In homogenised milk, cream stays in a "permanent emulsion".Explain why the cream in raw milk separates out but the cream in homogenised milk doesn't.

Raw milk and homogenised milk behave differently due to the process of homogenisation.

1. Raw Milk: In raw milk, the fat molecules are larger and less evenly distributed. Due to their size and buoyancy, these fat molecules, which constitute the cream, tend to rise to the top of the milk over time. This is due to the principle of density; the fat molecules are less dense than the rest of the milk, so they float to the top, causing the cream to separate out.

2. Homogenisation Process: Homogenisation is a mechanical process that breaks down the fat molecules in the milk into smaller sizes and distributes them evenly throughout the milk. This is usually done by forcing the milk at high pressure through small holes.

3. Homogenised Milk: In homogenised milk, the fat molecules are much smaller and more evenly distributed throughout the milk. Because they are smaller, they are less buoyant and do not rise to the top of the milk. This creates a permanent emulsion, where the fat molecules remain suspended in the milk, preventing the cream from separating out.

4. Stability of Emulsion: The stability of this emulsion in homogenised milk is also due to the proteins present in the milk. When the fat globules are broken down during homogenisation, the proteins in the milk can more easily surround and stabilise each fat globule, preventing them from clumping together and separating out from the milk.

So, the main reason why cream in raw milk separates out but the cream in homogenised milk doesn't is due to the size and distribution of the fat molecules, which is altered during the process of homogenisation.