Evaporation in Food Processing: Methods and Equipment

Every time you open a can of tomato puree, stir condensed milk into a recipe, or sip reconstituted orange juice, you’re enjoying a product made possible by evaporation. In the food processing industry, evaporation is the go-to method for removing water from liquid foods to create concentrated products. It extends shelf life, cuts transportation costs, and intensifies flavour – all in one operation. But there’s real engineering behind it. The type of evaporator, the way heat moves through the system, and the physical behaviour of the liquid being concentrated all determine whether the final product retains its quality or loses it. Let’s break down how evaporation works in food processing, what equipment is used, and what factors make or break efficiency.

What is evaporation in food processing? 🔗

Evaporation is a thermal separation process in which water is partially removed from a liquid food by heating it to its boiling point. The water turns into vapour and is carried away, leaving behind a more concentrated product. Unlike drying, the end result of evaporation is still a liquid – just a thicker, more concentrated one.

This operation is used across a wide range of food products: sugar syrups, fruit juices, milk, salt solutions, and vegetable pulps. In every case, the concentrated liquid is the desired product, and the removed water is discarded. Sometimes, evaporation also serves as a pre-treatment step before spray drying, since removing bulk water through evaporation is far more energy-efficient than doing it entirely through a dryer.

Why evaporation matters in the food industry 🔗

There are several practical reasons food manufacturers rely on evaporation:

Preservation: Reducing the water content lowers the water activity in food, which inhibits microbial growth and extends shelf life. Products like condensed milk and tomato paste owe their long storage life largely to this principle.

Cost reduction: Concentrated products weigh less and take up less volume. This directly translates to lower packaging, storage, and transportation costs – a significant consideration in global food supply chains.

Quality enhancement: Concentration intensifies flavour and colour. A well-made fruit concentrate, for instance, retains the taste profile of the original juice in a smaller volume, ready to be reconstituted later.

Intermediate processing: In many production lines, evaporation is an intermediate step. Concentrated juice is later reconstituted, or concentrated milk goes into a spray dryer to produce milk powder. As noted by the American Institute of Chemical Engineers (AIChE), combining evaporation with spray drying is a standard approach for making powdered food products.

Types of evaporators used in food processing 🔗

Not every food product can be concentrated in the same equipment. The choice of evaporator depends on the product’s viscosity, heat sensitivity, required throughput, and tendency to foul heat transfer surfaces. Here are the main types used in the food industry.

Batch pan evaporator 🔗

This is the simplest and oldest form of evaporator. It consists of an open or closed pan or vessel in which the food product sits while being heated – typically through steam jackets or coils from below. Water evaporates from the surface, and the vapour is either released into the atmosphere or condensed and removed.

Batch pan evaporators are flexible and easy to operate. They work well for small-scale operations where different products need to be processed in the same equipment, such as specialty sauces, fruit preserves, and small-batch dairy concentrates. However, their heat economy is poor, and running costs can be high compared to continuous systems.

Natural circulation evaporator 🔗

Natural circulation evaporators rely on the density difference between heated and cooler liquid to create circulation inside the equipment. In a typical vertical short-tube design, steam heats the outside of vertical tubes while liquid food passes through them. As the liquid heats up and boils, it becomes lighter and rises. Cooler, denser liquid flows downward through a central larger pipe called a “downcomer,” creating a continuous natural loop.

This type works well for low-viscosity solutions like sugar and salt where the natural buoyancy of heated liquid is enough to maintain good circulation. The tubes are usually 0.5 to 2.0 metres long with diameters of 25 to 75 mm. One limitation is that viscous products cannot climb easily through the tubes, which makes this design unsuitable for thick pastes or highly concentrated liquids.

Rising film (climbing film) evaporator 🔗

Rising film evaporators are a type of vertical long-tube evaporator. The liquid enters from the bottom and is fed into long, narrow tubes – typically 3 to 10 metres in length. As the liquid heats up, vapour bubbles form and push the liquid upward along the tube walls as a thin film. This rapid upward movement creates good agitation and high heat transfer rates.

The contact time between the liquid and the hot surface is very short, which minimises thermal damage. This makes rising film evaporators a good fit for low-viscosity, heat-sensitive products like fruit juices. However, they require the feed to be preheated close to its boiling point before entering, and they don’t handle viscous liquids well.

Falling film evaporator 🔗

The falling film evaporator is the most widely used evaporator type in the modern food industry. The liquid is fed to the top of a bundle of vertical tubes and distributed evenly using a specially designed distributor. It then flows downward as a thin film under gravity along the inner walls of the tubes, while steam heats the outer surface.

According to engineering references, the residence time of the liquid inside the tubes is measured in seconds, making falling film evaporators ideal for heat-sensitive products such as milk, fruit juice, and pharmaceutical solutions. The temperature difference between the heating medium and the product can be as low as 3-6 K, which further protects delicate compounds.

Key advantages include high heat transfer efficiency, very low pressure drop (making them suitable for vacuum operations), and energy savings when arranged in multiple-effect configurations. The main limitation is sensitivity to fouling – if the liquid contains precipitating solids, they can deposit on the tube walls and reduce performance. For this reason, falling film evaporators are best suited for clean, non-crystallising liquids.

Forced circulation evaporator 🔗

When the product is too viscous or prone to fouling for natural or film-type evaporators, a forced circulation system is used. Here, a pump pushes the liquid through the heat exchanger at high velocity, preventing deposits from building up on the tube surfaces. The liquid is heated under pressure inside the tubes so that boiling occurs only when it exits into the lower-pressure evaporation chamber.

Forced circulation evaporators are commonly used for concentrating tomato paste, thick sugar syrups, and other high-viscosity products. They are more expensive to operate because of the energy needed to run the circulation pump, but they deliver reliable performance where other evaporator types would fail.

Factors affecting evaporation efficiency 🔗

The efficiency of an evaporation system isn’t just about the equipment – several physical and operational factors play a critical role.

Boiling point elevation 🔗

As water is removed from a solution, the concentration of dissolved solutes (sugars, salts, proteins) increases. This raises the boiling point of the solution above that of pure water at the same pressure – a phenomenon known as boiling point elevation (BPE).

BPE is significant because the driving force for heat transfer in an evaporator is the temperature difference between the heating steam and the boiling liquid. When BPE rises, this temperature difference shrinks, and the rate of heat transfer decreases. For example, a 10% salt solution has a boiling point roughly 4°C higher than pure water. In sugar syrups and brine, BPE can be substantial enough to significantly impact evaporator performance.

This is one reason many food evaporators operate under vacuum. Reducing pressure lowers the boiling point of the liquid, which helps maintain a large temperature difference and also reduces thermal damage to the product.

Heat transfer rate 🔗

The rate at which heat moves from the steam to the liquid food determines how fast evaporation proceeds. It depends on three main factors: the temperature difference between the heating medium and the liquid, the area of the heat transfer surface, and the overall heat transfer coefficient.

The heat transfer coefficient itself is influenced by the liquid’s viscosity, the velocity at which it moves across the heated surface, and whether there are any fouling deposits on the tube walls. As concentration increases, the liquid becomes more viscous, which reduces the heat transfer coefficient and slows evaporation. Maintaining clean heat transfer surfaces through regular cleaning is essential for consistent performance.

Fouling and scaling 🔗

As liquid food concentrates during evaporation, some dissolved solids may exceed their solubility limits and precipitate on the heat transfer surfaces. These deposits – called scale or fouling – act as insulation, reducing the overall heat transfer coefficient and sometimes blocking tubes entirely.

Fouling is a particular problem in dairy evaporation (where protein deposits form) and sugar processing (where crystallisation can occur on hot surfaces). Proper equipment design, operating temperature control, and regular cleaning-in-place (CIP) procedures are essential for managing this issue.

Multiple-effect evaporation: improving energy efficiency 🔗

Evaporation is inherently energy-intensive because it requires large amounts of latent heat to convert water into vapour. A single-effect evaporator uses steam once and discards the resulting vapour. This is inefficient – roughly 1 kg of steam is needed to evaporate 1 kg of water.

Multiple-effect evaporators solve this problem by connecting two or more evaporator stages in series, each operating at a progressively lower pressure and temperature. The vapour generated in the first effect becomes the heat source for the second effect, and so on. This means the original steam input does much more work.

In a triple-effect evaporator – the most common configuration in the food industry – approximately 3 kg of water can be evaporated for every 1 kg of steam supplied. The trade-off is that adding more effects requires more capital investment in equipment, so there’s an economic optimum that balances energy savings against equipment cost.

Feed arrangements in multiple-effect systems 🔗

How the liquid feed moves through a multiple-effect system matters. In forward feed, the liquid enters the first (highest-pressure) effect and moves through subsequent effects in the same direction as the vapour. This is simple and works well for heat-sensitive products since the most concentrated – and therefore most heat-vulnerable – liquid ends up in the lowest-temperature effect.

In backward feed, the liquid enters the last (lowest-pressure) effect and is pumped toward the first. This is useful for viscous products, since the most concentrated liquid ends up at the highest temperature, where its viscosity is lower and heat transfer is better. However, backward feed requires pumps between each effect to move liquid against the pressure gradient.

Mixed feed and parallel feed configurations also exist, chosen based on specific product requirements and process constraints.

Vapour recompression: another route to energy savings 🔗

Beyond multiple-effect arrangements, vapour recompression offers an alternative way to reuse energy. In thermal vapour recompression (TVR), a steam jet ejector uses high-pressure steam to compress and re-energise the vapour leaving the evaporator, so it can be recycled as the heating medium. In mechanical vapour recompression (MVR), a compressor does this job electrically.

MVR systems can be extremely efficient – sometimes equivalent to 10 or more effects in terms of steam economy – but require reliable electrical power and are best suited for large-scale, continuous operations. TVR systems are simpler and have no moving parts, making them a practical choice when modest energy recovery is sufficient.

Common food products made using evaporation 🔗

Evaporation is behind many everyday products. Tomato puree and paste are made by concentrating tomato juice, typically using forced circulation evaporators to handle the viscous product. Fruit juice concentrates – orange, apple, grape – are produced using falling film evaporators that preserve natural flavour and colour. Condensed and evaporated milk are made by removing about 60% of the water from fresh milk using multiple-effect evaporator systems. Sugar syrups are concentrated from dilute cane or beet extracts in large-scale evaporation plants. And in many of these cases, evaporation is paired with downstream processes like spray drying, crystallisation, or packaging to produce the final consumer product.

Challenges in food evaporation 🔗

Despite its widespread use, evaporation in food processing comes with real challenges. Energy consumption remains a major cost factor, though multiple-effect systems and vapour recompression have improved this significantly. Thermal degradation of vitamins, volatile aroma compounds, and colour pigments is a constant concern, especially in products like fruit juices where freshness and flavour are valued. Equipment like falling film and vacuum evaporators help minimise this by operating at lower temperatures and shorter contact times. Fouling adds maintenance costs and downtime. And as products become more concentrated, their increased viscosity creates pumping challenges and reduces heat transfer efficiency.

Modern evaporator design increasingly incorporates automation, advanced heat exchanger materials, and optimised cleaning systems to address these issues. The trend is toward greater energy efficiency, better product quality, and more sustainable operations.

What do you think? Given the trade-offs between energy efficiency and product quality, how should food processors decide which evaporation system is right for a specific product? And as energy costs continue to rise globally, could newer technologies like membrane concentration eventually replace thermal evaporation for certain food applications?

References

1. https://ebooks.inflibnet.ac.in/ftp02/chapter/evaporation/
2. https://www.aiche.org/resources/publications/cep/2018/may/essentials-continuous-evaporation
3. http://wiki.zero-emissions.at/index.php/Evaporation_in_food_industry
4. https://vinanhatrang.com/comprehensive-guide-to-multiple-effect-evaporators-in-food-processing/
5. https://en.wikipedia.org/wiki/Falling_film_evaporator
6. https://www.coolplanet.io/blog/articles/multi-effect-evaporators
7. https://klarenbv.com/multi-effect-evaporators/
8. https://www.enerquip.com/how-evaporation-equipment-powers-food-pharma-energy-more/