Principles Behind Roller and Spray Drying Processes in Dairy

Converting liquid milk into powder is one of the most important preservation techniques in the dairy industry. It extends shelf life, reduces transportation costs, and makes milk available in regions far from production centres. The two commercially dominant methods for achieving this are roller drying (also called drum drying) and spray drying. Both methods rely on heat to evaporate water from pre-concentrated milk, but they differ significantly in how the milk contacts the heat source, the speed of drying, and the quality of the finished powder.

Why is milk dried in the first place? 🔗

Milk is highly perishable. Removing water from it inhibits the growth of microorganisms such as bacteria, yeasts, and moulds, thereby preserving the product for months or even years. Beyond preservation, dried milk powder is far lighter and more compact than liquid milk, which makes storage and shipping considerably cheaper. According to the Dairy Processing Handbook, drying is one of the oldest food preservation techniques known to humanity, and modern dairy powders can retain most of the nutritional quality, colour, and flavour of fresh milk when the process is carried out correctly.

Pre-concentration: the step before drying 🔗

Before milk enters either a roller dryer or a spray dryer, it is pre-concentrated using vacuum evaporators. This step removes a large portion of water at relatively low cost and energy compared to the drying stage itself. As noted by equipment manufacturer ANDRITZ, modern vacuum evaporator technology can remove over 55% of the water, bringing the dry solids content of the concentrate to roughly 40-45% before it is fed to the dryer. This makes the subsequent drying step faster and more energy-efficient.

Roller drying (drum drying): principle and process 🔗

Roller drying works on a straightforward principle: a thin film of pre-concentrated milk is spread onto the surface of a steam-heated, continuously rotating metal drum. The intense heat from the drum rapidly evaporates the water from the milk film. By the time the drum completes most of one revolution, the milk has dried into a thin sheet. A stationary scraper blade (also called a doctor blade) positioned against the drum peels off this dried film, which is then milled or pulverized into powder and packed.

Types of roller dryers 🔗

The most common construction used in the dairy industry is the atmospheric double drum dryer. In a double drum setup, two drums rotate in opposite directions with a small, precisely adjustable gap between them. The concentrated milk is fed into the reservoir (sump) formed between the two drums. As the drums rotate, the milk adheres to their surfaces as a thin film. Single drum dryers with applicator rolls or spray attachments also exist, but double drum dryers are more widely used for dairy applications.

Key factors affecting roller drying 🔗

Several process parameters determine how well roller drying works:

Drum surface temperature – The drums are heated internally by steam, and the surface temperature typically ranges from 120°C to 170°C. The temperature must be high enough to evaporate water quickly but not so high that the milk burns or undergoes excessive chemical changes.

Film thickness – The thickness of the milk layer on the drum is critical. A thinner film dries faster and more uniformly, but if it is too thin, there is a risk of scorching. If it is too thick, the milk may not dry completely in one drum revolution.

Drum speed – The rotation speed controls how long the milk film stays in contact with the hot surface. Faster rotation means shorter contact time, while slower rotation gives more thorough drying but increases the risk of heat damage.

Dry matter content of the feed – Higher solids concentration in the incoming milk means less water needs to be evaporated on the drum, improving efficiency.

Chemical changes during roller drying 🔗

Because the milk contacts a very hot metal surface directly, roller drying causes certain irreversible changes. The high temperature leads to caramelisation of lactose and Maillard browning reactions between lactose and the amino groups of milk proteins (especially lysine). These reactions give roller-dried milk powder a characteristic cooked or caramelised flavour and a slightly brownish colour. Additionally, protein denaturation occurs, which reduces the solubility of the powder when it is reconstituted with water.

Where roller-dried milk powder is used 🔗

Despite the flavour and solubility limitations, roller-dried powder has a well-established niche. The caramelised taste is actually desirable in products like milk chocolate, toffees, and certain bakery items. Chocolate manufacturers, for instance, specifically prefer drum-dried whole milk powder because the Maillard flavour compounds complement cocoa. Roller-dried powder is also used in some animal feed blends and in specific confectionery formulations where high solubility is not a priority.

Spray drying: principle and process 🔗

Spray drying is the dominant method in the modern dairy industry. Its basic principle involves converting concentrated milk into a fine mist of tiny droplets inside a large drying chamber, where the droplets meet a stream of hot air and lose their moisture almost instantly. The result is a fine, free-flowing powder. According to ScienceDirect, the very large surface area created by atomisation allows water to evaporate at relatively low particle temperatures, even though the surrounding air is hot, making the process well-suited for heat-sensitive dairy products.

Step 1: Atomisation 🔗

Atomisation is the heart of spray drying. It is the process of breaking the concentrated milk into millions of tiny droplets, typically ranging from 10 to 200 micrometres in diameter. This enormous increase in surface area is what allows rapid moisture removal. Two main types of atomisers are used in dairy spray dryers:

Rotary disc (centrifugal) atomisers – The concentrated milk is fed onto a high-speed spinning disc. Centrifugal force flings the milk outward in a fine spray perpendicular to the airflow. Rotary atomisers produce uniform droplets and handle high throughput well.

Pressure nozzle atomisers – The milk is forced through small orifices under high pressure (50-300 bar). Higher pressures produce finer particles with greater bulk density, while lower pressures yield coarser particles. According to the Dairy Processing Handbook, nozzle pressure plays a crucial role in determining the final particle size of the powder.

Step 2: Drying in the chamber 🔗

Once atomised, the milk droplets enter a large cylindrical drying chamber where they encounter hot air at inlet temperatures typically between 150°C and 200°C. Despite these high air temperatures, the milk droplets themselves never reach those extremes. As water rapidly evaporates from the droplet surface, the evaporative cooling effect keeps the particle temperature much lower than the surrounding air. The entire drying event for each droplet takes only a matter of seconds. This brief, gentle heat exposure is one of the main reasons spray-dried milk powder retains better nutritional value and solubility compared to roller-dried powder.

There are two main airflow patterns used in spray drying chambers. In co-current flow, the hot air and the atomised droplets move in the same direction – this is the most common setup for dairy and protects heat-sensitive components because the droplets encounter the hottest air when they contain the most moisture. In counter-current flow, air and droplets move in opposite directions, which gives more aggressive drying but is less suitable for heat-sensitive products.

Step 3: Powder separation and collection 🔗

After the droplets have dried into powder particles, the powder-laden air exits the drying chamber and enters separation equipment. The most common device for this purpose is the cyclone separator. Inside a cyclone, the air-powder mixture enters tangentially at high speed, creating a swirling vortex. The heavier powder particles are flung outward against the cyclone walls by centrifugal force, lose their momentum, and slide down to the bottom for collection. The cleaned air exits from the top. As noted by ScienceDirect, large dairy spray dryers often use batteries of multiple cyclones to ensure high recovery of powder and minimise product loss.

For very fine particles that escape the cyclones, additional equipment such as bag filters or wet scrubbers may be used downstream to capture remaining powder and meet emission standards.

Single-stage vs. multi-stage spray drying 🔗

In single-stage spray drying, all the moisture removal happens inside the drying chamber in one pass. However, most modern dairy plants use two-stage or three-stage systems. In a two-stage system, the powder exits the spray chamber with slightly higher moisture (around 5-10%) and then passes through a fluidised bed dryer where the remaining moisture is gently removed. Three-stage systems add an external vibrating fluid bed as a final drying and cooling step. Multi-stage drying is more energy-efficient and produces powder with better functional properties – including improved solubility and the ability to create instant milk powders that dissolve readily even in cold water.

Roller drying vs. spray drying: a quick comparison 🔗

Heat exposure – Roller drying subjects milk to direct contact with a hot metal surface (120-170°C), causing more protein denaturation and browning. Spray drying uses hot air (150-200°C), but the actual product temperature remains much lower due to evaporative cooling, resulting in gentler treatment.

Solubility – Spray-dried powder dissolves more readily in water, making it the preferred choice for reconstituted milk, infant formula, and beverages. Roller-dried powder has lower solubility due to protein denaturation.

Particle shape and bulk density – Spray drying produces spherical particles with smooth surfaces and relatively high bulk density. Roller drying yields irregularly shaped, flaky particles with lower bulk density.

Flavour – Roller-dried powder has a distinct cooked or caramelised flavour (an advantage in chocolate and confectionery). Spray-dried powder tastes closer to fresh milk.

Scale and economics – Spray drying requires expensive, large-scale equipment but offers far greater throughput. Modern spray drying plants can process over 100,000 kg of raw milk per day. Roller drying has lower capital costs and simpler operation, making it more suitable for smaller-scale or specialised production.

Applications – Spray drying dominates the production of skim milk powder, whole milk powder, infant formula, and dairy whiteners. Roller drying is used for chocolate milk powders, certain baby foods, and animal feed products.

Why spray drying dominates the modern dairy industry 🔗

Spray drying accounts for the vast majority of global milk powder production today. The reasons are clear: it delivers better product quality (higher solubility, lighter colour, milder flavour), is highly scalable, and allows precise control over powder properties through adjustments to atomisation parameters, air temperatures, and multi-stage drying configurations. As New Food Magazine notes, the capacity of tower spray dryers has more than doubled in recent decades, with the latest installations capable of processing several tonnes of water per hour.

However, roller drying has not become obsolete. Its simplicity, lower cost, and the unique flavour profile it imparts continue to make it valuable for specific product categories – particularly in the chocolate and confectionery sector where that caramelised dairy taste is a defining characteristic.

Key takeaways 🔗

Both roller drying and spray drying serve the same fundamental goal – removing water from milk to produce a stable, long-lasting powder. Roller drying achieves this through direct contact between a thin milk film and a steam-heated drum, producing a powder with distinctive flavour but lower solubility. Spray drying achieves it by atomising milk into fine droplets that dry almost instantly in hot air, yielding a high-quality powder with excellent solubility and nutritional retention. The choice between the two depends on the intended end use, production scale, and the specific properties desired in the final product.

What do you think? Given that spray drying dominates modern dairy processing, do you see a future where roller-dried milk powder becomes even more of a specialty product – or could advances in drum drying technology help it regain a larger share of the market?

References

1. https://dairyprocessinghandbook.tetrapak.com/chapter/milk-and-whey-powdered-ingredients
2. https://www.andritz.com/resource/blob/13596/684e09ac4454fee8c4d4c9ae9f090d7a/se-downloads-drum-dried-milk-powder-data.pdf
3. https://ebooks.inflibnet.ac.in/ftp04/chapter/technology-of-dried-milk-products-i-skimmed-and-whole-milk-powders-instant-milk-powders/
4. https://www.sciencedirect.com/topics/immunology-and-microbiology/spray-drying
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/cyclone-separator
6. https://www.newfoodmagazine.com/article/2090/spray-drying-of-dairy-products-a-review/