Pouring steamed milk into a fresh shot of espresso accomplishes a few things:
the sharp acidity is smoothed out
bitter flavours become less harsh
notes like chocolate and vanilla become more pleasant
a velvety foam forms at the surface of the beverage
An espresso macchiato may not cause a sensory jolt like straight espresso, but its coffee flavours will still be strong and clear.
A milkier flat white will be more mellow, smoother, and will have more of that lactose sweetness.
For the most part, alternative milks, like coconut, oat and soya, follow all the same rules. Proteins, sugars, and fats are still essential to nicely steamed milk. Of course, the milk flavours may be different, depending on the source.
