Food that Falls and Floats

Fig. 1. Interactive animation demonstrating Stoke’s Law. View in full screen for greater clarity (especially on phones) or read the source code to see the JavaScript implementation (if you’re so inclined)

Stoke’s Law describes the sedimentation of particles in suspensions (fruit pulp in orange juice) and the flotation of droplets in emulsions (creaming of milk fat). In food science it is invoked for a variety of purposes. One of the most simple techniques for measuring a liquid’s viscosity involves using Stoke’s Law to estimate the value from the rate at which a ball falls through the liquid. The equation is also used to identify the key parameters that influence how a group of particles fall or rise due to gravity, which can be used to develop strategies to make it happen quicker or slower. A version of the equation is shown below describing the separation velocity ($v$) of particles with diameter $D$ in a solvent with viscosity $\mu$ when subject to acceleration due to gravity $g$. The $\Delta \rho$ means the difference ($\Delta$) between the density ($\rho$) of the particle and the solvent.

$$v = \frac{D^2 \cdot(\Delta \rho) \cdot g }{18 \cdot \mu}$$

Techno-functionality #

Some of the technological solutions informed by Stoke’s Law are the source of routine confusion when “homogenised” is found on a product label or “guar gum” in an ingredients list. Homogenisation is simply a way of physically reducing the size of particles. As the particle diameter value ($D$) in Stoke’s Law is raised to the power of $2$ it has a significant influence on separation speed. Guar gum is one of a variety of food ingredients that has a purely techno-functional role, which typically involves increasing viscosity ($\mu$) so that friction slows particles down as they move through the liquid. Sometimes it might be desirable to promote instability, such as when whole milk is centrifuged to create skim milk and cream, replacing the acceleration due to gravity ($g$) with that due to centrifugal force ($\omega ^2 r$). These factors become more obvious if you play around with the particle diameter and solvent viscosity sliders in the animation. Try the other sliders and you might find some other interesting effects.

Intuition, Education & Commerce #

Most of us have a good intuition for these phenomena in a non-food context. We expect large stones to sink faster than small pebbles. We know that the air pockets in a wine bottle cork means that it will tend to float on water. In food, the particles involved are often microscopic, making the behavior less salient. Showing a static photograph of particles on a microscope slide does not give us a feel for the dynamics involved. If we are uncomfortable with equations then the mathematics of Stoke’s Law might not help. That’s why I developed a simple simulation that allows students to manipulate all of the major variables using sliders. I hope this simple “virtual experiment” can improve the intuition of students for the phenomena but also the equation that helps explain them.

Fig. 2. Micrograph of cream showing its emulsion structure. As the oil droplets are less dense than the water surrounding them they tend to float (credit: edibotopic).

In recent years it has become more common for companies to avoid homogenising their products and to stop adding stabilising ingredients. This is not because of any demonstrable health benefit but rather because these products can be marketed as being “more natural”. This is an interesting case of a technological decision (to employ stabilising technology) being justified on the basis of a metaphysical judgement (that technologies have degrees of naturalness) with real business consequences (there is a market for “natural” foods). In any case, whether we buy a destablised smoothie at a health food store or homogenised milk from the supermarket, we can understand both using Stoke’s law.