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Texture analysis is primarily concerned with the measurement of the mechanical properties of a food product, as they relate to its sensory properties detected by humans. Texture analysis is primarily concerned with the evaluation of mechanical characteristics where a material is subjected to a controlled force from which a deformation curve of its response is generated. These mechanical characteristics in food can be further sub-divided into primary and secondary sensory characteristics.

Primary Characteristics

Hardness Soft → Firm → Hard
Cohesiveness Crumbly → Crunchy → Brittle
Elasticity Plastic → Elastic
Adhesiveness Sticky → Tacky → Gooey
Viscosity Thin → Viscous

Adapted from Szczesniak, A., Classification of Textural Characteristics, Journal of Food Science 28, 981-985 (1965)

Secondary Characteristics

Brittleness Crumbly → Crunchy → Brittle
Chewiness Tender → Chewy → Tough
Gumminess Short → Mealy → Pasty → Gummy

Adapted from Szczesniak, A., Classification of Textural Characteristics, Journal of Food Science 28, 981-985 (1965)

Food texture vs rheology

There is considerable overlap between rheology and food texture. The deformation of a food item squeezed in the hand is both a textural property and a rheological property. The flow of the bolus of chewed food in the mouth, and the flow of fluid and semifluid foods, are both a textural property and a rheological property. However, the fracturing and grinding of solid foods that occurs during mastication is not a rheological phenomenon, and neither are the textural perceptions of particles, the release or absorption of moisture or oil. So food texture is partly rheology and partly non-rheology. Some rheological properties are probably not detected by the tactile sense, which means that some rheological properties are related to texture and some are not.

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Photo by Shohei Ohara from Pexels

Food rheology is the study of the rheological properties that is, the consistency and flow of food under tightly specified conditions. The consistency, degree of fluidity, and other mechanical properties are important in understanding how long food can be stored, how stable it will remain, and in determining food texture. The most important factor in food rheology is consumer perception of the product. This perception is affected by how the food looks on the plate as well as how it feels in the mouth, or "mouthfeel". Mouthfeel is influenced by how food moves or flows once it is in a person's mouth and determines how desirable the food is seen to be.

To study rheology, you have to be familiar with a few concepts that are used extensively within the world of food rheology. We will start with viscosity. All liquids and soft solids will have a viscosity value. This viscosity describes the resistance of the material when certain stress is applied. Another more visual way to explain viscosity is through “thickness”. A material with higher viscosity is thicker than one with a lower viscosity. For instance, honey is thicker than water and has a higher value for viscosity.

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The viscosity of material always depends on the temperature. For some materials, the viscosity at one temperature will be constant, no matter whether it’s being poured, mixed, etc. These are so-called Newtonian fluids. Water is an example of a Newtonian fluid.

However, for a lot of materials, their viscosity depends on external forces or stress. For instance, ketchup doesn’t flow when the bottle is hold upside down. Instead, you have to shake the bottle to initiate flow. The resistance towards flow, or thickness, has changed because of this shaking. A lot of foods behave like this, these are non-Newtonian fluids.

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Food rheology – Shear rate and shear stress

Two other very common terms within food rheology are the shear rate and shear stress. Within a lot of analysis techniques, these two terms will show up.

The easiest way to explain these concepts is by imagining two parallel plates on top of each other with a little space in between them. That space can be filled with the material you’d like to analyze. During an analysis, the top plate will move over the bottom plate. The shear rate describes at which rates these plates move alongside each other:

Shear\ rate = {speed\ of\ top\ plate\ (master\ per\ seconds,\ m/s) \over distance\ between\ the\ two\ plate\ (meters,\ m)}.

The reason the distance between the plates is included can be illustrated with a simple example. If the distance between two plates is very large, the material that sits in between won’t notice as much, the material has enough space to distribute that movement throughout the sample. In other words, the shear rate is low. If the distance is very small and the plates are very close to each other, the material in between will surely notice a movement.

 

In more practical terms: the shear rate is influenced by the speed at which you stir your food or the speed at which your mixing arm moves.

Now that we’ve found a way to describe how fast we try to move a material; we should define how much trouble it takes to move this. As we discussed, viscosity is the resistance against the flow. So if we know how much pressure it takes to apply a certain shear rate we can define the viscosity. That’s where shear stress comes in.

The shear stress describes how much force (per surface area) is required to apply that defined shear rate. In other words, it describes how much force you need to apply to move that peanut butter. 

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