Evaporators and Dryers: Reducing Moisture for Dairy Products

Turning liquid milk into shelf-stable products like condensed milk or milk powder requires one essential step – removing water. That’s where evaporators and dryers come in. These two categories of equipment work in sequence: evaporators first concentrate milk by partially removing water, and dryers then take over to reduce moisture even further, creating powdered dairy products. Together, they form the backbone of dairy processing lines worldwide.

Why removing water from milk matters 🔗

Fresh milk contains roughly 87% water. This high moisture content makes it highly perishable – bacteria and other microorganisms thrive in such an environment. By concentrating or drying milk, processors dramatically reduce water activity, which inhibits microbial growth and extends shelf life from days to months or even years. Concentrated and dried dairy products are also far cheaper to store and transport because of their reduced volume and weight.

Beyond preservation, water removal is necessary for manufacturing specific dairy products – evaporated milk, sweetened condensed milk, milk powder, whey powder, and infant formula all depend on controlled moisture reduction at various stages of production.

How evaporators work in dairy processing 🔗

An evaporator’s job is to boil off water from milk while keeping the valuable solids – proteins, fats, lactose, and minerals – intact. The critical challenge is that milk components are heat-sensitive. Proteins start to denature at high temperatures, and lactose can caramelise, causing off-flavours and browning. To solve this problem, dairy evaporators operate under vacuum conditions.

Under vacuum, the boiling point of water drops significantly. Instead of needing 100°C at normal atmospheric pressure, milk can boil at temperatures between 50°C and 60°C inside a vacuum evaporator. This gentle heating preserves nutrients and prevents undesirable changes in flavour and colour. However, the temperature should not drop below 45°C, as that could allow harmful microorganisms like Staphylococci to survive.

Through evaporation, milk is typically concentrated to around 40-50% total solids depending on the product. For whey, concentration can go even higher – up to 58-65% solids.

Batch-type vacuum pans 🔗

The simplest type of dairy evaporator is the batch vacuum pan. It consists of a large, jacketed vessel where milk is heated while under vacuum. Steam circulates through the jacket, providing uniform heating. In the dairy industry, a single-effect evaporator is often referred to as a vacuum pan.

Batch vacuum pans work well for small to medium-scale operations. Operators can easily monitor concentration levels and make real-time adjustments. The total solids content is typically tracked using a Baumé hydrometer, with density checks performed continuously during processing. Their simplicity and reliability make them suitable for manufacturers producing condensed milk or smaller volumes of concentrated dairy products.

However, batch processing has limitations. It is less energy-efficient than continuous systems, and throughput is naturally restricted by the vessel size and cycle time.

Continuous falling-film evaporators 🔗

For large-scale dairy operations, falling-film tubular evaporators are the industry standard. In these systems, milk is distributed as a thin film on the inner surface of vertical heated tubes. As gravity pulls the film downward, water evaporates rapidly from this thin layer, and the concentrate is collected at the bottom.

The key advantage of falling-film evaporators is their very short residence time – often less than one minute. This means the milk spends minimal time in contact with heat, resulting in better product quality with minimal protein damage. The thin-film design also enables excellent heat transfer efficiency.

These evaporators are typically configured as multiple-effect systems – with two, three, four, or even six effects operating at progressively lower pressures. In a multi-effect setup, the vapour produced in the first effect serves as the heating medium for the next effect. This dramatically reduces steam consumption. For example, a six-effect falling-film evaporator uses only about 0.08 kg of steam per kilogram of water evaporated, compared to 0.25 kg in a single-effect system.

Modern plants also use mechanical vapour recompression (MVR) technology, where an electric compressor raises the temperature of the vapour from the evaporator by a few degrees, allowing it to be reused as a heating medium. This further cuts energy costs significantly.

Rising-film and forced-circulation evaporators 🔗

Besides falling-film types, two other evaporator designs deserve mention. In rising-film evaporators, the liquid enters at the bottom of vertical heated tubes. As it heats up, vapour bubbles form and push the liquid upward as a thin film. This design provides good heat transfer for products with moderate viscosity.

For highly viscous or fouling-prone products, forced-circulation evaporators use pumps to push the liquid through heat exchangers at high velocities. This prevents settling, burning, and scale formation on heating surfaces. According to dairy equipment specialists, hybrid evaporators that combine film-type and forced-circulation features are among the most energy-efficient options available today.

From concentrate to powder: the role of dryers 🔗

Evaporators can only take concentration so far – typically to 40-50% solids for milk. To produce milk powder with a final moisture content of just 2-5%, drying equipment is needed. The two main types of dryers used in the dairy industry are drum (roller) dryers and spray dryers.

Drum dryers (roller dryers) 🔗

Drum drying is one of the oldest methods for converting liquid milk into powder. In this process, pre-concentrated milk is spread as a thin film onto the surface of steam-heated rotating drums. As the drum rotates, the water evaporates rapidly from the film. After less than one full revolution, the dried film is scraped off by a metal blade (called a doctor blade), producing flakes that are then milled into powder.

The drum surface temperature typically ranges from 150°C to 180°C, which is considerably higher than the temperatures used in evaporators. Drums are usually 61 to 122 cm in diameter and can be up to 3.6 metres long. The rotation speed, usually between 6 and 24 RPM, determines how long the milk stays on the heated surface.

Types of drum dryers 🔗

Drum dryers come in several configurations. The double-drum atmospheric dryer is the most common type used in the dairy industry – two drums rotate toward each other with milk fed into the gap between them. Single-drum dryers are also available, often used with vacuum enclosures. In a vacuum drum dryer, the drum is enclosed in a housing where reduced pressure lowers the boiling point, allowing drying at lower temperatures and producing better quality powder.

Advantages and limitations of drum drying 🔗

Drum dryers are energy-efficient for viscous and pulpy dairy products and are simpler and cheaper to operate than spray dryers. They require less floor space and can handle high-fat products effectively.

However, drum drying has significant drawbacks for milk powder production. The high contact temperatures cause protein denaturation and lactose caramelisation, which results in a darker colour, a cooked or scorched flavour, and reduced solubility. The powder particles are also larger and more irregular compared to spray-dried powder, making reconstitution harder. For these reasons, drum drying accounts for only about 4% of global milk powder production. It is now mainly used for speciality applications like confectionery ingredients and animal feed.

Spray dryers 🔗

Spray drying is the dominant technology for producing milk powder worldwide. The basic principle involves atomising concentrated milk into extremely fine droplets and exposing them to a stream of hot air inside a large drying chamber. The moisture evaporates almost instantly from each droplet, leaving behind fine powder particles.

How a spray dryer works 🔗

The process begins with pre-concentrated milk (typically 40-50% solids from the evaporator stage) being pumped to an atomiser. There are two main types of atomisers – rotary (wheel) atomisers, which spin at high speed to break the liquid into droplets, and high-pressure nozzle atomisers, which force the liquid through a small opening. The resulting droplets are typically 10 to 200 micrometres in diameter.

These tiny droplets enter a large drying chamber where they come into contact with hot air at inlet temperatures of 150-200°C. Despite these high air temperatures, the milk itself does not overheat because the rapid evaporation of water has a strong cooling effect – the milk solids typically stay around 70-80°C. The entire drying process inside the chamber takes only about 20 to 60 seconds.

The dried powder is then separated from the air using cyclone separators or bag filters, which capture the fine particles while allowing the air to exit the system.

Multi-stage spray drying 🔗

Modern dairy plants often use two-stage or three-stage drying systems. In these setups, the initial spray drying is followed by one or two additional stages using fluidised bed dryers – either integrated inside the chamber or external vibrating beds. These additional stages gently finish the drying at lower temperatures and can also agglomerate the powder particles, improving their instant dissolving properties. Three-stage systems now dominate the dairy powder industry.

Why spray drying is preferred 🔗

Spray drying offers several advantages over drum drying for milk powder production. The rapid, low-temperature drying preserves proteins, vitamins, and minerals far better than the high-heat drum process. The resulting powder has excellent solubility, fine and uniform particle size, and good reconstitution properties – meaning it dissolves easily when mixed with water.

Spray-dried milk powder also has a longer shelf life, typically 12 to 24 months under proper storage, thanks to its very low moisture content (usually 2-4%). The technology is highly scalable – modern tower spray dryers in countries like New Zealand and Australia can remove 10 to 15 tonnes of water per hour.

The versatility of spray drying extends beyond regular milk powder. It is used to produce skim milk powder, whole milk powder, whey powder, infant formula, and various specialised dairy ingredients with specific functional properties.

Drum drying vs. spray drying: a quick comparison 🔗

The choice between drum and spray drying depends on the product requirements, budget, and production scale. Spray drying wins on product quality – it produces powder with superior solubility, colour, flavour, and nutritional retention. It is the clear choice for products like infant formula and high-quality skim milk powder intended for direct consumption or reconstitution.

Drum drying, while less common for milk powder today, remains useful where the caramelised flavour is actually desirable (such as in chocolate manufacturing), where budgets are tighter, or where the product is naturally viscous. It also uses less energy than spray drying in certain applications, particularly for thick, paste-like feeds.

The evaporation-drying connection 🔗

It is worth emphasising that evaporation and drying are not competing processes – they are complementary stages in milk powder production. Removing water by evaporation before drying is far more energy-efficient than trying to dry liquid milk directly. Evaporating water costs roughly one-tenth the energy of removing the same amount of water through spray drying. That is why milk is always pre-concentrated in an evaporator (to 40-50% solids) before it enters the dryer. This approach reduces the dryer’s workload, lowers energy costs, and improves the overall quality of the final powder.

Key takeaways 🔗

Evaporators and dryers work together as a two-step moisture removal system in dairy processing. Vacuum evaporators – whether batch-type vacuum pans for smaller operations or multi-effect falling-film systems for industrial plants – gently concentrate milk while preserving its nutritional value. Dryers then take the concentrated milk to its final form. Spray dryers dominate the industry for their efficiency, scalability, and ability to produce high-quality, easily reconstituted powders. Drum dryers, though largely replaced by spray dryers for standard milk powder, continue to serve niche applications where their unique characteristics are valued.

What do you think? As dairy processing becomes more energy-intensive globally, how can evaporator and dryer technologies evolve to reduce their environmental footprint while maintaining powder quality? And for smaller dairy operations in developing regions, could simpler drum drying still offer a viable entry point into powder production?

References

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4. https://www.basu.org.in/wp-content/uploads/2020/03/Different-Types-of-Evaporators-Used-In-Dairy-Industry.pdf
5. https://foodtechprocess.com/blog/post/46_vacuum-evaporation-and-concentration-of-liquid-products
6. https://www.researchgate.net/publication/313797160_Drum_Drying
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