming together as lubricant (water) is forced out | Fixed viscosity due to constant molecular structure |
| Example | Crème anglaise, baked custard | Cornflour and water mixture, oobleck | Water, oil, alcohol |
Conclusion
Understanding what kind of fluid custard is reveals a surprising truth about this humble dessert. It is not a simple liquid but a complex non-Newtonian fluid whose behavior changes dramatically depending on its preparation and the forces applied to it. Cooked custard is a thixotropic, shear-thinning substance that becomes thinner when stirred, while its uncooked starch-based sibling is a dilatant, shear-thickening mixture that can momentarily turn solid. This dual nature is a perfect example of the fascinating science hidden within everyday foods, proving that even a simple dish can be a subject of intriguing physical phenomena.
Resources
For more in-depth information on the physics of custard and non-Newtonian fluids, check out the article on ScienceDirect that investigates the effect of various components on the rheological properties of custard: Effect of components and heating on rheological properties of custards containing wheat starch.
