Texture vs viscosity: Understanding their roles and differences

When it comes to food science and sensory analysis, texture and viscosity are two fundamental attributes and while these terms are often used interchangeably, they describe different properties and behaviours of food.

Why should you care about texture?

• It signals freshness and quality. That snap when you bite into a fresh vegetable or the tenderness of a perfectly cooked steak tells you a lot about the food's condition.
• It enhances flavour perception. The way food breaks down in your mouth can intensify or mute flavours, directly impacting your overall enjoyment.
• It adds excitement to your meals. Varied textures can make eating more interesting and satisfying, preventing food boredom.

What is texture?

Texture is a multisensory marvel and refers to the physical properties of food that are perceived by the sense of touch in the mouth or hands. It encompasses a wide range of attributes such as hardness, chewiness, crispiness, and stickiness. Texture can be both a sensory and a mechanical property, influencing how a food feels and how it deforms under various forces.

Key aspects of texture

Hardness: The resistance of food to deformation or penetration. For example, a carrot is hard, while a banana is soft.

Chewiness: The extent to which food needs to be chewed before swallowing. Chewy foods include gummy candies and steak.

Crispiness: The brittleness and fracturability of food. Foods like potato chips and fresh lettuce are crispy.

Stickiness: The adhesion of a food's surface to e.g. teeth, important for products like caramels and bakery toppings.

What is viscosity?

While texture grabs the spotlight, viscosity works behind the scenes, especially in liquid and semi-liquid foods. It's all about how easily a fluid flows, and it describes how thick or thin a liquid is, which affects how it pours, spreads, and coats surfaces. So, why should you pay attention to viscosity?

• It impacts mouthfeel. A too-thin milkshake or an overly thick soup can disappoint your taste buds before you even swallow.
• It affects flavour release. The right viscosity ensures that flavours are released gradually, enhancing your tasting experience.
• It's crucial for food stability. Proper viscosity can mean the difference between a smooth salad dressing and one that separates unappealingly.

Key aspects of viscosity

• Flow behaviour: Viscosity determines how a liquid moves. For instance, water has low viscosity and flows easily, while honey has high viscosity and flows slowly.
• Shear rate dependence: Viscosity can change with the rate of applied shear force. This property is crucial for understanding how foods behave under different conditions, such as mixing or swallowing.
• Temperature dependence: Viscosity typically decreases with increasing temperature. For example, heated chocolate syrup flows more easily than when it is cold.

Texture vs. viscosity: Key differences

State of matter

Texture: Applies to both solid and semi-solid foods.

Viscosity: Primarily concerns liquids and semi-liquids.

Perception

Texture: Perceived through touch and bite. It involves physical attributes experienced during chewing and swallowing.

Viscosity: Perceived through flow behaviour and mouthfeel, influencing how a liquid feels as it moves and spreads in the mouth.

Measurement

Texture: Measured using a Texture Analyser which provides a wide range of quantitative data, depending on the test being performed and the material being analysed.

Here are some common units obtained from a Texture Analyser:

Force (N, g, kg, lbf)

• Newton (N) – The primary unit of force measurement.
• Gram-force (g) / Kilogram-force (kg) – Alternative units for smaller/larger force measurements.
• Pound-force (lbf) – Common in the US market.

Distance (mm, in)

• Millimetres (mm) – The typical unit for measuring sample deformation.
• Inches (in) – Sometimes used for larger deformations in non-metric regions.

Time (s, ms)

• Seconds (s) – Standard unit for measuring test duration.

Stress (Pa, MPa, kPa)

• Pascal (Pa), Megapascal (MPa), Kilopascal (kPa) – Used when calculating mechanical properties like compressive or tensile stress.

Strain (%)

• Expressed as a percentage (%) of deformation relative to the original sample height or dimension.

Work / Energy (mJ, J)

• Millijoules (mJ) / Joules (J) – Measures the area under the force-distance curve, representing the total energy required to deform a sample.

Viscosity: Measured using viscometers or rheometers that determine flow rates and resistance. Here are some common units obtained from a viscometer/rheometer:

Viscosity (Pa·s, mPa·s, cP)

• Pascal-second (Pa·s) – The SI unit of dynamic viscosity.
• Millipascal-second (mPa·s) – Often used for lower-viscosity fluids (1 mPa·s = 1 cP).
• Centipoise (cP) – A common non-SI unit, where 1 cP = 1 mPa·s.

Shear rate (s⁻¹)

• Measured in inverse seconds (s⁻¹).
• Indicates how quickly adjacent layers of fluid move relative to each other.
• Higher shear rates simulate faster movement (e.g., stirring or pumping).

Shear stress (Pa)

• Measured in Pascals (Pa).
• Represents the force per unit area required to move the fluid.

Yield stress (Pa)

• Also measured in Pascals (Pa).
• Indicates the minimum force needed to initiate flow in a structured material (e.g., gels, pastes).

Torque (mN·m, N·m)

• Measured in millinewton-meters (mN·m) or Newton-meters (N·m).
• Important for rotational viscometers and rheometers.

Rotational speed / Angular velocity (rpm, rad/s)

• Revolutions per minute (rpm) – Common in rotational viscometers.
• Radians per second (rad/s) – SI unit for rotational speed.

Elastic (Storage) modulus, G' (Pa)

• Measured in Pascals (Pa).
• Represents the elasticity or solid-like behaviour of a material.

Viscous (Loss) modulus, G'' (Pa)

• Also measured in Pascals (Pa).
• Represents the liquid-like, energy-dissipating behaviour of a material.

Complex viscosity (Pa·s)

• Measured in Pascals-seconds (Pa·s).
• Used in oscillatory rheometry to describe viscosity under dynamic conditions.

Phase angle (°)

• Measured in degrees (°).
• Indicates the balance between elastic and viscous behaviour (0° = purely elastic, 90° = purely viscous).

When to use a Texture Analyser

A Texture Analyser is used when measuring the mechanical and textural properties of solid, semi-solid, or structured materials. You would choose a Texture Analyser when:

You need to assess bulk physical properties, including firmness, hardness, or crispness.
Example: Testing the firmness of a fruit or the crispness of a biscuit.

The material is too solid or structured for a viscometer or rheometer.
Example: Measuring the chewiness of a protein bar or the elasticity of a gel capsule.

The material is not homogeneous.
Example: jam with inclusions or hair products with air pockets.

You need to measure deformation, compression, or breaking strength.
Example: Determining the fracture force of chocolate or the tensile strength of packaging film.

Your test requires penetration, cutting, stretching, or compression.
Example: Adhesive strength of glue, spreadability of butter, or compaction force of tablets.

You need to analyse adhesion or tackiness.
Example: Testing stickiness of chewing gum or adhesive force of a medical bandage.

You are measuring relaxation or recovery over time.
Example: Dough extensibility or resilience of foams.

When to use a viscometer or rheometer

A viscometer or rheometer is used when measuring the flow and deformation behaviour of liquids, semi-liquids, or soft materials. You would choose a viscometer/rheometer when:

You are working with a flowing liquid or semi-liquid.
Example: Testing the viscosity of honey, paint, or shampoo.

Shear rate and viscosity are important factors.
Example: Measuring the shear-thinning behaviour of ketchup (how it flows more easily when squeezed).

You need to measure yield stress (when a material starts to flow).
Example: Evaluating the spreadability of mayonnaise or cream.

You need to analyse viscoelastic properties (both solid and liquid behaviour).
Example: Measuring the gel strength of yogurt or the setting behaviour of cement paste.

Oscillatory testing is required to measure elasticity and viscosity.
Example: Testing the gelation of hydrocolloids or the structural stability of emulsions.

Your material is highly sensitive to applied forces and needs precise, low-stress measurements.

Example: Studying the rheological properties of blood plasma or polymer solutions.