Understanding the Properties of Solid Food Materials in Food Engineering

Every food product you pick up – whether it’s a grain of rice, a block of cheese, or a fresh apple – has a set of measurable physical characteristics that determine how it behaves during processing, transportation, and storage. These are called the properties of solid food materials, and they form the backbone of food engineering. Understanding these properties helps food engineers design efficient processes, maintain product quality, and deliver consistent results. In this post, we’ll break down the three main categories of these properties: physical, thermal, and rheological.

Why do the properties of solid food materials matter? 🔗

Food is not a uniform, simple substance. It’s a complex, multi-component system made up of solids, liquids, and gases. A single apple, for instance, has a skin, flesh, air pockets, and moisture – all behaving differently under various conditions. Food engineers need to measure and predict how these materials respond during operations like drying, freezing, heating, grinding, and packaging. Without accurate knowledge of a food material’s properties, it becomes nearly impossible to design equipment, set processing parameters, or ensure the final product meets quality standards. The engineering properties of foods broadly include geometrical, physical, thermal, and rheological characteristics, all of which play interconnected roles in food quality and safety.

Physical properties of solid food materials 🔗

Physical properties describe the fundamental measurable characteristics of a food material – things you can quantify without changing the food’s chemical composition. These are the starting point for any food engineering calculation.

Density 🔗

Density is the mass of a material per unit volume, expressed in kg/m³. It sounds simple, but in food engineering, density is more nuanced than it appears. Because most foods contain multiple phases – solid matter, water, and air – a single density value isn’t always enough. Food scientists work with several types of density. True density (or substance density) refers to the density of the solid material alone, excluding any pores or air. Particle density accounts for internal pores but not the spaces between particles. Bulk density includes both the material and the air spaces between individual particles, which matters greatly when you’re filling a container with grain or flour. The density of a food influences how much energy is needed to move it, how it settles in storage containers, and how it interacts with liquids during processes like blanching or brining. For example, denser fruits sink in water during washing, while less dense ones float – a principle used in quality sorting of food products.

Specific gravity 🔗

Specific gravity is the ratio of a food material’s density to the density of water (at a standard temperature, typically 4°C). It’s a dimensionless number. A specific gravity greater than 1 means the material is denser than water, and less than 1 means it’s lighter. This property is widely used in separation and grading operations in the food industry. Impurities, stones, and damaged grains can be separated from good-quality produce based on differences in specific gravity. It’s also useful in determining the concentration of solutions – for instance, measuring the sugar content of a fruit juice by checking its specific gravity.

Porosity 🔗

Porosity refers to the volume fraction of void spaces or air within a food material. It directly affects how a food interacts with air, water, and heat during processing. A highly porous food like bread has many air pockets, which makes it dry faster and absorb liquids more readily. A dense food like a raw carrot has low porosity, meaning moisture moves through it more slowly. Porosity is critical in processes like drying, frying, and baking. During frying, for example, oil enters the food through its pore structure – more porous foods absorb more oil. During drying, porous structures allow moisture to escape more easily, reducing drying time. According to food engineering research, porosity also affects the bulk density and pore size distribution of food powders and granular materials, which in turn impacts packaging and flowability.

Size, shape, and surface area 🔗

While density and porosity get much of the attention, size, shape, and surface area are equally important. Fruits and vegetables are routinely graded by size and shape for market standards. In heat transfer calculations, the surface area of a food item determines how quickly it can gain or lose heat. A small, round berry will cool faster than a large, irregularly shaped mango – simply because of the ratio of surface area to volume. Sphericity – a measure of how closely a food item’s shape resembles a sphere – is used in heat and mass transfer calculations and in designing conveying systems.

Thermal properties of solid food materials 🔗

Whenever food is heated, cooled, frozen, or dried, thermal properties come into play. These properties determine how fast and how uniformly heat moves through a food product. Getting thermal processing wrong – undercooking, overheating, or uneven heat distribution – can compromise both safety and quality. Accurate thermal property data is essential for engineering and process design in the food industry.

Specific heat 🔗

Specific heat (Cp) is the amount of heat energy required to raise the temperature of one kilogram of a material by one degree Celsius. It’s measured in J/(kg·°C). Water has a very high specific heat (about 4,186 J/kg·°C), which is why water-rich foods like tomatoes and cucumbers take longer to heat up – and also longer to cool down – compared to dry foods. Fats and oils have a specific heat roughly half that of water, and dry solids have even lower values, around one-third to one-quarter. This means that the moisture content of a food has a major influence on its specific heat. As moisture increases, so does the energy needed for heating. This relationship is approximately linear, making it possible to estimate specific heat from a food’s composition if direct measurement isn’t available.

Thermal conductivity 🔗

Thermal conductivity (k) measures how well a material conducts heat, expressed in W/(m·K). Materials with high thermal conductivity transfer heat quickly; materials with low thermal conductivity act as insulators. Metals like aluminium have high thermal conductivity, which is why they’re used in cookware. Food materials, however, are poor conductors of heat – their thermal conductivity is typically 20 to 30 times lower than that of stainless steel. Like specific heat, thermal conductivity in foods increases with moisture content. In porous, low-moisture foods, thermal conductivity becomes a non-linear function of moisture because changes in bulk porosity significantly alter how heat flows through the material. One important practical implication: thermal conductivity in fibrous foods like meat can vary by 15-30% depending on whether you measure along or across the muscle fibres.

Thermal diffusivity 🔗

Thermal diffusivity (α) is a combined property that tells you how fast a heat front moves through a material. It’s calculated as the ratio of thermal conductivity to the product of density and specific heat: α = k / (ρ × Cp). Its unit is m²/s. A material with high thermal diffusivity heats up quickly throughout its volume, while a material with low thermal diffusivity heats slowly with large temperature gradients between its surface and centre. This property is critical in processes like canning, sterilisation, and freezing, where you need to ensure the coldest or slowest-heating point in the product reaches a target temperature. Without accurate diffusivity data, calculating how long to process a food for safety would be guesswork.

Rheological properties of solid food materials 🔗

Rheological properties describe how food materials respond to applied forces – how they deform, flow, or break. While rheology is often associated with liquid foods (think ketchup or honey), it’s equally important for solid and semi-solid foods. The way a food feels in your mouth, how it responds to cutting or pressing, and how it behaves during manufacturing are all governed by its rheological characteristics. These properties are also the foundation of what we perceive as food texture.

Hardness 🔗

Hardness is the resistance of a food material to deformation – specifically, the force required to compress or penetrate it. A raw carrot is hard; a ripe banana is soft. In food engineering, hardness is one of the primary parameters measured during Texture Profile Analysis (TPA), a standardised double-compression test that mimics the chewing action of the mouth. Hardness values help food manufacturers set quality benchmarks. For instance, a biscuit manufacturer needs to ensure every batch falls within a specific hardness range so that consumers experience the expected crunch. Hardness data is also important in designing size reduction equipment – grinding a hard grain requires different machinery than mashing a soft fruit.

Elasticity (springiness) 🔗

Elasticity – now more commonly called springiness in TPA terminology – refers to a material’s ability to return to its original shape after a deforming force is removed. A marshmallow bounces back when you press it; a cracker does not. In food processing, elasticity matters for products that must maintain their shape during handling, packaging, and transport. Bread dough, for example, needs a certain level of elasticity to rise properly and hold its structure during baking. Cheese is another good example: mozzarella has high elasticity (it stretches without breaking), while a hard cheddar has much lower elasticity. The original TPA parameter “elasticity” was renamed “springiness” because elasticity already had established definitions in engineering and physics, which could cause confusion. Springiness in TPA is expressed as a ratio of the height recovered between the first and second compressions.

Cohesiveness 🔗

Cohesiveness measures how well a food holds together under deformation. It’s the ratio of the work done during the second compression to the work done during the first compression in a TPA test. A cohesive food maintains its structure even after being compressed – think of a gummy bear that deforms but doesn’t crumble. A food with low cohesiveness falls apart easily, like a dry cookie. Cohesiveness data helps food engineers understand how a product will behave during slicing, portioning, and eating.

Chewiness and gumminess 🔗

Chewiness and gumminess are secondary TPA parameters derived from the primary ones. Chewiness applies to solid foods and is calculated as hardness × cohesiveness × springiness – it represents the energy needed to chew a food until it’s ready to swallow. Gumminess applies to semi-solid foods and is calculated as hardness × cohesiveness. These parameters are particularly useful in product development, where food engineers need to match a product’s chewing characteristics to consumer expectations.

Viscoelasticity 🔗

Many real foods don’t behave purely as solids or purely as liquids – they exhibit both viscous (liquid-like) and elastic (solid-like) behaviour. This is called viscoelasticity. Wheat flour dough is a classic viscoelastic material: it stretches (elastic response) and also slowly flows when left under its own weight (viscous response). Dairy cream, ice cream mixtures, and cheese also fall into this category. Viscoelastic properties are measured using techniques like stress relaxation tests (apply a strain and watch the stress decay), creep tests (apply a constant stress and measure deformation over time), and dynamic oscillatory tests (apply small oscillating strains and measure the storage modulus G’ and loss modulus G”). The storage modulus G’ represents the elastic (stored) energy, while the loss modulus G” represents the viscous (dissipated) energy. The ratio of G” to G’ (called the loss tangent, tan δ) tells you whether a food behaves more like a solid or a liquid.

How these properties connect in real food processing 🔗

These three categories – physical, thermal, and rheological – don’t exist in isolation. They interact constantly during food processing. Consider what happens when you bake bread. The dough’s viscoelastic properties determine how it rises and holds shape. Its thermal conductivity and specific heat determine how quickly the interior reaches the temperature needed for starch gelatinisation and protein denaturation. The porosity of the final loaf affects its texture, crumb structure, and how quickly it dries out during storage. Meanwhile, the density and hardness of the crust versus the crumb create the sensory experience we associate with good bread.

Similarly, during the freezing of fruits or vegetables, thermal diffusivity determines how fast the product freezes, porosity affects ice crystal formation, and elasticity influences whether the product retains its texture after thawing. The physical state of food solids – affected by temperature, moisture, and phase transitions – has a significant effect on mechanical properties, especially in low-moisture and frozen foods where even small environmental changes can cause major shifts in texture.

Measuring these properties 🔗

Accurate measurement of solid food properties requires specialised instruments. Density is commonly measured using pycnometers or displacement methods. Porosity can be estimated from the ratio of bulk density to true density, or measured using gas pycnometry. Thermal conductivity is typically measured using thermal probe methods, while specific heat is determined via differential scanning calorimetry (DSC). Thermal diffusivity can be calculated from conductivity, density, and specific heat data, or measured directly from temperature history data during heating or cooling experiments.

For rheological properties, the Texture Profile Analysis using a texture analyser remains the most widely used method for solid foods. Developed in the 1960s by Dr. Alina Szczesniak and colleagues at the General Foods Corporation, TPA provides multiple textural parameters – hardness, springiness, cohesiveness, chewiness – from a single double-compression test. For more fundamental characterisation, rheometers are used for oscillatory and creep tests to study viscoelastic behaviour in detail.

Practical applications in the food industry 🔗

Knowledge of solid food properties feeds directly into practical decisions across the food industry. In grain handling, bulk density and porosity determine storage bin design and aeration strategies. In thermal processing, specific heat and thermal diffusivity data determine sterilisation times for canned foods. In snack manufacturing, hardness and crispiness targets guide formulation and frying parameters. In dairy processing, the viscoelastic properties of cheese determine how it performs during slicing, shredding, and melting. In bakery products, dough elasticity and cohesiveness affect everything from mixing times to final product texture.

Even packaging decisions depend on these properties. A porous, fragile product like a puffed rice cake requires very different packaging (cushioning, moisture barriers) compared to a dense, hard product like a nut bar.

What do you think? How might a better understanding of these material properties help reduce food waste during processing and transportation? And if you’ve ever noticed how the same fruit can feel different depending on its ripeness, which property do you think is changing the most – hardness, elasticity, or porosity?

References

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