Milk Chemistry In Espresso Drinks

Recently I switched the type of (cow's) milk I use in espresso-based drinks, from Arla's Cravendale (which is marketed as, and rumoured to be the barista's favourite in the UK) to milk from my local dairy. The milk has approximately the same fat content of just under 2% (semi-skimmed), but it is visibly much more of a solid white rather than having that slightly translucent appearance that Cravendale semi-skimmed has. It also tastes more like whole milk... more flavoursome ... more wholesome.  In a traditional 6oz cappuccino it makes fewer bubbles, smoother microfoam, and results in a more delicious coffee drink. As a barista I just find it to be a better ingredient.

So if we assume a 1oz shot (I'm using volume rather than mass just to make it easier to communicate) and 3.5oz of milk (pre-stretch), that cappuccino consists of (very approximately):

• 2.2% coffee solids
• 20% water
• 77.8% milk

I think that as a breed, we barista are learning more about the smallest part of this cup, the coffee... about the fruit and the beans themselves. Great. Some of us are also learning about the next largest part, brew water... how water TDS is just the tip of the iceberg and different waters dramatically affect the covalent bonds that are broken, and compounds that are created & extracted during the brewing process.  Even better.  So to me a logical progression is to understand more about the ingredient that comprises the largest part of what is in many customers' cups - the milk.

I have so many questions. For example:

1. Obviously, what is it that makes one milk taste better than another? Cow diet, no doubt, but which dietary components lead to which milk flavours? Shouldn't we be speaking more closely with milk suppliers regarding this? 
2. Why does my new milk have less bubbles when steamed? (This one has been discussed on forums and it has been suggested that it relates to the use of supplements in cow feed during winter. I'd like to find other possible reasons.) 
3. What chemical reactions are happening during the steaming process, and how do each of these affect flavour? 
4. The whole "don't stretch the milk after 100F" thing is such an established part of barista training, but what is it all about (what IS denaturing of proteins)?

There is a lot to uncover, it is going to get technical, and I am by no means a natural when it comes to chemistry! So in the meantime maybe it's a good idea to get some 'quick wins'... snippets of info that might open this up a bit.

87% of milk is... water! So even in the average milky cappuccino, around 88% of the cup is actually H2O!!

Lactose is a sugar found only in milk. It is often said that milk sweetness in a cappuccino or flat white comes from the lactose.  However, lactose is about 30 times less sweet than standard cane sugar.

On arrival at a dairy, up to 10% of the content of milk is gas... including CO2 and nitrogen. A source of bubbles, perhaps?

 UPDATE: It seems Morten Münchow is the man when it comes to milk chemistry relating to coffee. Let's see how I get on...