Food & Beverage Analysis
Rotational Viscometry as Key to Customised Taste: How Flow Behaviour Affects Food Products
Judith Hartmann, Anton Paar GmbH
Modern food production is highly industrialised and aims for constant quality. A product’s fl ow behaviour is relevant not only for quality control, both of incoming raw materials and the fi nal product, but also for formulating the product to customise it for the intended target group. This article provides an overview of the basic parameters describing fl ow behaviour and explains why viscosity is a relevant physical quantity for food. Application examples illustrate why rotational viscometry is the tool of choice when analysing fl ow behaviour.
Flow behaviour and viscosity
The fl ow behaviour describes how the viscosity (colloquially, ‘thickness’) of a substance changes under stress, where ‘stress’ means any force acting on the material. Viscosity is the physical quantity that defi nes a substance’s internal fl ow resistance, or, simply put, how easily a substance fl ows. Water is a typical example for low viscosity, while mayonnaise or peanut butter are at the high end of the scale. To add further to the complexity, external stress is not the only cause of viscosity changes. Viscosity strongly depends on temperature and also on pressure. By assuming that temperature and pressure remain constant, three different types of fl ow behaviour can be discerned:
• Curve 1: Newtonian (named after Sir Isaac Newton) - in this case, the viscosity is independent of an external force. Water and oil are typical examples.
• Curve 2: Shear-thinning - the viscosity decreases with increasing stress. Yogurt, most creams, and sauces show this behaviour.
• Curve 3: Shear-thickening - the viscosity increases with increasing stress. This behaviour is rare, but can be observed in highly concentrated starch solutions or dough.
The force F moving the upper plate divided by the area A of this plate is the shear stress. The movement is accounted for by the shear rate, which is the result of the upper plate’s velocity v divided by the distance h between the two plates.
Reasons to analyse fl ow behaviour
in food production During production processes, most steps involve some kind of force infl uencing the fl ow behaviour and consequently the viscosity of the food material: This involves stirring, pumping, pouring, or extruding the food material.
Furthermore, fl ow behaviour is equally important from the consumer point of view.
• The product should be easy to pour or squeeze out of its container without unnecessary force or undesirable splashing and staining.
• Some products require the right viscosity for spooning or spreading on e.g., bread.
• Texture and taste are closely connected: If consumers do not enjoy the expected mouthfeel, they perceive the product to be inferior compared to one that has the right structure.
Notably, the expected mouth feeling and food texture vary in relation to regional preferences as well as over age cohorts [2].
• More critical than mouthfeel is swallowing as another shear force. Due to certain clinical conditions, older or sick people sometimes cannot swallow nourishment if it is too liquid [3], while highly viscous food is diffi cult to swallow for small infants.
The following table gives a short overview of the shear rates occurring during food production and consumption:
Table 1 [4]: Typical Shear Rates in Food-Related Processes Process
Sedimentation of particles Figure 1: Viscosity curves. Newtonian fl ow behaviour (1), shear-thinning (2), shear thickening (3).
In correct physical terms, viscosity is defi ned as shear stress tau divided by shear rate gamma-dot. Equation:
This mathematical defi nition is best explained by the two-plates model [1], which consists of two solid plates containing the viscous substance between them. The lower plate stands still, while the force F moves the upper plate slowly sideways as shown in Figure 2.
Sagging of coatings, fl ow under gravity 0.01 to 0.1 Dip coating
Chewing, swallowing Spreading Extrusion
1 to 100 10 to 100
10 to 1,000 10 to 1,000
Shear Rates [s-1] Example ≤0.001 to 0.01
Fruit juices
Chocolate coatings (couvertures) Dip coatings, candy masses Baby food, yogurt, cheese Peanut butter, butter, jam Dough
The yield point as parameter for mouthfeel
To make mouthfeel measurable, another parameter needs to be introduced: the yield point or yield stress. Simply put, some substances subjected to shear stress behave like a solid up to a certain value. Once this critical value - the yield point - has been exceeded, they start to fl ow [5].
The yield point is determined by measuring the viscosity while increasing the shear rate. From the resulting curve, the yield point is calculated using empirically developed mathematical model functions. This approach has resulted in several different models, which all return the yield point as an approximated value. For this reason, the yield point depends on the measuring and calculation method used. It is no material constant. The most commonly used models are Bingham, Casson, Herschel-Bulkley, or Windhab.
Figure 2: The two-plates model as tool to defi ne shear rate, shear stress, and viscosity.
While a higher yield point results in a creamier mouthfeel, the pressure required to squeeze a substance out of its tube (e.g., mayonnaise) also increases with the yield point.
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