A good explanation of the difference between ordinary pasteurised milk and UHT milk from the European Food Information Council. Interesting reading as a lot of organic milk is UHT-treated in order to ensure it remains safe for customers during transport etc. –

The pasteurisation process heats milk to 70 – 75 °C for 15 seconds, inactivating or killing all of the disease-producing microorganisms in milk while preserving most of the nutritional value of milk. The UHT process ensures total microbial and enzyme inactivation, because the milk is heated up to 150 °C for 5 seconds.

Cow’s milk contains a number of vitamins, particularly group B vitamins (e.g.: riboflavin, vit B12, vit B1) and vitamin A. Although pasteurisation or UHT may result in small loses in some of these vitamins (below 20% on average), this will have a negligible effect on the nutritional value of a balanced diet. Vitamin losses resulting from processing are sometimes offset by the addition of vitamins. Some milk is also enriched with multiple vitamins.

During both processings there is little physical or chemical change in the milk fat and a denaturation of milk proteins; however the denaturation process does not affect their nutritional value. The flavour of pasteurised milk is more similar to that of fresh milk than that of UHT milk. This is due to a certain caramelisation of milk sugar that happens in the UHT processing.

One thing we should not forget is that the nutritional value is also dependent on the fat content of milk. Full-fat milk contains considerable quantities of vitamin A, B and D and calcium. Semi-skimmed or skimmed milk contain the same levels of proteins, calcium, magnesium, phosphorous, potassium and zinc and B vitamin as full-fat milk, but the amount of vitamin A and D is either halved or almost non-existent.

Click eufic.org to read the full article.