Peanut butter is a viscoelastic food that exhibits both solid and fluid behaviors. It consists of ground up peanuts and may contain additional additives, such as stabilizers, sugars, or salt. Its characteristic soft, spreadable texture can be further defined through rheology – the study of flow and deformation of matter, affecting texture, consistency, and mouthfeel. Specifically for peanut butter, rheology can be used to more accurately define characteristics, such as spreadability and grittiness.
Soft matter context In a soft matter context, peanut butter can be considered as a colloidal dispersion, where solid, insoluble peanut particles are suspended in liquid oil. There are two types of peanut butter, and at room temperature, these two types of peanut butter behave differently. Non-stabilized peanut butter, also known as "natural" or "100%" peanut butter, consists only of ground peanuts and peanut oil and may contain seasonings, such as salt. In natural peanut butter at room temperature, the insoluble peanut particles separate from peanut oil, and the difference in density causes the peanut oil to float upwards. Stabilized peanut butter contains additional ingredients, such as vegetable oil, to prevent the grounded peanuts and peanut oil from separating into two layers. During the grinding process, the peanuts release oils, forming a peanut paste consisting of peanut oil and peanut grounds. The grinding process also causes an increase in the overall product temperature, and at this point a stabilizer might be added, such as hydrogenated vegetable oils. At this temperature, the stabilizer melts, uniformly dispersing into the peanut paste. This oil then crystallizes once the product returns to ambient temperatures, and the formed crystalline lattices trap the stabilizer particles within the paste. This prevents the final peanut butter from separating into two separate phases. Without the stabilizer, the peanut oil alone is not enough, as it is unable to crystallize at room temperature. The melting point of peanut oil is 3 °C (37 °F). At room temperature, the oils in natural peanut butter remain liquid, causing a phase separation. Within the stabilized peanut butter, the microstructural features are able to remain well-dispersed in a matrix of stabilized oil due to crystallization, while in the unstabilized peanut butter, the features are not able to retain the same uniformity.
Methods to characterize peanut butter rheology For most viscous semi-liquid foods, rheological characteristics are determined in shear flow using a coaxial viscometer. However, as peanut butter is not only a highly viscous material, it is also self-lubricating, meaning it releases oils under shear. If placed in a typical coaxial viscometer, the resulting flow pattern is a distorted shear flow or plug flow. For accurate data, rheometers typically require no-slip, and the properties of peanut butter do not satisfy this condition. This causes it to be particularly difficult to study its rheology. There have been a few methods devised to overcome this.
Squeezing flow viscosimetry Squeezing flow viscosimetery uses two parallel plates to compress a fluid uniaxially. This method can be used to better understand the viscoelastic properties of peanut butter. Peanut butter samples can be placed between two lubricated plates, and samples can be subjected to either uniaxial deformation at various constant displacement rates, or to uniaxial creep deformation under various constant loads. As the plates compressed the sample, if the sample retained a cylindrical shape without bulging, this is evident that there is a lack of shear flow. Using this method, peanut butter has been determined to be a power-law fluid with shear thinning properties. In other words, under high shear rates, there is a lower apparent viscosity. This is likely due to the size difference in peanut and oil particles. The larger peanut particles likely form loosely bound aggregates that break down as shear rate increases (e.g. mixing), which allow the oil to better disperse between peanut particles, resulting in a reduced viscosity.
Rough plates with parallel plate rheometers Another way to overcome the wall-slip effects, is to rough up the contact surface of parallel plate rheometers using a material such as sandpaper. In order to determine if this method sufficiently reduces the wall-slip effects, stress growth experiments can be conducted. If the stress over time is independent of gap size, then wall slip has been successfully reduced.
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