Pasteurization is the process of heating a food – usually a liquid – to a specific temperature for the purpose of slowing microbial growth, extending the shelf life of a product by slowing the spoilage process. The technique was first developed to prevent wine and beer from souring quickly, but it is very widely used today with dairy products because pasteurization actually halts the growth of potentially harmful bacteria in milk. In pasteurization, milk is heated to a temperature of 161°F for 15–20 seconds, then quickly chilled to 40°F or lower. The target temperature for pasteurization is below the boiling point of milk because milk will curdle if it boils.
The label “ultra pasteurized” also appears frequently in the dairy aisle, primarily on cartons of heavy cream and half and half. These products are subject to an ultra high temperature (UHT) pasteurization process where they are heated to at least 275°F for a very short period of time. Ultra pasteurized dairy products (the process is used for many products, including juices, etc.) have a much longer shelf life than their pasteurized counterparts because the process eliminates a much larger percentage of bacteria than regular pasteurization does.
In general it does not matter if you use pasteurized or ultra pasteurized in a recipe that calls for milk or cream and there is virtually no nutritional difference between the two. That said, since cream is the product subject to UHT more frequently than milk (in the US, this is the case, while in many other parts of the world UHT is common with milk products to make them shelf-stable without refrigeration for several months), it is worth noting that there can be some small drawbacks to it. UHT can impart a slightly cooked taste to dairy, which might not be noticeable in a baked product but can be detectable in whipped cream. The ultra pasteurized heavy and whipping creams also do not whip up quite as well – although they still will whip up to a fluffy cream – as their pasteurized counterparts and seem to take slightly longer to do so.