I once had a conversation with a colleague about the correct way to make tea. He stated that adding milk to hot tea "scalds the milk".
Won't having hot water poured into some milk "scald" it just as much?
(For the record, I've never been able to tell the difference. I'm amazed so many people are so passionate about this question.)
Yes. The demonstration of this fact is actually a famous anecdote in the history of statistics. Ronald A. Fisher, one of the towering figures in the history of statistics, gave the case as an example in the second chapter of his book The Design of Experiments (1951), without mentioning it being based on a true story. Some references can be found on the Wikipedia page for Lady tasting tea. A detailed description of the experiment can be found here, where one can find the following account from R. A. Fisher: The Life of a Scientist (1978) by Box:
Already, quite soon after he had come to Rothamstead, his presence had transformed one commonplace tea time to an historic event. It happened one afternoon when he drew a cup of tea from the urn and offered it to the lady beside him, Dr. B. Muriel Bristol, an algologist. She declined it, stating that she preferred a cup into which the milk had been poured first. “Nonsense,” returned Fisher, smiling, “Surely it makes no difference.” But she maintained, with emphasis, that of course it did. From just behind, a voice suggested, “Let’s test her.” It was William Roach who was not long afterward to marry Miss Bristol. Immediately, they embarked on the preliminaries of the experiment, Roach assisting with the cups and exulting that Miss Bristol divined correctly more than enough of those cups into which tea had been poured first to prove her case.
Source - How to make a Perfect Cup of Tea, Royal Society of Chemistry
Milk should be added before the tea, because denaturation (degradation) of milk proteins is liable to occur if milk encounters temperatures above 75°C. If milk is poured into hot tea, individual drops separate from the bulk of the milk and come into contact with the high temperatures of the tea for enough time for significant denaturation to occur. This is much less likely to happen if hot water is added to the milk.
The British Standard for making tea (BS 6008) also states that tea should be added after the milk:
Prepare the liquor as described in 7.2.1 but pour it into the bowl after the milk, in order to avoid scalding the milk
