Analyzing the Flow Properties of Liquid Foods in Food Engineering

Every liquid food you encounter in a processing plant – from milk flowing through stainless steel pipes to honey being pumped into jars – behaves according to specific physical rules. These rules, known as flow properties, determine how efficiently a liquid can be transported, heated, mixed, and packaged. For food engineers, getting these properties right is not optional; it’s the difference between a smooth production line and a costly bottleneck. The two most critical flow properties are density and viscosity, and together they govern nearly every decision made in liquid food processing.

Why flow properties matter in food processing 🔗

In any food manufacturing facility, liquid foods must move – through pipelines, pumps, heat exchangers, and filling machines. Rheological data are essential for developing industrial plant designs, including the selection of pumps and pipes, as well as for calculations involving extruders, mixers, and homogenizers. Without accurate knowledge of how a liquid food flows, engineers risk choosing undersized pumps, designing inefficient heat exchangers, or creating products with inconsistent texture. Flow properties also directly affect heat transfer during pasteurization and sterilization, mixing efficiency during blending operations, and product quality in the final package.

Understanding density of liquid foods 🔗

Density is one of the most fundamental physical properties of any material. It is defined as the mass per unit volume of a substance, typically expressed in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³). The basic formula is straightforward:

Density (ρ) = Mass (m) / Volume (V)

For liquid foods, density tells engineers how “heavy” a given volume of product will be. This information is critical when sizing tanks, calculating the load on conveyor systems, designing pipeline networks, and determining how liquids will separate or stratify during storage.

How temperature affects density 🔗

Temperature has a direct and measurable effect on the density of liquid foods. As temperature rises, most liquids expand, meaning their volume increases while mass stays the same – resulting in lower density. When a fluid changes temperature, the resulting changes in density establish natural convection currents, which influence how heat moves through the liquid during processing. This relationship is particularly important during thermal operations like pasteurization, where heated liquid rises and cooler liquid sinks, creating circulation patterns that affect processing uniformity.

Research has shown that density decreases in a linear fashion as temperature increases for most liquid foods across typical processing temperature ranges. For example, milk, fruit juices, and sugar solutions all become slightly less dense when heated. This is why accurate temperature control during density measurement is essential for reliable results.

Other factors influencing density 🔗

Beyond temperature, several other factors influence the density of liquid foods. Composition plays a major role – liquids with higher sugar or solid content tend to have greater density. Structural properties of foods are affected by factors such as moisture content, material morphology, and processing conditions. For instance, concentrated fruit juices are denser than diluted ones because the proportion of dissolved solids is higher. Fat content also matters – higher fat in milk reduces its density because fat is lighter than water. Dissolved gases, suspended particles, and overall chemical composition all contribute to the final density value of a liquid food.

Viscosity: the resistance to flow 🔗

While density tells you how heavy a liquid is, viscosity tells you how easily it flows. Viscosity is defined as a fluid’s internal resistance to flow – or more practically, how “thick” or “thin” a liquid feels. Water has low viscosity and pours quickly. Honey has high viscosity and moves slowly. The standard unit of viscosity is the Pascal-second (Pa·s), though the centipoise (cP) is also commonly used in food science.

All liquids and soft solids have a viscosity value, which describes the resistance of the material when a certain stress is applied. In food engineering, viscosity determines how a liquid behaves when it is pumped, stirred, poured, or forced through a heat exchanger. Getting viscosity wrong can lead to pumps that cavitate, uneven mixing, or products that don’t coat or fill containers properly.

Temperature and viscosity 🔗

Temperature has a major effect on viscosity, with viscosity decreasing significantly as temperature increases. At higher temperatures, molecules move more energetically and spend less time interacting with their neighbours, which reduces internal friction. This is why honey flows more easily when warmed and why chocolate needs precise temperature control during enrobing operations. For food engineers, this means viscosity measurements must always be taken at a specified temperature to be meaningful, and processing temperatures must be carefully controlled to maintain desired flow behaviour.

Newtonian fluids in food processing 🔗

Liquid foods can be classified into two broad categories based on how they respond to applied force: Newtonian and non-Newtonian fluids. This classification is central to food engineering because it determines the mathematical models, equipment choices, and process designs used for each product.

Newtonian fluids exhibit a linear relationship between shear stress and shear rate, which means their viscosity remains constant regardless of how fast or how hard they are being stirred, pumped, or poured. In practical terms, a Newtonian fluid flows the same way whether you stir it gently or vigorously – only the speed of flow changes, not the fluid’s resistance to flow.

Examples of Newtonian liquid foods 🔗

Common Newtonian liquid foods include water, milk, clear fruit juices, sugar syrups, vegetable oils, and honey (at a constant temperature). These liquids are relatively predictable in processing environments. Engineers can calculate pressure drops, pump sizes, and flow rates using straightforward equations because viscosity does not change with processing conditions. Milk, for example, flows with consistent resistance whether it is being gently moved through a pasteurizer or rapidly pumped through a filling line.

Shear stress and shear rate explained 🔗

To understand the Newtonian model, you need two key concepts. Shear stress is the force per unit area applied to a fluid to make it flow – essentially, how hard you push. Shear rate is the velocity gradient across the fluid – essentially, how fast the layers of fluid slide past each other. The shear rate describes how fast the fluid layers move alongside each other, while the shear stress describes how much force is required to achieve that movement. For Newtonian fluids, dividing shear stress by shear rate always gives the same viscosity value, which is why these fluids are the simplest to model in engineering calculations.

Non-Newtonian fluids in food processing 🔗

Most real-world liquid foods, however, do not behave as neatly as Newtonian fluids. A non-Newtonian fluid does not follow Newton’s law of viscosity – its viscosity changes depending on the stress or shear rate applied to it. This means the same food can feel thick in one situation and thin in another, depending on how it is being handled. For food engineers, this complexity demands more sophisticated equipment, more careful process design, and more detailed testing.

Shear-thinning (pseudoplastic) fluids 🔗

The most common type of non-Newtonian behaviour in food is shear thinning, also called pseudoplastic behaviour. These fluids become less viscous – they flow more easily – as the shear rate increases. Fluids are shear thinning if the viscosity decreases as the shear rate increases. Ketchup is a well-known example: it sits thick in the bottle but flows freely when you squeeze or shake. Other shear-thinning foods include tomato paste, fruit purees, yogurt, salad dressings, and many concentrated juices. This behaviour occurs because the internal structure of the fluid – such as tangled polymer chains or suspended particles – gets disrupted and aligned under force, reducing resistance.

Shear-thickening (dilatant) fluids 🔗

The opposite behaviour, shear thickening, is less common in foods but still important. Here, the fluid becomes more viscous as force increases. A common example of shear thickening is a mixture of cornstarch and water: stir it slowly and it flows like a liquid, but try to stir it fast and it resists like a solid. Certain concentrated starch suspensions and some candy mixtures can exhibit this behaviour during processing.

Bingham plastics and yield stress 🔗

Some liquid foods require a minimum amount of force – called a yield stress – before they begin to flow at all. Several examples of Bingham plastics include clay suspensions, toothpaste, mayonnaise, chocolate, and mustard. In these fluids, the material behaves like a solid under low stress and like a liquid once the yield stress is exceeded. Mayonnaise, for instance, holds its shape on a spoon but flows when you spread it with a knife. This property is critical in product design – it keeps products like sauces and spreads stable in the jar but easy to use once force is applied.

Time-dependent flow behaviour 🔗

Some non-Newtonian foods also exhibit time-dependent flow behaviour. Thixotropic fluids become thinner over time under constant shear – yogurt is a common example, becoming runnier the longer you stir it. Rheopectic fluids, which are rare in food, become thicker over time under constant shear. For the same type of fluid food, different rheological behaviour can be observed depending on particle size, degree of concentration, or addition of macromolecules as thickeners. This means even slight changes in a food’s recipe or processing conditions can shift it from one flow category to another.

Measuring flow properties 🔗

Accurate measurement of density and viscosity is essential for process control and product quality. Several instruments are used in the food industry for this purpose.

Density measurement tools 🔗

The most common tools for measuring the density of liquid foods include hydrometers (floating devices that indicate density based on displacement), pycnometers (precise glass vessels for measuring volume and mass), and digital density meters (which use oscillating tube technology for rapid, accurate readings). Density of aqueous food solutions can be accurately predicted within very tight margins using fitted predictive equations based on composition and temperature, making it possible to design automated monitoring systems on production lines.

Viscosity measurement instruments 🔗

Viscosity is measured using several types of instruments depending on the fluid and the level of detail required. Capillary viscometers measure viscosity by timing the flow of a liquid through a narrow tube. Rotational viscometers (such as the widely used Brookfield viscometer) measure the torque required to rotate a spindle immersed in the fluid. Rheometers are advanced instruments that can measure viscosity, yield stress, and other flow properties under a wide range of shear conditions. The use of controlled stress/strain rheometers and advanced optical Fourier transformation rheometry has increased the precision and reliability of rheological data in the food industry.

Practical applications in food engineering 🔗

Understanding flow properties has direct, real-world consequences in food manufacturing. Here are some key applications:

Pipeline design: Engineers must know the viscosity and density of a product to calculate the correct pipe diameter, pump capacity, and pressure required to move it through a processing plant. A product like tomato paste, which is thick and shear-thinning, needs different pipeline specifications than water or clear juice.

Heat exchanger performance: Viscosity affects how efficiently heat transfers through a liquid. Thicker fluids form larger boundary layers that slow heat transfer, requiring longer exposure times or larger heat exchange surfaces. Knowing whether a product is Newtonian or non-Newtonian helps engineers predict and optimise thermal processing.

Quality control: Food rheology is important in quality control during food manufacture and processing. Monitoring viscosity during production ensures batch-to-batch consistency. A change in viscosity can signal problems with ingredient quality, mixing errors, or temperature deviations.

Product texture and consumer perception: The acceptability of food products to the consumer is often determined by food texture, such as how spreadable and creamy a food product feels. Flow properties directly influence mouthfeel – the sensory experience of food in the mouth – making them critical to product development and consumer satisfaction.

Key mathematical models for flow behaviour 🔗

Food engineers use several mathematical models to describe the flow behaviour of different liquid foods. For Newtonian fluids, Newton’s law of viscosity is sufficient: shear stress equals viscosity multiplied by shear rate. For non-Newtonian fluids, more complex models are needed.

The Power Law (Ostwald-de Waele) model is one of the most widely used, describing both shear-thinning and shear-thickening behaviour using two parameters: a consistency coefficient (K) and a flow behaviour index (n). When n is less than 1, the fluid is shear-thinning; when n is greater than 1, it is shear-thickening; and when n equals 1, the fluid is Newtonian.

The Herschel-Bulkley model extends the Power Law by adding a yield stress term, making it suitable for fluids like mayonnaise and chocolate that need a minimum force before they begin to flow. The Casson model is specifically recognised in the chocolate industry as a standard for measuring yield stress and plastic viscosity. Proper modelling of flow or viscosity curves as a function of shear rate is a useful tool in any engineering activity related to food processing.

Bringing density and viscosity together 🔗

In practice, density and viscosity are not isolated properties – they work together to determine how a liquid food behaves in a processing system. The Reynolds number, a dimensionless quantity that predicts whether flow will be laminar (smooth) or turbulent (chaotic), depends on both density and viscosity along with flow velocity and pipe diameter. Laminar flow is common with thick, viscous foods in narrow pipes, while turbulent flow is more typical with thin, low-viscosity liquids at high speeds. Knowing the flow regime helps engineers design systems that minimise energy use, prevent product damage, and ensure uniform processing.

By mastering both density and viscosity – and understanding how temperature, composition, and shear conditions affect them – food engineers can design processing lines that are efficient, consistent, and capable of delivering high-quality products to consumers.

What do you think? How might changing the temperature of a liquid food during processing create unexpected challenges in pipeline design? Can you think of liquid foods in your daily life that clearly behave as non-Newtonian fluids?

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