0 Rates

Rheological investigation of food

Many of the textural properties that we perceive when we consume food are related to rheology — the  creaminess, juiciness, smoothness, brittleness, tenderness, and hardness of food often results from its rheological properties and behavior. Characterization of the rheological properties of food is of increasing importance for evaluating the quality of raw materials, predicting materials’ behavior during processing, and for meeting storage and stability requirements.

Rheological behavior of food

Structure strength / behavior as a suspension

Knowledge of the structural strength of food is important for predicting its de-mixing behavior and sedimentation tendency, for example. Food toppings such as salad dressings and vinegar often contain suspended herbs and vegetable pieces. If well suspended, this food mixture will stay stable without the consumer having to shake the container to distribute the particles. A stable food topping delivers the desired taste profile and appearance. Structure properties also give a food product the sensory properties required by the consumer, such as a flavor profile, the body of a beverage, the creaminess of an ice cream, the stickiness of a sweet, or the mouthfeel when food is swallowed. To gain insight into all of these properties, food materials can be tested and analyzed with the help of rheology.

Flow behavior

Knowledge of the flow behavior is essential for predicting the processability and filling behavior of food materials. Flow behavior also plays an important role in predicting how the food structure will change as the consumer chews and swallows it. Flow behavior may also be influenced by temperature changes, from refrigeration or ambient storage temperature to elevated temperatures in the mouth.

Investigating food using rheological tests

Rheological tests are useful to:

  • Investigate the fluid and structural properties of raw materials used in food production
  • Improve the texture, taste, and mouthfeel of food
  • Predict the materials' performance during processing
  • Measure product quality (quality control of food products)

Chocolate

Chocolate needs to have certain rheological properties in order to meet producers’ as well as consumers’ needs. While the raw material must flow smoothly during production, the finished product should only melt in the mouth and not at room temperature. Furthermore, the way chocolate sticks to the tongue and palate is of great importance for the consumer’s experience of mouthfeel and taste.

Rheological tests on chocolate

Two important rheological properties of liquid chocolate are its viscosity and yield pointViscosity is the measure of the chocolate’s resistance to flow. This has to be considered when the liquid chocolate is poured into a mold (for bars, figures, etc.). The yield point is the shear stress that must be applied to initiate flow. This is relevant when it comes to pouring and decorating, but it also influences how the chocolate melts in the consumer’s mouth and hands. Both values depend on temperature.

Chocolate should melt on the tongue between 29 °C and 33 °C. If not, it may taste waxy or be too soft at room temperature.

Rheological measurements can be used for quality control of chocolate melts. A perfect solution for this is a flow curve at 40 °C. The flow behavior of chocolate is influenced by its various ingredients. By checking parameters like yield point and viscosity, consistent quality can be guaranteed.

This test requires a rheometer equipped with a Peltier temperature control system.

Coffee creamer

Coffee creamer is an extremely convenient substitute for milk because it doesn’t need to be refrigerated, has a very long shelf life, and is easily transported. However, the creamer must be produced as a homogeneous and flowing powder to ensure it retains these good properties. Such qualities cannot easily be measured by visually inspecting the creamer but they can be measured using powder rheology.

Rheological tests on coffee creamer

How a powder flows has a big impact on the ease of processing and packaging, and also on the powder’s long-term stability. Therefore, understanding how a powder flows is crucial for efficient production and quality control. Quantitative information about the factors governing powder flow can be gained by means of powder rheology. For example, how strongly the particles stick together is determined by the cohesion strength; a high-end rheometer with a powder cell can provide reliable measurements of the cohesion strength, which can then act as a direct measure for quality control.

This test requires a rheometer which can handle powder rheology.

Fruit jam/marmalade

To obtain good pumping and filling behavior of fruit jam or marmalade, its viscosity needs to be low at high shear rates. On the other hand, if a jam’s viscosity is too low it will run off the bread. The structure of the sweet spread also influences its fruity-sweet taste when it melts on the tongue. The most important factor for the viscosity of fruit jams is the type and amount of pectin added (in combination with the calcium concentration). 

Rheological tests on fruit jam

A major difficulty when measuring the viscosity of fruit jam is that it contains many large fruit particles. Removing them would be both time-consuming and would falsify the results when determining flow behavior. In such cases a rotational rheometer with a ball measuring system is required. Because the ball encounters fresh, unsheared material only during the first rotation, the entire measurement must be completed in a single turn of the measuring system. This calls for a rheometer that is able to control the rotational speed very quickly for each single measuring point.

This test requires a rheometer equipped with a ball measuring system.

Ketchup

Ketchup is essentially a combination of tomatoes and spices. Consumers expect not only a certain taste, but also a certain flow behavior from their ketchup. During the manufacturing process it is important that the ketchup is easily mixed and filled into bottles. On the other hand, consumers want ketchup that flows easily out of the bottle and then remains on the hot fries without running off.

Rheological tests on ketchup

The main factor responsible for ketchup’s flow out of the bottle when squeezed is the yield stress or yield point. It is assumed that ketchup is in a solid state until its yield point is reached; at stresses above this point it starts to flow. After the stress is released, the ketchup returns to its former state.

To simulate flow and leveling behavior after pouring, a step test is carried out consisting of three intervals. The first interval describes the sample’s behavior at rest. The second interval (pouring) shows structural decomposition and the third interval shows structural regeneration. Using oscillatory tests, the settings in the first and third interval are a constant angular frequency and a constant deformation value within the linear viscoelastic range. The second interval is performed by means of rotation at a constant shear rate that simulates the shearing process when the sample material is poured out of or filled into containers.

This test requires a rheometer equipped with a Peltier temperature control system.

Powdered milk

Powdered milk is commonly used as a basic food supply item in situations in which fresh milk is not available, so consistent and reliable quality is crucial to the product’s viability. Nonetheless, its quality can vary significantly, depending on the raw product (the fresh milk) and the manufacturing process. On its way to becoming a stable powder, the milk must pass through a number of processing steps, which present different technological challenges. The last of these is discharging the powder from the spray-drying tower. This discharge step often causes problems because milk powder is in general rather cohesive, and even more importantly, the walls of the hopper can be a source of friction, giving rise to interrupted flow.

Rheological test on powdered milk

One way of monitoring the flow behavior is to use a rheometer with a powder cell for measuring the wall friction. Wall friction measurements enable the wall-friction angle to be calculated, which indicates whether a powder can be discharged from a hopper homogeneously. A lower wall-friction angle indicates that a powder will adhere less to the walls, and be discharged more easily.

This test requires a rheometer which can handle powder rheology.