From Raw Milk to Skim Milk: Separation and Treatment
Before the dehydration process can begin, liquid milk must be prepared. This initial phase is crucial for ensuring the final product's quality, safety, and functional properties.
Milk Reception and Separation
Upon arrival at a dairy factory, raw milk undergoes a series of quality checks for temperature, hygiene, and composition. Once accepted, it is immediately cooled and stored in large silos. To make skim milk powder, the first critical step is to separate the raw milk into skim milk and cream. This is achieved using a centrifugal cream separator, which spins the milk at high velocity. The centrifugal force causes the heavier, fat-poor skim milk to separate from the lighter cream, which is then siphoned off. The fat content of the remaining skim milk is reduced to less than 0.1%.
Pasteurization and Preheating
Once separated, the skim milk is pasteurized to eliminate harmful microorganisms and increase its shelf life. The standard method involves heating the milk to a specific temperature, typically 72-75°C, and holding it for 15-20 seconds. Following this, a more intensive preheating step is often performed. Temperatures for preheating can range from 85-95°C for several minutes, a process that denatures the whey proteins and helps to improve the powder's solubility and stability.
Concentration: Removing Water Efficiently
Dairy plants employ highly energy-efficient methods to remove the majority of the water content from the skim milk before the final drying stage.
Evaporation
The pasteurized and preheated skim milk, which is about 91% water, is sent to an evaporator. Evaporation involves heating the milk under a vacuum at a low temperature, typically below 72°C. Modern evaporators are often multi-stage or "multi-effect," using the vapor from one stage to heat the next, which maximizes energy efficiency. This process removes about 85% of the milk's water content, leaving a concentrated liquid with a total solids content of 45-52%.
Drying and Finishing: Creating the Powder
The final and most critical stages of the process transform the concentrated liquid into the fine, stable powder form.
Spray Drying
Spray drying is the most common method for converting the concentrated milk into powder. In this process, the concentrated milk is atomized into a fine mist of tiny droplets inside a large, hot drying chamber. Hot air, with temperatures ranging from 150-220°C, is circulated in the chamber. The high surface area of the droplets allows for almost instantaneous evaporation of the remaining moisture. The milk solids form a fine powder with a moisture content of around 6%. The hot air is then exhausted from the chamber, and the fine powder particles settle at the bottom.
Post-Drying Treatments
- Secondary Drying: Following the initial spray drying, the powder often undergoes a secondary drying step in a fluid bed to reduce the moisture content further, typically to 2-4%. This helps prevent caking and ensures product stability.
- Agglomeration (Instant Powder): To create instant skim milk powder that dissolves more readily, the powder is agglomerated. This involves rewetting the fine powder particles with steam to make them sticky, causing them to clump together into larger, porous granules. The agglomerated particles are then re-dried, resulting in a less dusty, free-flowing powder with improved dispersibility.
- Sieving and Packaging: The powder is sieved to ensure a uniform particle size and remove any lumps before packaging. It is then packed into moisture-proof and airtight bags or containers, often with a nitrogen flush to reduce oxygen exposure and preserve quality.
Skim Milk Powder vs. Whole Milk Powder Processing
While both skim milk and whole milk powders are produced using a similar fundamental process of dehydration, a few key distinctions set them apart, largely due to their differing fat content.
| Feature | Skim Milk Powder (SMP) | Whole Milk Powder (WMP) |
|---|---|---|
| Starting Point | Raw milk separated to remove nearly all fat. | Whole milk with its natural fat content retained. |
| Fat Content | Very low (max 1.5%). | High (typically 26-30%). |
| Homogenization | Not required due to low fat content. | Often applied to break down fat globules and prevent fat separation. |
| Preheat Treatment | High-heat treatment can be used to achieve specific protein denaturation for functional properties. | A specific preheat regime (e.g., 90-95°C for 30-60s) is used to create antioxidants and protect the fat from oxidation during storage. |
| Shelf Life | Long shelf life due to minimal fat content. | Shorter shelf life due to fat content being prone to oxidation. |
| Instantizing | Agglomeration with water is sufficient. | Requires the addition of an emulsifier, like lecithin, to help the surface fat particles mix with water. |
Conclusion
The industrial process of making skim milk powder is a highly controlled and multi-stage operation. Starting with the mechanical separation of fat from raw milk, the process moves through pasteurization and efficient vacuum evaporation. The final conversion into a dry powder is primarily accomplished through spray drying, with options for secondary drying and agglomeration to enhance properties like solubility and flowability. This comprehensive manufacturing journey results in a shelf-stable and versatile dairy product used widely across the food industry for everything from baked goods to recombined dairy items.
