Viscosity describes a fluid's internal resistance to flow. A fluid with a higher viscosity would pour slower and seem thicker than a fluid with less viscosity. When there is a change in a material’s property such as molecular weight and density, both of which affect how a liquid flows, the viscosity changes, and the quality is altered.
The viscosity of liquid food is an important parameter as it can be used as an indicator of quality by the consumer, in some instances a thicker liquid being thought of as superior quality when compared to a thinner product. Viscosity is also a characteristic of the texture of food. This means that the viscosity of a product must be controlled and measured in production so that each batch is consistent from day today.
Newtonian behavior is displayed by simple liquids consisting of small molecules that do not interact or form any connected structure. However, it must be pointed out that long-chain polymers at low concentrations can also show Newtonian behavior. An easy way to demonstrate Newtonian behavior is to double the shear stress during a viscosity test and this should result in doubling of the shear rate. If this is not observed then the liquid is non-Newtonian.
The viscosity of some fluids is dependent on the rate used to shear the material, a high rate of shear making the fluid thinner compared with the fluid that was sheared more slowly. This is referred to as time-independent (steady-state) flow and materials showing this type of behavior are called pseudoplastic. Another type of vicious behavior exhibited by fluid foods and polymer systems is thixotropy which is again shear-thinning of the material with increasing rates of shear but is also dependent on the duration of shear.
The viscosity is also dependent on concentration and the relationship is not usually linear. For example, a small increase in the concentration of a hydrocolloid may increase viscosity a little, but once a critical concentration is exceeded the viscosity can increase exponentially.
The temperature has a major effect on viscosity; the viscosity decreasing significantly with an increase in temperature. As the temperature increases the molecules in the liquid move about more, and therefore spend less time in contact with each other, thus the internal friction of the liquid decreases.
By far the most researched food in terms of viscosity is chocolate. The flow behavior of chocolates is important both during processing as well as for organoleptic reasons. Chocolates have different flow properties depending on application and products are made for enrobing and for making blocks. However, the flow behavior is complex due to the fact that a number of ingredients, namely sugar, cocoa butter, cocoa particles, and milk products, need to be finely dispersed. The viscosity plays a crucial role as it affects texture, that is, how it flows in the mouth.
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The measurement of chocolate viscosity is highly specialized and requires a specific type of viscometer for the measurement to be performed accurately. The flow properties are usually described by the Casson flow curve and for this, it is necessary to make measurements at different rotational speeds so that shear stresses at different shear rates can be determined. The Casson method gives information about the yield stress and the plastic viscosity. Yield stress is defined as the shear stress required to initiate the flow of the chocolate and hence gives information about potential enrobing properties, whereas the plastic viscosity relates to the shear stress required to maintain a constant flow. The latter thus relates to the way the chocolate will flow in a mold or perhaps in the mouth. The viscosity test is typically carried out at 40°C and temperature control within a narrow range is important in order to perform the test accurately.
Viscosity is also important in the production of bakery products that are made from dough and batter. Such systems typically consist of a number of dispersed phases such as flour, fat, water, and air. Sufficient viscosity is required to stop phase separation during mixing, floor time, and baking in the oven. For systems such as bread doughs, there is usually sufficient viscosity to stop phase separation and to trap and retain air during bread production. However, it is a different matter when it comes to batter systems. Less viscous batter systems such as wafers batters and batters typically used to make Yorkshire puddings can result in both losses of air beaten into the batter during mixing as well as separation of solids. This can be detrimental to end-product quality. Cake batters also need to be sufficiently viscous to prevent loss of gas bubbles that are incorporated during mixing as these bubbles are recipients of the gas produced by raising agents and steam, which cause expansion and help to reduce batter density. If these gas bubbles are lost, then the expansion of the batter will be restricted as new bubbles cannot be created after the mixing has stopped. Viscosity is also temperature-dependent, hence in the oven as the batter is heated it becomes thinner. The likelihood of phase separation of the denser components such as starch granules is greater and such components can sink to the bottom of the baking tin. The result would be a gummy layer at the bottom and a fragile coarse open structure at the top. Fruit pieces, if added to the recipe, can also sink to the bottom when the batter becomes thinner as the temperature increases in the oven. The separation of recipe components, therefore, needs to be avoided by maintaining a certain level of viscosity up to the point where the starch gelatinization occurs and the structure is set.
The quality of foods such as soups, sauces, and gravies depend heavily on the structure-forming properties of materials such as starch. After starch material is cooked, the granules swell and become fragile and can break down under shear and lose viscosity. This can have disastrous consequences on the viscosity and hence on the characteristic eating quality of the food. The effect of temperature on the viscosity of starch-based foods can be investigated by techniques such as the Brabender Amylograph and its derivative the Rapid Visco Analyser. Both techniques are based on a rotational viscometer that continually records the viscosity of a sample while the temperature is changed in a controlled way. The resistance to the flow of the test sample is measured by a rotating paddle at a known speed. The continued heating and shearing of the flour batter damage the starch granules, resulting in the release of water initially absorbed by the granules when the viscosity first increased, lowering the viscosity. A minimum in hot viscosity is then achieved, which then begins to increase as the temperature is gradually reduced. The starch is subjected to constant shear throughout the test and this provides useful information about stability during the processing of food materials that rely on starch for their physical characteristics and quality.
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