Rheological Characterization of Food by Anton Paar

Topics Covered

Background
Rheological Behavior of Food
Understanding Consistency and Flow Behavior of Food
     The Importance of Knowing the Rheological Properties of Food
Applications of Rheology in Food Industry
     Chocolate
     Jam
     Starch
     Ketchup
     Cheese
     Salad Dressing
     Ice Cream

Background

Anton Paar GmbH produces high-end measuring and laboratory instruments for industry and research. It is the world leader in the measurement of density, concentration and CO₂ and in the field of rheometry. Anton Paar GmbH is owned by the charitable Santner Foundation.

Over 1180 employees at the headquarters in Graz and the thirteen sales subsidiaries worldwide ensure that Anton Paar products live up to their excellent reputation. The core competence of Anton Paar – high -precision production – and close contact to the scientific community form the basis for the quality of Anton Paar's instruments.

Rheological Behavior of Food

Food sustains us. Its rheological properties make us able to bite it, chew it and swallow it, while appealing to our senses. These properties are referred to as the "texture" of the food. Rheological characterization of food becomes increasingly important for evaluating the quality of both the raw materials and the end product and predicting its behavior during processing.

In practice, the following information is important regarding the rheological behavior of food:

• The consistency or structural strength at rest
• The flow behavior
• The time-dependent and temperature-dependent behavior

Understanding Consistency and Flow Behavior of Food

Knowledge of the consistency is important in order to predict the dispersion or gel stability, expected stability in storage and tendency to sedimentation.

The Importance of Knowing the Rheological Properties of Food

Knowledge of the flow behavior as well as behavior over time is essential for predicting the processability and filling behavior. Here, the structure of the material and its structural changes during technological processes must be known. Using this knowledge a process can be designed and calculations for the technological equipment can be carried out.

The structure also gives the product the sensory properties required by the consumer, such as the flavor and body of a beverage or creaminess of an ice cream. It provides insight into the stickiness of a sweet or the feeling when swallowing.

Applications of Rheology in Food Industry

Typical applications of rheology in the food industry are highlighted in the following sections on:

• Chocolate
• Jam
• Starch
• Ketchup
• Cheese
• Salad Dressing
• Ice Cream

Chocolate

The flow behavior of food plays an important role in the production process and in achieving acceptance from the consumer. The yield point influences the way the chocolate levels out during manufacturing and how it melts in the mouth and on the consumer's hands.

Chocolate makers are very interested in the spatial distribution of the cocoa butter molecules. The chocolate should melt on the tongue at temperatures between 29 and 33 °C. If not, it tastes waxy or is too soft at room temperature.

For quality control of chocolate melts at 40°C a flow curve approximation model is used. The analysis according to Windhab is recommended for the measuring range ? = 2 to 50 1/s by the IOCCC (International Office of Cocoa, Chocolate and Sugar Confectionary). The results are the yield point, a specific shear rate value in the range of "shear induced restructuring" and the equilibrium viscosity at high shear rates.

The viscosity of the chocolate melt is influenced by a number of ingredients. As the amount of fat in the chocolate increases, for example, the viscosity and yield point value decrease. Consistent quality can be guaranteed by checking the flow behavior and viscosity.

Jam

The viscosity of jam must be known for a range of applications. For good pumping and filling of jam the viscosity should be low at high shear rates. For the consumer the viscosity is also of great importance: if the viscosity at rest is too low the jam will run off the bread.

Jam, like other food, often contains large particles. Removing these particles is time-consuming and also leads to falsified results when determining the flow behavior. For samples with large particles the typical measuring systems such as parallel plate, cone/plate or cylinder geometries are unsuitable due to their small measuring gap. To make it possible to measure samples with large particles there is a ball measuring system. This moves the ball of the measuring bob rotating in a circular path through the sample. The entire measurement is completed in just one circuit, which ensures that each measuring point is recorded in fresh, nondeformed material.

Starch

Starch is not only important for nutrition, it is often used in the food industry due to its physical properties. The gelification of starch is widely used to improve the appearance, stability, texture and quality of the food. Monitoring of the gelatinization process is difficult due to the different phases at different temperatures. Therefore, a special test geometry and an electrical temperature chamber with rapid heating and cooling rates have been developed. This enables an accurate measurement of the gelification process. If higher pressures are required, a pressure cell can be used.

Viscosity curves differ widely depending on the starch type. At 35 °C starch is not soluble in water, so the measuring geometry must keep the starch granules in suspension. When heated, the granules begin to swell and this increases the viscosity considerably. Depending on the starch type, the viscosity maximum and therefore also a volume maximum is reached at a specific temperature. At this point the starch granules are destroyed and the viscosity decreases. When the starch cools down, the viscosity increases again due to the gelification of the starch.

Ketchup

During the manufacturing process, ketchup should be easy to mix and fill into bottles. However, the consumer demands a ketchup which flows easily out of the bottle but then remains on the chips, without running off.

To simulate the flowing and leveling behavior after pouring out and the structural regeneration after filling and leaving the bottle, a step test is carried out. This consists of three intervals. The first interval determines the behavior at rest. The second interval determines the structural decomposition and the third the structural regeneration. The settings in the first and third intervals are a constant angular frequency and a constant deformation within the linear-viscoelastic range. The second interval consists of rotation at a constant shear rate that simulates shearing during pouring out or filling.

Despite its higher viscosity at rest, Ketchup 1 has a lower viscosity during shearing (pouring out of the bottle) than Ketchup 2. The structural regeneration of Ketchup 1 takes longer than that of Ketchup 2. This means that Ketchup 1 runs for a longer period of time and therefore has a thinner layer thickness than Ketchup 2.

Cheese

The consistency as a function of the temperature is an important property of cheese products. For example, cheese spread should be spreadable straight from the refrigerator but at room temperature it should have a structure that prevents it from flowing too much. For a good feeling in the mouth, the cheese spread should have the structure of a soft solid when at body temperature.

Due to the extended gel structures and high consistency at room temperature, a parallel plate measuring system is recommended. A constant normal force should be applied to the sample to ensure it has a good grip on the measuring plate.

As this type of sample tends to flow out of the gap, even at very low shear rates, oscillatory tests are preferable. In addition, oscillatory tests reveal information about both the viscous and elastic behavior.

Whether the material characteristics meet the above-mentioned requirements can be investigated using an amplitude sweep. In this test the deformation (strain) is increased logarithmically while keeping a constant angular frequency. The amplitude sweep is carried out at a number of temperatures.

Salad Dressing

Salad dressings and vinegar often contain suspended herbs and vegetable pieces. This has the advantage that the consumer does not have to shake the container to distribute the particles and also improves the appearance of the product. For manufacturers the challenge is to select the right additives in order to modify the viscoelastic behavior so that the particles do not separate out while ensuring that the salad dressing or vinegar remains liquid enough to pour out and distribute on the salad. Amplitude sweeps and frequency sweeps are well suited for checking these properties.

In a frequency sweep the frequency is varied and the amplitude kept constant. The deformation is first determined in an amplitude sweep and must be within the linear viscoelastic range. Measurements at high frequencies represent the short-term behavior of a sample. The long-term behavior (stability in storage) is described by the measuring data at low frequencies.

Additives change the Newtonian flow behavior into a shear thinning one. The pure viscous character becomes a weak gel character, e.g. the storage modulus G' lies slightly above the loss modulus G''. This property keeps the particles in suspension and prevents sedimentation.

Ice Cream

The temperature-dependent rheological behavior is an important property of ice cream. For example, an ice cream should have a low rigidity at low temperatures. This enables good scoopability. On the other hand, ice cream should be creamy and feel good in the mouth after melting at higher temperatures. As it melts in the mouth, the consumer does not like to have a feeling of too much coldness, requiring a slow melting phase.

Due to extended crystal structures in ice cream and its stiff consistency at low temperatures, using a parallel plate measuring system is recommended. In order to prevent slip the geometries should be profiled.

Temperature tests on ice cream should be carried out using oscillation. In these tests angular frequency is kept constant and the very low deformation is within the linear-viscoelastic range. The temperature is steadily increased from low to high values.

To keep the temperature gradients at an optimally low level, the measuring cell P-PTD200 with Peltier temperature control and actively heated Peltier controlled hood H-PTD200 is used. Besides temperature controlling the lower plate, the temperature of the measuring area around the sample is controlled from above. Without the Peltier hood the results for G' and G'' are considerably lower in the low temperature range, i.e. the real average temperature of the sample is higher than shown. The resulting curve is also less steep for G' and G'' during the melting process, meaning this process is also simulated inaccurately and does not reflect reality.

Source: Anton Paar GmbH.

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