Lots of questions :-)
First, "sweet whey" refers to whey produced by using rennet to curdle the milk. "Acid whey" refers to using acid. The amount of sugar left over in "acid whey" depends heavily on how you produce it. For example, if you make yogurt and drain the whey from it, it is "acid whey", but it will have a lot less sugar than if you just add citric acid to milk (because the yogurt culture eats the sugar and produces lactic acid. I don't know what the USDA uses for their "acid whey", but I think it is the "citric acid" kind. In a previous thread I compared it to the milk entry and I think the sugar numbers pretty much add up (you could check that in case I'm wrong).
Your premises are basically correct, but I think you are missing some nuances. When you curdle milk, the casein stacks together (either by making bonds with calcium, in the case of rennet cheeses, or by physically sticking together in the case of acid coagulated cheeses). The casein traps fat and whey in the mesh that it builds. How much fat and whey is trapped is dependent entirely on the type of cheese you are making. Parmesan has very little whey in it, while something like cream cheese has a lot. For rennet cheeses, you "cook" the curds -- the heat provides osmotic pressure to expel whey from the curds. The longer the cook the curds, the less whey they have (and the drier the cheese). Some cheeses are "washed curd" cheeses. You remove something like half of the whey from the cooking pot and replace it with hot water. Because they whey contains a lot of salts, you get an exchange of liquid in this case -- whey is transported out of the curds and replaced with the diluted whey. In this way (ha ha) you end up with curds that have a high moisture content, but less whey.
On top of that, after you drain the whey, the cheese continues to ferment the lactose and other sugars. First, before you add salt, the starter cultures are munching away at the lactose and producing lactic acid. For some cheeses, you might ferment it down all the way to a pH of 4.7. For others you might stop at 5.3. Parmesan, in particular, is famous for fermenting until there is essentially no sugar left. Even after you salt the cheese (which slows down the starter cultures to almost no activity), mould and bacteria in the cheese still work away digesting lactose, lactic acid and galactose. So washed rind cheeses, blue cheeses, bloomy rind cheeses, etc, etc will be slowly fermenting away the sugar. Long aged cheeses, even if they don't have mould or bacteria added, still slowly ferment and long aged cheddars, etc have very little sugar left when you eat them.
So, no, the numbers are not reasonable except in a very general way. You will have *completely* different numbers based on the kind of cheese you make. This is why the cheeses taste different ;-)
For the rest of the questions:
- Using rennet doesn't necessarily change anything for sugar. It's the external use of acid, or the use of acid from starter cultures that makes a difference. So if you coagulate the milk with citric acid or lemon juice or vinegar, and add no other starter culture, *all* of the sugar in the trapped whey will be present. If you add a starter culture, then the amount of sugar depends on how long you fermented the cheese before you added salt. There is also a difference between acid coagulated cheeses and rennet cheeses. Rennet formed curds are stronger and the cheese will create a rind if you dry it out. Acid formed curds are really weak (because they don't benefit from the calcium bonds that are possible with rennet formed curds). The cheese does not form a rind and so it will transpire liquid really easily. Finally, acid formed curds do not trap fat as well as rennet formed curds. If you are careful with temperature (keep it as low as possible -- i.e. not over 42 C) you can keep a "normal" amount of fat, but I've noticed that fat tends to drain out pretty easily at higher temps.
- Fat needs to be incorporated with the milk in order for it to get trapped in the casein mesh. You can add a limited amount of cream and suspend it in the milk, but I don't think you will ever incorporate butter appropriately. It tends to simply melt and float on top of the milk. Because of that, it doesn't get trapped in the curds. I've heard of people adding butter to skim milk powder and making a kind of cheese, but I think it would be pretty difficult. I've experimented a bit with butter but gave up pretty quickly. The normal way of increasing the amount of fat in cheese is to start with milk that is naturally high in fat -- either because it is late season milk (which is higher in fat naturally) or because it comes from an animal that naturally produces more fat (Jersey cows often produce up to 6% fat milk, sheep and buffaloes 8%, etc).
- Curds + Whey = Milk. There are no nuclear processes in cheese making :-). However, sugars are converted to lactic acid by the starter cultures.
- I think they basically guess. Most nutrition labels are fiction, unfortunately. Even the USDA database is notoriously filled with errors. You can test to a certain extent, but it is expensive. I don't know how often they test and to what degree of accuracy/precision.
- It *really* depends on how the acid whey was produced. I could write an entire blog post on this subject, but I'll try to keep it brief :-). If you take unhomogenised milk and add citric acid, the whey will contain a fair amount of fat and protein. If you use homogenised milk, you will have much less fat and protein left in the whey (you get about 20% greater yield in the curds, although you need about 30% more acidity to achieve it) I don't completely understand why this is the case, but I think it has to do with the homogenisation process. When milk is homogenised, the fat is broken down into very small bits (the "globs" are broken). The fat often adheres to the proteins. Ironically, this makes it difficult for rennet to do its job (because rennet is an enzyme and it's job is to cut apart the protein). When you use homogenised milk with rennet, you end up with fragile curds and potentially lower yield. For acid formed curds, it is exactly the opposite. It seems that the fat sticking to the protein allows it to stick together better.
So that's one possible reason -- they were measuring with homogenised milk and so almost all of the fat ended up in the curd with the acid whey. My other speculation is that their acid whey is the product of yogurt. If you make yogurt, put it in a fine mesh bag and drain the whey, it has very little fat compared to just adding acid (in unhomogenised milk). I don't exactly know what the reason is, but I suspect it has to do with yogurt having smaller curds.
I can tell you that from experience, adding acid to unhomogenised milk (at 30-40 C) results in a yield similar to using rennet. I can't test it, but I think the protein and fat content is similar. When you make rennet cheese, you can then (almost) boil the resulting whey and add acid to get most of the remaining fat/protein (making ricotta cheese). The resulting whey is very similar to to whey produced from homogenised milk that you add acid to. Possibly if you make a high temperature ricotta with whole unhomogenised milk, you might get more yield in the curds, but I haven't experienced that (I only tried once, mind you).
Hope that helps you get some ideas. Surprisingly quite a lot of the chemistry for cheese making is still a matter of research (nobody knows how cesein micells are put together, for example!) You are unlikely to get precise answers, but it is still fun to look :-)
