Viscosity in Everyday Foods

This all with a simple question: what determines if a food item is denominated in weight or volume? My first thought went to viscosity - are viscous fluids like ketchup, maple syrup and honey measured by weight or volume? What’s the threshold or critical viscosity (if it exists ) that the FDA considers too viscous for a volume measurement.

Raiding my pantry quickly, I found that condiments, such as ketchup and mustard, were sold by net weight, not the net volume. Generally speaking, a mass mesurement is always the most convinient. Using a taring procedure to measure the weight of a weighing vessel before and after the object of interest is a simple process. This makes weighing ingredients much more precise than using volumetric methods for activities like baking.

What is Viscosity?

Viscosity is a concept that is much easier to develop intuition for from a simple demonstration (most of us had our first few interactions with viscosity in the proxmity of a kitchen!) rather than a more formal definition. Similar to a measure of a liquid’s “thickness”, it measures the internal force within a liquid.

Formally, viscosity is defined as the relationship between the shear displacement in response to an external shear force, $$ \tau = \mu \frac{\partial u}{\partial x} $$ where $\tau = F/A$ is the shear stress and $\frac{\partial u}{\partial x}$ is the shear velocity . This is an example of an empirical relationship, similar to Ohm’s law governing electrical current flow. The viscosity can be thought of analogous to resistance, as it is the internal friction of a viscous fluid that transfers the applied shear force into a shear displacement. Of course, for those who want to get into the detailed math, viscosity can be broken out into a tensor form in a similar way the electrical conductivity can be expressed as a tensor.

Viscosity and Food Science

Viscosity is an important aspect of food science and plays a large role in how we perceive the textures of food. The solute volume can change the viscosity dramatically, as is the case with sugar water solutions (sucrose), which for high enough concentrations eventually become simple syrup.

Suspensions and Non-Newtonian Fluids

Many of our favorite condiments in the USA are composed of suspensions, which dramatically increase the complexity. Many of these condiments (ketchup, mustard, mayonnaise) are strongly non-Newtonian fluids, such that the viscosity is no longer linear with the applied shear velocity. A Newtonian fluid exhibits the same viscosity as a function the applied stress; the viscosity is captured in the linear relationship between stress and shear velocity. Similar to the difference between a light bulb and a resistor in an electrical circuit, non-Newtonian fluids have a nonlinear dependence on the applied stress. Oobleck and ketchup fit in this category, although oobleck is a shear-thickening (more viscous under applied stress), whereas ketchup is shear-thinning (less viscous under applied stress).

There are also fluids with a well-defined viscosity, but a threshold stress that is required before the fluid will deform. Materials in this class are called Bingham plastics and is explemplified in the common condiment, mayonnaise (hence why a jar of mayo or homogenized peanut butter will retain their knife marks from the previous users).

Conclusion

I have always found viscosity fascinating, and its role in food is always a great place to engage with people on asking simple but complex questions about why things are the way they are. I am glad the FDA had the foresight to anticipate the complexities of dealing with foods across a wide range of viscosities. Texture is a major part of how we experience food, and just digging around your pantry and fridge may cause you ask more questions about the viscosity of food than you ever had imagined!