Info:

Tangzhong (also known as a ‘water roux’) is a method used in breadmaking to create soft and fluffy bread which was originated by the Japanese. However, it was popularised throughout south-east Asia in the 1990s by a Chinese woman called Yvonne Chen who wrote a book called The 65° Bread Doctor. Using this method also allows bread to stay fresh for longer without needing to use artificial preservatives.

To make the tangzhong, you mix together one part flour with five parts liquid (by weight) to make a smooth paste. This is usually water, but can be milk or a mixture of both. The mixture is then heated in a saucepan until it reaches exactly 65°C , removed from the hob, covered and left to cool until it is down to room temperature, when it will be ready to use. It would be useful have a digital thermometer with a probe when making this as other types of thermometer tend to be too large. If you are not making your bread immediately, the tangzhong will keep in the fridge for a couple of days, but will need to be brought up to room temperature before use. The tangzhong is added to the main flour with the liquid and mixed in and kneaded as normal.

Source:

https://www.cookipedia.co.uk/recipes_wiki/Tang_zhong