Tang zhong (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 tang zhong, 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 (149°F), 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 tang zhong will keep in the fridge for a couple of days, but will need to be brought up to room temperature before use. The tang zhong is added to the main flour with the liquid and mixed in and kneaded as normal.
The amount of tang zhong used should be about 35% of the weight of the main flour. It is best to make a little extra, because the liquid will evaporate slightly during heating. To make a loaf weighing about 1kg, I would suggest using 480g flour, 200g liquid and 170g tang zhong (made with 30g flour and 150g liquid), which will give a hydration of about 68%. You can of course adjust the amount of liquid either side of the 200g, but the tang zhong proportions should not be adjusted.
Note for Thermomix owners
Tang zhong can be made very easily in the new Thermomix TM5 because it will heat to exactly 65°C. Just weigh in the flour and water 3-4 minutes / 65°C / Speed 4, reducing to Speed 2 about halfway through, leave to cool, then add your ingredients for the main dough as usual. However, please note that as TM31 temperatures only go up in degrees of 10°C , users of this model will need to set the temperature at 70°C. Once it has hit the 60°C mark, leave for half a minute or so then keep checking the temperature with a digital probe until it has reached 65°C.