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 buoyancy and size, these fat molecules, which constitute the cream, rise to the top and separate from the rest of the milk over time. This is due to the principle of density; the fat molecules are less dense than the water content of the milk, causing them to float to the top.

2. Homogenisation Process: Homogenisation is a mechanical process that breaks down the fat molecules in milk to a smaller size so that they remain dispersed evenly throughout the milk. This is done by forcing the milk under high pressure through small holes.

3. Homogenised Milk: In homogenised milk, the fat molecules are smaller and more evenly distributed throughout the milk. This prevents the cream from separating and rising to the top. The smaller fat globules are not buoyant enough to overcome the viscosity of the water content of the milk and rise to the surface. This results in a "permanent emulsion" where the cream remains mixed in with the rest of 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.