Peanut butter is a staple known for its thick, creamy, and spreadable texture. While it appears solid when sitting in a jar, it flows easily when scraped with a knife or chewed. This unusual behavior challenges the basic categories of physical states, as peanut butter exhibits characteristics of both traditional solids and liquids. Its complex nature places it in a specialized category of materials studied by physicists and food scientists.
How Scientists Define a Solid
The traditional definition of a solid is based on a material’s resistance to external forces. A true solid maintains a fixed volume and a definite, unchanging shape, regardless of the container it occupies. Its constituent particles are typically held in a rigid arrangement, providing high resistance to shear stress—a force applied parallel to a surface.
In contrast, a true liquid possesses a definite volume but takes the shape of any container it is poured into. Liquids and gases are collectively known as fluids because they continuously deform when subjected to even a small amount of shear stress. The atoms in a liquid are free to slide past one another, allowing the substance to flow easily. Since peanut butter retains its shape in a jar, it initially suggests a solid state, but its ability to be easily spread points toward a fluid classification.
The Internal Structure of Peanut Butter
Peanut butter is a complex mixture, classified as a colloidal dispersion. It is specifically a suspension where solid particles are distributed throughout a liquid medium. The solid phase consists of finely ground peanut particles, including protein and fiber. These insoluble particles are suspended within the liquid phase, which is the peanut oil and fat released during the grinding process.
In stabilized peanut butter, hydrogenated vegetable oils are often added during manufacturing. These stabilizers melt during processing, disperse throughout the mixture, and then crystallize at room temperature. This crystallization forms a robust, three-dimensional network that traps the peanut oil and solid particles, preventing them from separating. This interwoven network gives the peanut butter its solid-like appearance and its ability to resist flow when at rest.
Peanut Butter as a Non-Newtonian Fluid
The ultimate classification of peanut butter is that of a complex fluid, specifically a non-Newtonian fluid. This means its flow behavior does not follow the law of constant viscosity observed in Newtonian fluids like water. Instead, peanut butter’s viscosity—its resistance to flow—changes based on the amount of force, or shear stress, applied to it.
Peanut butter exhibits a property called shear-thinning, where its apparent viscosity decreases as the shear rate increases. When scooping the substance with a knife, the applied force breaks down the internal structural network of the solid particles and crystallized fat. This structural breakdown allows the trapped oil to lubricate the particles, causing the peanut butter to flow and spread easily, behaving more like a liquid.
The material also demonstrates thixotropy, meaning its viscosity decreases over a period of time when it is continuously worked or stressed. If the peanut butter is stirred continuously, it becomes progressively runnier. When the stress is removed and the substance is left undisturbed, the internal structure gradually rebuilds itself over time, allowing the peanut butter to recover its initial thick, solid-like state in the jar. This dual nature—appearing solid at rest but flowing under force—makes peanut butter a viscoelastic material, a classification that accurately describes its simultaneous display of both solid and fluid characteristics.