Anne Wilkinson, of Oamaru, asks :-

Why is it that some milk freezes very well, green top is fine but blue top separates out and is not appealing?

Chris Erikson, a industrial food microbiologist, responded.

Without any intervention the fat in milk normally separates from the water and collects at the top. In the old days farmers left milk from the family cow in a shallow vessel, then skimmed the thick cream off.

Homogenization is the process of breaking up the fat globules to such a small size that they remain suspended evenly in the milk rather than separating out and floating to the surface. Homogenization allows the production of non-separating milk at any fat specification.

Most pasteurized milk available has been homogenised and standardised to specific fat contents governed by regulation under sections of the Food Standards Code Australia New Zealand (FSANZ).

You will have noticed that when thawing, frozen milk looks quite yellow. This is because of the fat separating out. Once thawed it returns to its normal colour. The freezing process separates out the fat (almost a reverse of the homogenisation process) and can affect the mouthfeel of the milk. There is nothing lost in the nutrient content so if the mouthfeel is not liked then use thawed milk for cooking, baking, and hot drinks.

"Green top" skim milks which, depending on brand, can have anywhere between 0.1 to 0.5 % fat have a minimal fat content compared to "blue top" standard milks which are usually standardised to 3.2% minimum fat. Hence the affect is far more pronounced in blue top milks.

Each summer I go to Totaranui every year with a large group so we freeze both sorts of milk We thaw initially on the bench, transferring part way through the process to the fridge. I've noticed lots of people in passing give the containers a shake just to help speed up the thawing process.

Shaking the milk slightly during thawing and before use helps to blend the fat back into the milk. The safest method is to thaw milk in the fridge and this can take up to 2 -3 days. It can be safely speeded up slightly by being thawed in cold water in the sink.