Rheological properties play an important part in developing new food as well as beverages. This article will discuss the rheological analysis of common beverages.
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Rheology is the study of the flow phenomenon of matter (solids, soft solids, liquids, or gasses). However, typically, it is used to measure the flow property of fluids. The term “flow” has been defined as a continuous deformation of materials over time when forces or stresses are applied. This field of science is extremely important in a wider spectrum of research that includes medicine, material design, etc.
Rheological Properties of Beverages
It is important to study the rheological properties of beverages as it helps to evaluate their quality and behavior. For determining the characteristic features of wine, scientists have performed chemical and rheological analyses of various types of wine. They determined the level of acidity, reduced sugar, pH, alcoholic content, volatile acids, and total and free SO2 of wines.
The wine samples were also subjected to a rheological test to evaluate their viscosity in relation to temperature and time. The shear rate of the wine was described using the Herschel-Bulkley mathematical model. The majority of the tested wines behaved as non-Newtonian fluids at a low temperature, i.e., 5°C. However, this behavior changed into Newtonian at a temperature greater than 10°C.
Scientists have also measured the rheological properties of milk-based beverages, such as lassi, milkshake, malted milk, buttermilk, milk coffee, etc. They found that the viscosity was associated with how a milk-based beverage would feel in the mouth.
Recently, researchers in the journal Food Science and Technology studied the rheological properties of plant-based beverages under different temperatures using models of Newton, Ostwald-de Waele, Herschel Bulkley, and Bingham. In this study, they analyzed beverages made from Bertholletia excelsa (Brazil but), Dipteryx alata (Baru), and Macadamia integrifolia (Macadamia).
They further reported that the nut-based beverage made from Brazil nut and Macadamia presented lower viscosity compared to the beverage made from Baru and that it required lesser tension to flow.
Rheological Analysis of Tea
As tea is one of the most common beverages, the remainder of this article will focus on the rheological analysis of tea. Recently, researchers in the journal Physics of Fluids have focussed on the science of making tea and studied the thin film that is formed on its surface when it is left to steep.
A long time ago, this coating was thought to be a residual waxy coating from tea leaves. However, in the 1990s, scientists revealed that this thin film is formed by calcium carbonate that comes from the water. This is because in many regions, tap water comes from limestone aquifers and the harmless calcium carbonate alters the taste of the water. Therefore, the addition of citric acid may result in a less visible formation of a film on the tea-air interface.
Further Reading: Using Rheology to Consider Exopolymer Gels
The film only forms at the air-water interface, when the tea is left to cool after steeping. Interestingly, the film formed on the surface of a tea, made from only tea (Camellia sinensis) leaves and water, comprises oxidized tea polyphenols, calcium carbonate, and other salts. However, scientists believe that the surface film contains additional compounds that are still not known. The tea film also forms cracks that appear to be similar to ice cracks.
Some studies used rheology to determine how different brewing parameters, i.e., hardness of water, tea concentration, milk, acidity, and sugar, influenced the formation of the film. These studies investigated interfacial rheology by using a metal device on the surface of the tea.
The strength of the film was determined by evaluating the resistance experienced while rotating the device in a controlled manner. Scientists stated that water resistance has the maximum influence on the formation of the film.
Therefore, if one makes tea using perfectly pure water, the film will not form at all; however, the tea will taste bitter. Additionally, the ion content of the water used to brew tea influences the mass of tea film generated. For instance, deionized water does not form any collectible film.
Recently, scientists have also investigated the rheological properties of tea using different concentrations of calcium carbonate. The optimal CaCO3 concentration for ideal tea brewing had been estimated to be between 17 and 68 mg CaCO3/L; otherwise, the tea could lose its flavor or become astringent. The addition of milk results in thicker film formation. Rheological measurements help understand the brittle nature of the film.
In a recent study, researchers showed that the addition of citric acid, sugar, and milk affects the elastic and viscous properties of the tea film. A greater interfacial elastic or viscous modulus is associated with a stronger tea film. Previous studies reported that the addition of sugar in the tea has a thinning effect on the physical thickness of the tea film.
However, the recent research revealed that the addition of sugar does not significantly affect the interfacial elastic or viscous moduli and this finding is not in line with reports from previous studies. Also, the current study has strongly emphasized that a thin film does not mean it is rheologically weaker.
Experimental results proved that reduction in the film thickness did not alter the physical strength. As stated above, the addition of milk increases the physical thickness of the film but does not increase the strength. This might be due to the influence of milk lipids or proteins altering the chemical composition of the film. The most important conclusion of this study is that tea film thickness and strength are determined by interfacial rheology and not surface energy.
References and Future Reading
Giacomin, E.C. and Fischer, P. (2021) Black tea interfacial rheology and calcium carbonate. Physics of Fluids. 33, 092105. https://doi.org/10.1063/5.0059760
Pushpadass, A.H. (2019) Rheological Properties of Milk-Based Beverages. Milk-Based Beverages. 9. pp. 373-396. https://doi.org/10.1016/B978-0-12-815504-2.00011-6
Trávníček, P. et al. (2016) Study of rheological behaviour of wines. Int. Agrophys. 30(4), pp.509-518. https://doi.org/10.1515/intag-2016-0018
SILVA, Kamilla et al. Rheological behavior of plant-based beverages. Food Science and Technology [online]. 2020, v. 40, n. Suppl. 1 [Accessed 30 December 2021] , pp. 258-263. Available from: <https://doi.org/10.1590/fst.09219>. Epub 20 Dec 2019. ISSN 1678-457X. https://doi.org/10.1590/fst.09219
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