By Food Materials Research Team
Today’s highly competitive global marketplace has created huge awareness for the private sectors on the development of the products and services. The company’s critical aspect of development, therefore, is to produce better quality products and provide more efficient services, as well as foster innovation or create a differentiation.
Rheology is the science of deformation and flow of materials, e.g., liquids, soft matter, and especially food. It is very important for food texture quality, such as the hardness, softness, smoothness, juiciness, formability, as well as the product flowability through the pipeline in the production process, packaging, and product usage, such as spreading.
In this article, we will explain the science of rheology by focusing on the extensional rheology and its importance to food product design. There are various examples of the extensional rheology related to daily life, such as a suction of liquid from a tube (as shown in Figure 1A), spreading butter or jam on bread slides (as shown in Figure 1B), and chewing food in the cavity to produce bolus (the lumps of food that combine with saliva) with good extensional properties. That is to say, if the food has a high viscosity with low extensional properties, it will be difficult to swallow, thus creates a tightness in the epiglottis when swallowed. On the contrary, if the food has a very low viscosity, e.g., water, when rapidly swallowed, it can cause suffocation especially for the elderly or patients with dysphagia.
The modification of viscosity and the extensional rheology properties of food to be suitable for patients’ age or conditions, therefore, will facilitate the food to flow safely into the esophagus as shown in Figure 1C. Furthermore, the bread that made from doughs with the extensional properties would be bulge more properly in the fermentation process, which created soft and fluffy breads’ properties as shown in Figure 1D. Also, the food design with Molecular gastronomy by mixing polysaccharides powder into sauces or juices to make it flow, and once the foam forms, it will be stable for several minutes as shown as Figure 1E.
The aforementioned “flow properties” is technically known as “the extensional or elongational rheology”
Figure 1: Some examples of the extensional rheology involved in daily life; (A) a suction of liquid from a tube, (B) spreading butter or jam on bread slides, (C) the swallowing of food bolus in the pharynx, (D) bread making, and (E) the food design with Molecular gastronomy. |
Extensional rheology has found applications in various industrial processes
The extensional or elongational rheology studies the rheological properties in relation to a pull resistance, and normally exhibits the elongational viscosity from the product manufacturing processes and materials forming in various industries. It typically involves in complex flow processes, especially the extensional flow of multicomponent materials and the different elongation properties, such as the extrusion flow, coating, fiber spinning, ink jetting, or flow in microfluidics system.
In the food industry, there are various manufacturing processes related to the extensional flow mechanisms, such as the encapsulation, bioactive substances using microfluidics technique or electrospinning, sheeting and calendaring dough in bakery industries, the extrusion-based 3D printing, as well as the development of textured vegetable protein products from plant protein using extrusion process for the vegetarians as shown in Figure 2.
Figure 2: The examples of various processes related to the extensional properties; (A) the channel-flow in the microfluidics system, (B) sheeting and calendaring dough, (C) the extrudate expansion and fiber spinning, and (D) the extrusion-based 3D printing.
Thus, the food manufacturers need to understand the extensional behaviors and properties of food and raw materials, and the final products in order to reduce waste in the production process and produce standardized and high-quality products to consumers.
Examples of the extensional rheological properties’ importance on the development of food products and personal care products
We have placed importance on the extensional flow behavior to the food and other industries by giving the study of the extensional rheology of food products and personal care products as the examples. In the food industry, our research is related to the development of thickening powders for people with dysphagia, which the commercial thickening powders were divided into 2 main groups, namely, gum-based and modified starch-based.
Our work on the rheological properties of the thickening powders added to the drinking water indicated that the shear viscosity of the two samples of the thickening powders in the gum-based group (Thickener A and Thickener B) analyzed by a rotational rheometer had a similar as shown in Figure 3.
However, when the two samples were analyzed for the extensional flow with the HAAKE CaBER1 rheometer, it was found that the samples had distinctly different extensional actions. That is, the filament of Thickener A lacked faster than Thickener B as shown in Figures 3 and 4. It was found that the intermolecular interactions within Thickener B were stronger than Thickener A, which could be due to the molecular size or structure of the elements in the thickening powders.
Furthermore, the breakage of filaments at different durations had also provided the cohesiveness information of different samples. Therefore, the products with thickening powders suitable for dysphagia people should have good extensional and elongational rheology to reduce the risk of suffocation, which could link to pneumonitis.
Figure 3: The extensional flow properties show the diameter change of the filament (Y-axis) during the time (X-axis) of the two commercial thickening powders products mixed with drinking water. The D is the size of the diameter of filament changes as the extensional time, while the D0 is the diameter of the initial filament. (Insert photo: the shear rheological properties show the similar thickening value of the two thickening powders samples.)
Figure 4: The characteristic of thinning filaments of the thickening powders samples as a time function during the testing the Capillary Break-up Extensional Rheometry (CaBER) technique.
In the development of personal care products, such as sachet shampoo products, it is necessary to control the flow behavior during the production process. The extensional rheological properties, therefore, are required to accurately control the amount of shampoo contained in each sachet and reduce the waste during the production process.
Results from the rheological properties testing of the two samples (A and B) indicated that the shear viscosity values did not differ significantly as shown in Figure 5. On the contrary, when the extensional properties were analyzed, both samples exhibited a distinct breakage of filaments at different times as shown in Figure 5. The filament of shampoo A was broken earlier than shampoo B, which made easier and faster packaging.
Figure 5: The extensional flow properties show the diameter change of the filament (Y-axis) during the time (X-axis) of the two commercial shampoos. The D is the size of the diameter of filament changes as the extensional time, while the D0 is the diameter of the initial filament. (Insert photo: the shear rheological properties show the similar thickening value of the two shampoos samples.)
The filament of shampoo B was broken slower than shampoo A and could create a string formation, e.g., shampoos with a greasy flow that affect the sachet sealing.
The sachet shampoo products, therefore, are necessary to contain an appropriate additive to reduce the string formation effect and waste in the production process, as well as increase the production rate.
Guidelines for extensional rheological properties testing
The food rheology research had employed materials modelling for the forming process, production, and sensory qualities of food products. The shear flow and the extensional flow were important behaviors in rheology. The extensional viscosity, technically known as , strongly correlated with the molecular weight and structure of the large macromolecular chains, such as polysaccharides or the protein that was the main food component. Thus, the extensional viscosity is also important parameters apart from the shear viscosity ().
Various techniques and equipment for the testing of the extensional viscosity and analyzing the extensional rheology had been developed, such as Entrance capillary flows, two- or four-roll mills, thread-line rheometry, Filament-Stretching Extensional Rheometry (FISER), Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER), Dripping-onto-Substrate (DoS) rheometry, and Capillary break-up Extensional Rheometry (CaBER). Each technique was appropriated for a certain range of the extensional flow properties of the samples based on the zero shear viscosity of the samples and the deformation rate of that technique as shown in Figure 5.
However, most techniques were developed primarily for laboratory research, so they were not widely used, except the CaBER technique. The CaBER technique has found extensively use in the analysis of the extensional rheological properties of the materials and food products over the past 10 years.
Figure 6: The extensional flow analysis technique and the analyzed range based on the zero shear viscosity of the samples and the deformation rate of each techniques.
The CaBER test is an easy technique and requires a small sample volume depending on the size of the test plate used, which usually have a diameter in the range of 4-8 mm. The tested liquid was injected between the upper and lower plates at the appropriate gap size. Then, the upper plate would move rapidly (in an interval of 10-20 mm.), causing the extension of the samples, and the formation of the filament. The diameter of the filament would be smaller as a function of time, which depended on the capillary force, surface tension, elasticity of the samples. The samples were analyzed with a laser micrometer as shown in figure 7A, which created the filament break-up time as shown in figure 7B, as well as the extensional viscosity as shown in Figure 7C.
Figure 7: (7A) The extensional rheology testing by the Capillary Break-up Extensional Rheometry (CaBER) technique with a CaBER1 (Thermo Haake GmbH, Karlsruhe, Germany) and the sample results interacting between (7B) the diameter of the filament as a function of time and (7C) the extensional viscosity is function of Hencky strain value for the sample of Newtonian fluid, Boger fluid, or adhesive.
In fluid mechanics, it is found that various materials may response to the shear stress with a similar shear flow, but they could show different elongation properties or responses. The elongation analysis of materials or products, employs a technique that provides information of additional necessary materials properties of the shear flow properties using a rotary rheometer, which can be used to analyze the properties and performance of the interested materials or products.
From the aforementioned examples, it can be seen that the extensional rheological properties are very critical to food industry in terms of structural analysis, design, production process controlling, and product quality. Furthermore, the extensional rheological properties are also related to other industries, such as coating process and part plating, film forming, inks developing, spraying, and oil production. Thus, the analysis of appropriate rheological properties for products and production processes can increase a production efficiency, as well as control and maintain products’ quality.
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