Rheology is the study of the deformation and flow of matter. Rheological properties are based on the flow and deformation responses of a substance when subjected to stress. All materials have rheological properties, and the area is relevant to many fields of study such as plastics processing, polymers and composites and food. Food rheology enables to understand the flow behavior which is crucial for optimizing product development, processing methodology and final product quality. Food can be classified in different ways, such as solids, gels, homogeneous liquids, suspensions of solids in liquids and emulsions. Foods that do not retain their shape but take the shape of their container are fluids. Such materials may contain amounts of dissolved or suspended solids, and exhibit non-Newtonian behavior, which classifies them as semi-fluids. In contrast, if fluids contain dissolved low-molecular-weight compounds and no polymer or insoluble solids, they may show Newtonian behavior. A small amount of dissolved polymer can substantially increase the viscosity and alter material behavior from Newtonian to non-Newtonian. (Ian, Fotios and Philip 2011, p.8)
Deformation of a substance may be viscous flow, elastic deformation or a combination of the two. Viscous flow is an irreversible deformation, which means that when the stress is removed the material does not return to its original form. Figure 1 shows common type of flow curve, which plotted as shear stress against shear rate. Shear-thinning (pseudoplastic) and shear-thickening (dilatant) flow are shown by curves and Newtonian flow by a straight line. The yield stress (interception on the shear axis) must be exceeded before flow starts.
Figure 1 Figure 2
Shear stress is a force tending to cause deformation of a material by slippage along a plane or planes parallel to the imposed stress. Shear rate is the continuing change of the shear angle deformation over time. Viscosity is a measure of the resistance to flow of a fluid. It is defined as the ratio of applied shearing stress to the rate of shear strain. According to the Newton’s law, for an ideal viscous liquid, the applied stress is proportional to the rate of shear strain but is independent of the strain. In other words, the viscosity is a constant. If the flow behavior curve does not pass through the origin of the flow curve, it is non-Newtonian flow behavior, where the viscosity is shear-rate dependent.
Types of fluid
Newtonian or Non-Newtonian fluid
Example of foods
When there is a linear relationship between shear stress and resulting shear rate, the flow behavior is called ideal viscous or Newtonian.
Viscosity decreases as the shear rate increases.
Concentrated fruit juices and creams
Viscosity increases as the shear rate increases.
Liquid chocolate and Corn flour suspension
No flow until a critical shear stress is reached.
Viscosity decreases over time at a constant shear rate. This is due to the decrease in the intermolecular interactions within the molecular structure of the material. Divided into positive and negative thixotropy.
a) Positive Thixotropy- When pressure applied it becomes more fluid.
b) Negative Thixotropy- When pressure applied it becomes more viscous.
Margarine and custard
Viscosity increases over time at a constant shear rate. This is due to the intermolecular interaction causing friction to increase with time at constant shear rate within the molecular structure of the material.
Food Products – Milk
From rheological perspective, milk can be categorized as a typical fluid because it deforms and flow immediately when subjected to a stress. Knowledge of the flow behavior of milk is important to design and operation of dairy processing equipments involved mixing, storage and pumping. Rheological properties of milk closely correlate with sensory perception of texture.
The rheological characterization of a fluid can be described by using the relationship between shear stress (?) and shear rate (. Newtonian fluid displays a linear relationship between shear stress and shear rate which independently of time. The major component of milk is water so milk will show similar rheological properties as water (a Newtonian flow behavior)
Where ? is the Newtonian viscosity, which is measurement of the resistance of fluid to flow due to the internal friction. Newtonian viscosity can be affected by composition of the fluid and temperature (Ian, Fotios and Philip 2011, p.134).
Type of fluid
Newtonian Viscosity, mPa·s
The composition of milk has a determining influence on the rheological characteristics of dairy products. The fat content has a greater effect on dairy rheology. In this way, products with high concentration of fat have different behaviors. The milk protein mainly casein also influences the viscosity of dairy products. However, these compounds can be modified by some factors, as thermal process and mechanical operations, which significantly change the rheological characteristics (da Silva VB and da Costa MP, 2017). Newtonian viscosities of milks and creams of different fat contents increase with decreasing temperature. Milk fat globules can undergo crystallization transition and cold agglutination at temperature below 40 °C. When milk is concentrated, the particle-to-particle interactions can contribute significantly to viscosity. A transition from Newtonian to non-Newtonian flow occurs when the solid concentration of milk reaches a certain level.
Power law model is used to describe a non-Newtonian fluid when the flow behavior is time-independent and shows no yield stress. Where K is consistency coefficient and n is the dimension flow behavior index. Milk products show shear-thinning (pseudoplastic) behavior with 0
Newtonian, non-Newtonian and viscosity of fluid was discussed. Rheological properties in foods are important as quality control during food manufacture processing.