The preparation of sugar of milk, also known as lactose, involves isolating and purifying lactose from milk. The process typically involves these steps:

1. Collection of Milk: Start by collecting fresh milk from a reliable source. The milk can be sourced from cows, goats, or other mammals.

2. Separation of Fat: If the milk contains high fat content, it’s often necessary to separate the fat. This can be done through processes like centrifugation or skimming.

3. Heating and Acidification: The milk is then heated to around 40-50°C (104-122°F) and acidified by adding a food-grade acid like lemon juice or citric acid. This causes the proteins in the milk to coagulate and separate from the liquid.

4. Straining: The coagulated proteins are separated from the liquid portion, which is whey. Straining through a fine cloth or filter helps to remove the coagulated proteins.

5. Evaporation: The whey is then heated to evaporate off the water content. This results in the concentration of lactose and other solids.

6. Crystallization and Drying: As the whey concentrate cools down, lactose crystals start to form. The crystals are separated from the remaining liquid and then dried to remove any remaining moisture.

7. Grinding and Sieving: The dried lactose crystals are ground into a fine powder and then sieved to achieve a consistent particle size.

8. Packaging: The sugar of milk, or lactose powder, is packaged in airtight containers to prevent moisture absorption and maintain its quality.

Lactose obtained through this process is commonly used as a filler in various pharmaceutical and food products due to its low sweetness and minimal impact on flavor.