Gilbert van Reenen, of Wanaka, asks :-

Why does UHT milk boil faster in a microwave oven than the same volume of standard homogenised milk with identical composition.

Clive Bleaken, the principal technologist with the Fonterra Co-operative Group, responded.

I haven't checked your observation but a scientific test would require identical conditions and well-characterised milk because there are various differences. Let’s look at the things which are presumed to be the same, and check that they actually are.

Standard homogenised milk and Standard 3.3 per cent fat UHT milk should have very similar gross composition, but could have minor composition differences, due to time of season, area it is sourced from etc. Milk composition varies with season, climate, type of feed the cows eat, and time since calving. While most producers standardise milk for fat and protein, there can be slight variations from nominal values, and lactose along with other minor components can vary. To eliminate this factor you would have to use exactly the same milk, process some by normal pasteurisation, and some by UHT.

The question states ‘same volume’, which is not the same mass, as milk density (normally about 1.032 g/ml) can vary due to slight compositional changes and also dissolved air. Remember milk is about 88 per cent water which contains significant dissolved air. Mass is the important factor when considering the absorption of microwave energy.

Were the containers actually identical, as they will also absorb energy, mostly by conduction from the heated medium? The shape and mass of the containers are important factors.

Were the containers in exactly the same position in the oven, so that they were exposed to exactly the same microwave energy? Also was the power supply voltage to the microwave exactly the same in both trials.

The question states ‘time to boil’. How was the point of boiling measured? It would be more accurate to measure the milk temperature after a constant heating exposure.

The proteins in the UHT milk have been subjected to heating, typically 144C for 4-6 seconds, which will cause partial denaturation, especially the whey proteins. However I wouldn’t have thought this makes much difference to their ability to absorb microwave energy.

Without knowing precise details of the process under which it was manufactured (they can be significantly different), it is probable the milk has been subjected to a deaeration step in the UHT sterilisation process which will remove most of the dissolved air. This does not occur with normal pasteurisation process. On balance, I suspect this is the most likely factor, as it will also impact on the density and mass.