The water content of sterilised milk shouldn't be any different than that of pasteurised milk. The main difference is that sterilised milk is heated for longer and the milk sugars are partially caramelised. Sterilised milk was an early form of homogenised milk, i.e. no cream layer on top, typical of bottled pasteurised milk in the time we are taking about. Perhaps that was what made the difference?I was advised by a young Yorkshire girl on how to make perfect Yorkshire Pudding. Always use sterilised milk the less water content the better and make the mixture at least 24 hours before and keep it in the fridge. Beef dripping in oven till it is on the verge of smoking. IT WORKs.
Batters should be allowed to stand for at least an hour to allow the proteins and starches to absorb water.