The Role of Evaporation and Air-Vapour Mixtures in Food Processing

Every time you open a can of condensed milk, stir tomato paste into a sauce, or dissolve instant coffee, you’re using a product shaped by evaporation – one of the most important unit operations in food processing. Alongside evaporation, there’s another less talked-about but equally vital concept: air-vapour mixtures. Together, these two topics form the backbone of how we concentrate, dry, and preserve food at an industrial scale. Let’s break them down.

What is evaporation in food processing? 🔗

Evaporation in the food industry refers to the removal of water from a liquid food product by converting it into vapour using heat. The goal is not to dry the product completely – that would be dehydration – but to concentrate it. The end product of evaporation is still a liquid, just with a much higher solids content. Evaporation differs from dehydration because the final output remains in liquid form, whereas dehydration yields a solid product.

Common examples include concentrating fruit juices, producing condensed milk, thickening tomato paste, and refining sugar syrups. The process increases dry matter content, enhances flavour richness, and improves stability of the final product.

Why evaporation matters for the food industry 🔗

Evaporation offers several practical and economic benefits that make it indispensable in modern food processing:

Extended shelf life: By reducing the water content of a product, evaporation lowers water activity – a key factor that controls microbial growth. Products with lower water activity are far more resistant to spoilage by bacteria, yeasts, and moulds.

Reduced transportation and storage costs: Concentrated products weigh less and take up less space. Shipping concentrated orange juice instead of regular juice, for instance, dramatically cuts freight expenses. The product can be reconstituted with water at the destination.

Improved product quality: When done correctly – especially under vacuum – evaporation can preserve the original colour, flavour, and nutritional profile of food while producing a more intense taste.

The basic principle behind evaporation 🔗

At its core, evaporation is a heat-driven process. Steam or another heating medium supplies thermal energy to the food liquid, raising its temperature to the boiling point. Once boiling begins, water transitions from liquid to vapour and is carried away, leaving a more concentrated liquid behind. The three essential components of any evaporator are a heating unit (where heat is transferred), a vapour separation area (where steam is separated from the liquid), and a condenser (where vapour is cooled back into liquid for removal).

An important concept here is boiling point elevation. As a solution becomes more concentrated – say, as sugar levels in juice increase – its boiling point rises above that of pure water. This means more energy is needed as the process progresses, and evaporator design must account for this shift.

Types of evaporators used in food processing 🔗

Different food products have different properties – viscosity, heat sensitivity, tendency to foul surfaces – so the food industry uses several types of evaporators. Here are the most common ones.

Batch pan evaporators 🔗

This is the oldest and simplest type. The food is placed in a steam-jacketed vessel and heated until the desired concentration is reached. Batch pan evaporators are still used in limited applications such as making jams, jellies, and some pharmaceutical products. They offer flexibility for small-scale operations but have a relatively small heat transfer area, leading to long residence times – often several hours. For heat-sensitive products, a vacuum system is added to lower the boiling point.

Natural circulation evaporators 🔗

These evaporators contain short vertical tubes (typically 1-2 metres long and 50-100 mm in diameter) arranged inside the vessel. As the product heats up, it naturally rises through the tubes due to density differences. The concentrated liquid falls back to the base through a central section. Natural circulation systems are commonly used for products like fruit juices and dairy concentrates, offering better efficiency than batch pans without needing external pumps.

Falling film evaporators 🔗

In a falling film evaporator, the liquid is distributed as a thin film that flows downward along the inner walls of heated vertical tubes. Because the film is thin, exposure to heat is very brief – sometimes just seconds. This makes falling film evaporators the top choice for heat-sensitive products like milk, fruit juices, and coffee extracts. They offer high heat transfer efficiency and are widely used in the food and dairy industries. However, they require careful design to ensure the film remains thin and even throughout the tube length.

Rising film evaporators 🔗

Here, the liquid enters the bottom of vertical tubes and is heated from the outside. As it heats up, vapour bubbles form and push the liquid upward as a thin film on the tube walls. Rising film evaporators work well for low-viscosity liquids and are often paired with falling film units – the rising film handles pre-concentration, while the falling film completes the process.

Forced circulation evaporators 🔗

When a product is very viscous, prone to crystallising, or tends to form deposits on heating surfaces, forced circulation evaporators are the answer. A pump pushes the liquid through the heat exchanger at high velocity, creating turbulence that prevents fouling. These systems are commonly used for tomato paste, sugar solutions, and salt brines.

Agitated thin-film evaporators 🔗

These evaporators use mechanical rotor blades to spread the product as a thin film on a heated cylindrical wall. The agitation improves heat transfer and prevents burning, making them suitable for highly viscous or particle-laden products such as fruit pastes, creamy vegetable concentrates, and products with viscosities up to 100 poise. The short residence times and high heat transfer rates protect heat-sensitive food components.

Single-effect vs. multiple-effect evaporators 🔗

A single-effect evaporator uses steam once and discards the resulting vapour. This is simple but energy-intensive. Multiple-effect evaporators (MEEs) address this by reusing the vapour produced in one stage as the heating medium for the next stage. Each subsequent stage operates at a lower pressure and temperature than the previous one.

For example, a triple-effect evaporator can achieve roughly three times the evaporation capacity per unit of steam compared to a single-effect unit. This makes MEEs the standard choice for large-scale food processing where energy efficiency is critical. Another energy-saving approach is mechanical vapour recompression (MVR), where the vapour is mechanically compressed to raise its temperature and then reused as the heat source.

Vacuum evaporation: protecting heat-sensitive foods 🔗

Many food products – fruit juices, dairy, honey, herbal extracts – contain vitamins, volatile aroma compounds, and pigments that degrade at high temperatures. Vacuum evaporation solves this problem elegantly. By reducing the pressure inside the evaporator, the boiling point of water drops significantly. At 0.2 bar, for instance, water boils at approximately 60°C instead of 100°C.

This lower temperature means less thermal damage to sensitive nutrients and flavours. Vacuum systems also remove dissolved air from the product, reducing oxidation and creating a denser, more stable concentrate. Products concentrated under vacuum typically have lower water activity, fewer microbubbles, and reduced oxygen content – all of which contribute to a longer shelf life.

Vacuum evaporation is widely used to produce fruit concentrates, condensed milk, whey protein concentrates, honey, and sauce bases.

Factors affecting evaporation efficiency 🔗

Several factors determine how efficiently an evaporator operates:

Heat transfer rate: This depends on the temperature difference between the steam and the product, the surface area available for heating, and the heat transfer coefficient of the material.

Boiling point elevation: As concentration increases, the boiling point rises. Products with high sugar or salt content experience a greater boiling point elevation, requiring more energy input.

Product viscosity: Thicker liquids resist flow and reduce heat transfer efficiency. Forced circulation or agitated film evaporators are better suited for viscous products.

Fouling: Deposits on heating surfaces act as insulation, reducing heat transfer. Regular cleaning and the right evaporator type minimise this issue.

Understanding air-vapour mixtures and psychrometry 🔗

While evaporation deals with concentrating liquids, many food processing operations – particularly drying, air conditioning, and storage – require a deep understanding of air-vapour mixtures. This is where psychrometry comes in: the study of the thermodynamic properties of moist air (a mixture of dry air and water vapour).

Atmospheric air is never completely dry. It always carries some amount of water vapour. The behaviour of this air-water vapour mixture directly affects how efficiently food can be dried, how storage environments should be controlled, and how cooling and heating systems should be designed in food plants.

Key properties of air-vapour mixtures 🔗

Dry-bulb temperature (DBT): This is the standard temperature measured by an ordinary thermometer. It indicates the sensible heat content of the air.

Wet-bulb temperature (WBT): Measured by a thermometer with its bulb wrapped in a wet cloth, WBT reflects the cooling effect of evaporation. It indicates how much moisture the air can still absorb. When WBT equals DBT, the air is fully saturated – it cannot hold any more water vapour. A device called a sling psychrometer measures both DBT and WBT by spinning two thermometers in the air.

Dew point temperature: This is the temperature at which water vapour in the air begins to condense. If the air cools below its dew point, moisture will form on surfaces – a critical consideration in cold storage and packaging facilities.

Humidity ratio (absolute humidity): The actual mass of water vapour present per unit mass of dry air, expressed in grams or kilograms of moisture per kilogram of dry air.

Relative humidity (RH): The ratio of the actual vapour pressure in the air to the maximum (saturated) vapour pressure at that temperature, expressed as a percentage. Air at 30°C and 40% RH holds more moisture than air at 20°C and 40% RH because warmer air has a higher moisture-holding capacity.

Enthalpy: The total heat content of moist air, including both sensible heat (from temperature) and latent heat (from water vapour).

The psychrometric chart 🔗

All these properties are linked together in a single graphical tool called the psychrometric chart. The chart plots dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis. Curved lines represent relative humidity levels, and diagonal lines represent wet-bulb temperatures. The upper curved boundary represents 100% relative humidity or saturated air.

By knowing just two properties – say, DBT and WBT from a sling psychrometer – you can locate a point on the chart and read off all the other properties: relative humidity, dew point, humidity ratio, enthalpy, and specific volume. This makes the psychrometric chart an essential design tool for food engineers.

How air-vapour mixtures apply to food processing 🔗

Food drying 🔗

Drying is the most direct application of psychrometry in the food industry. Hot air is passed over moist food (grains, fruits, vegetables, spices), and the moisture migrates from the food into the air stream. This is essentially an adiabatic saturation process – the air’s dry-bulb temperature drops as its moisture content increases, but the total enthalpy remains roughly constant. On the psychrometric chart, this process follows a constant wet-bulb temperature line.

Understanding this allows engineers to calculate exactly how much air and energy is needed to dry a given quantity of food to the target moisture level. For example, when designing a grain dryer, the initial air conditions (temperature and humidity) and the desired final grain moisture content determine the air flow rate and heating power required.

Air conditioning in food plants 🔗

Food processing and storage facilities often need precise climate control. Chocolate factories, for instance, require low humidity to prevent blooming, while cheese aging rooms need specific temperature and humidity combinations. Psychrometric principles guide the design of HVAC systems that maintain these conditions.

Cooling and dehumidification 🔗

When moist air passes over a cold surface (like an evaporator coil), its temperature drops. If the temperature falls below the dew point, water condenses out of the air, reducing its humidity ratio. This cooling and dehumidification process is critical for creating the dry air used in food dryers and controlled-atmosphere storage.

Mixing of air streams 🔗

In many food plants, air from different sources – say, hot dry air from a heater and cooler ambient air – needs to be blended. The resulting mixture’s properties can be predicted using mass and energy balance equations or simply by plotting the two air states on a psychrometric chart and finding the mixing point on the straight line connecting them.

The connection between evaporation and air-vapour mixtures 🔗

Evaporation and psychrometry are deeply interrelated in food engineering. When water evaporates from a food product during drying, it enters the surrounding air as water vapour, changing the air’s psychrometric properties. The rate of evaporation depends on the difference between the vapour pressure at the food’s surface and the vapour pressure in the surrounding air – which is, in turn, determined by the air’s temperature and humidity.

In evaporators, the vapour produced must be efficiently condensed and removed. Understanding air-vapour mixture behaviour helps engineers design condensers and manage the thermodynamic conditions inside and around evaporation equipment. In spray drying – which combines evaporation and drying – hot air simultaneously provides heat for evaporation and carries away the resulting moisture. The entire process can be mapped on a psychrometric chart.

Practical importance in the food industry 🔗

Together, evaporation and air-vapour mixture knowledge enable food processors to concentrate fruit juices without destroying vitamins, dry grains to safe moisture levels for long-term storage, produce milk powder and instant coffee with consistent quality, and design storage environments that prevent spoilage. Psychrometry is considered one of the most important subjects for food engineers working in drying, post-harvest handling, and grain storage applications. Without a solid grasp of these fundamentals, optimising any thermal food process would be nearly impossible.

What do you think? How might the growing demand for minimally processed, nutrient-rich foods push the development of evaporator technology toward even lower operating temperatures? And in your view, should psychrometric training be a mandatory skill for everyone working in food quality management – not just engineers?

References

1. https://www.sciencedirect.com/topics/food-science/evaporation-process
2. https://foodtechprocess.com/blog/post/73_vacuum-evaporation-in-food-processing
3. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/evaporator
4. https://www.andritz.com/separation-en/insights/evaporators-guide
5. https://vinanhatrang.com/comprehensive-guide-to-multiple-effect-evaporators-in-food-processing/
6. https://en.wikipedia.org/wiki/Psychrometrics
7. https://ebooks.inflibnet.ac.in/ftp02/chapter/introduction-to-psychrometry/
8. https://extension.psu.edu/psychrometric-chart-use
9. https://www.sciencedirect.com/topics/engineering/psychrometrics
10. https://www.sciencedirect.com/science/article/abs/pii/B9780128184738000098