The Step-by-Step Manufacturing Process
The creation of nonfat dry milk (NFDM), also known as skim milk powder (SMP), is a highly controlled, multi-stage industrial process designed to remove the water content from milk while retaining its nutritional value. The process is energy-intensive but results in a product with a significantly longer shelf life and reduced weight, making it easier and cheaper to transport and store.
Step 1: Receiving and Separating
The process begins with fresh, raw milk arriving at the processing plant. It is rigorously tested for quality, including checks for antibiotics and bacterial count. Once approved, the raw milk is separated into skim milk and cream using a high-speed, centrifugal separator. This machine spins the milk at high velocity, causing the lighter cream to separate from the denser skim milk. The skim milk, which is virtually fat-free, continues on the production line, while the cream is diverted for other dairy products like butter.
Step 2: Pasteurization and Preheating
Next, the skim milk is pasteurized to eliminate any harmful bacteria. This involves heating the milk to a specific temperature for a set period, such as 72°C for 15 seconds, followed by rapid cooling. After pasteurization, the milk undergoes a preheating stage, where it is heated again to a higher temperature (e.g., 75-120°C) and held for a specified time. This crucial step serves several purposes:
- It destroys additional bacteria and inactivates enzymes.
- It can denature whey proteins, which is a key factor in determining the powder's final properties, such as its solubility.
Step 3: Evaporation
Evaporation is the next major phase, where a substantial amount of the remaining water is removed. The milk is concentrated in a multi-stage evaporator, which boils the milk under a vacuum at a low temperature, typically below 72°C. The low pressure significantly reduces the milk's boiling point, preventing the milk solids from scorching and preserving the milk's natural flavor and nutritional content. In this step, over 85% of the water can be removed, increasing the total solids content from around 9% to 45-52%. This is a highly efficient process, using significantly less energy than the final drying stage.
Step 4: Drying
The concentrated milk is then transported to the spray dryer, which is the final and most critical drying stage. The liquid is atomized—either through a high-pressure nozzle or a spinning disc—into a fine mist of droplets inside a large drying chamber. Simultaneously, hot air (up to 200°C) is introduced into the chamber, which instantly evaporates the water from the surface of the milk droplets. The powder particles, though exposed to high temperatures, remain cool due to the evaporative cooling effect. This results in a fine powder with a moisture content of around 6%.
Step 5: Secondary Drying and Agglomeration
To achieve the required final moisture content of 2-4%, the powder undergoes secondary drying, often in a fluidized bed. In this process, hot air is blown through a layer of the powder, causing it to behave like a fluid and removing the last traces of moisture. For instantized powders, this stage also includes agglomeration, where powder particles are wetted with a fine mist and caused to stick together, forming larger, more porous granules. This structure allows water to penetrate more easily, making the powder dissolve instantly.
Step 6: Packaging and Storage
Finally, the nonfat dry milk is packaged in airtight containers or multi-wall bags, often with an inert gas like nitrogen for added protection. This packaging protects the powder from moisture, oxygen, and light, all of which can degrade quality over time. Correct storage is essential to maintain the powder's long shelf life. Some manufacturers may also fortify the milk powder with vitamins A and D.
Comparison of Drying Methods
While spray drying is the industry standard for producing nonfat dry milk due to its ability to preserve quality, other methods exist. Here is a comparison of the most common ones:
| Feature | Spray Drying | Drum Drying | Freeze Drying |
|---|---|---|---|
| Equipment | Large, complex spray dryer towers and fluid beds | Heated rotating metal drums | Freezing chambers and vacuum chamber |
| Heat Exposure | Rapid, low thermal exposure to milk solids | High, direct heat contact on metal surfaces | Low temperature, sublimation process |
| Product Quality | Excellent solubility, flavor, and color | Poorer solubility, cooked/caramelized flavor | High retention of nutrients and flavor |
| Energy Cost | High energy cost, but very efficient | Moderate energy cost | Very high energy cost due to freezing |
| Powder Structure | Fine, spherical particles; can be agglomerated | Flaky, irregular shape | Light, porous structure |
| Industrial Use | Most common for NFDM and SMP | Less common for milk powders due to heat damage | Specialty products, not common for milk powder |
Conclusion
The industrial process for creating nonfat dry milk is a sophisticated combination of physical separation, gentle heat treatments, and highly efficient dehydration techniques. By starting with fresh, high-quality skim milk and carefully controlling the evaporation and spray-drying phases, manufacturers can remove nearly all the water content while preserving the milk's nutritional profile. The result is a long-lasting, versatile, and nutrient-dense powder that can be easily reconstituted or used in a wide variety of food applications, from baking to infant formula. Modern methods, especially spray drying, ensure excellent solubility and flavor, making nonfat dry milk a staple ingredient in food production worldwide. You can learn more about dairy processing and food preservation by visiting the Dairy Processing Handbook.
