For people who can't do it (and sometimes even for those of us who can), latte art can seem like the hardest thing there is to do in coffee. Well, it's the second hardest. The hardest thing in coffee is getting that damn sticky mucilage off the parchment.
I'm talking, of course, about coffee processing. If you don't know what I'm referring to with mucilage and parchment, Wikipedia has a fairly good, succinct introduction to coffee processing.
I want to talk about the origins of sweetness in the processing stage, but it might be more accurate to talk about the preservation of sweetness. Processing has a huge influence on the flavor of a coffee, but most of the time it's a matter of not ruining the coffee. You can put in the ripest, most delicate and beautiful high-grown coffee cherries in one end of the coffee mill and get rotten, spoiled, fermented coffee out the other side.
Geoff Watts, green buyer for Intelligentsia and all-around coffee savant has an amusing and apt analogy for the creation/preservation of quality in coffee. He says the bean is like a cowboy walking down the main street of an Old West town in a shoot-out. It's trying to get from the farm to your cup, and the whole way it's being shot at. The coffee has to dodge many bullets to make it to your cup in good condition. Well, the processing stage is like the saloon where most of the bad guys are hiding out... the coffee's got more dangerous bullets to dodge in that stretch of road than anywhere else.
In the processing stage, the two main goals are to get the fruit off the coffee, and to dry the coffee out. There are too many variations in how coffee is processed to get into it in detail here, but there are some universals. If the fruit doesn't come off cleanly or efficiently, you have problems with fermentation and dirty flavors in general. If the coffee is not dried properly, you have problems with phenolic tastes, mold, and a whole host of other problems. And all during these stages, there is the possibility of contaminating the coffee with unclean equipment and/or storage facilities.
So to sum up, sweetness is not created during processing, it is only ever really destroyed or preserved. Good processing is all about preserving the wonderful sweetness that Mother Nature and the farmers cultivated in the first place.
There is one possible exception to this rule, and that is "natural" processed coffee. (I'll use that term to encompass the varied natural styles, like sun-dried natural, "pulp natural," etc.) In these cases, the fruit is being dried onto the parchment coffee. As you can imagine if you've ever eaten a raisin or a dried apricot, it tends to concentrate the sticky sweet sugars in the fruit. The bean inside this fruit then tends to "soak up" some of this sticky sweetness, resulting in the unique flavor characteristics you get from natural-process coffees.
There's a lot of controversy surrounding natural coffees in the specialty industry. Some consider their flavors to be, by definition, defective. I don't particularly feel like wading into that quicksand right now, but I will say that I happen to adore well-cared-for natural coffees, and some of the greatest coffee experiences I have ever had were with natural coffees. Any discussion of sweetness and processing would be incomplete without mentioning naturals.
In general, though, with naturals as with washed coffee, it's all about not getting shot, so to speak. That bean has a lot farther to go before it ends up in your mouth as sweet, delicious coffee....