Quark is one of my favourite foods and I've sorely missed it while not living in Estonia. It is incredibly versatile — you can use it as is, mix it into other things, mix other things into it, use it as a condiment or main meal, make it into a sauce or a cake or other things. There are two recipe books published in Estonia that only have quark recipes in them. I need to take a peek in the older one and buy the new one when I visit Estonia next time.
Wikipedia defines quark as a type of fresh cheese (Quark (cheese)), which is sort of right, but misleading for folks who think of cheddar or parmesan when they hear the word cheese. Quark is nothing like that.
Wiki:
Quark (or qvark) is a type of fresh cheese of Central European origin. Dictionaries usually translate it as curd cheese. It is soft, white and un-aged, similar to Fromage frais. It is not the same thing as cream cheese or cottage cheese. It is distinctly different from ricotta because ricotta (Italian: recooked) is made from scalded whey. It also differs from the other cream cheeses in its usually much lower fat content (about the same as yoghurt), and it is completely salt free.
As you notice, the description is more about what it's NOT like than what it's like. The difficulty is that any description of quark anyone can come up with will sound like some kind of cheese they are familiar with, but quark is not really like those cheeses. Confusing, isn't it?
If it's not really like any other cheese what it is like, then? It is a dairy product made from soured milk. There are two commercially produced varieties that I'm aware of (I'm generalising here, bear with me) — smooth and grainy. The smooth variety is made into multitude of products mostly meant to be consumed as a snack or a dessert, a few are meant for cooking. The smooth variety resembles ricotta in texture, but not in flavour. The Swedish Kesella is one of the smooth ones.
The grainy variety is usually more sour than the smooth one and is mostly used for cooking, but can be made into desserts that mimic the store bought smooth ones. The grainy variety does not have grains as large as cottage cheese, the grain size is 1mm at largest and mostly less than that. It is grainy only compared to the smooth variety. I haven't seen the grainy variety sold in Swedish, Finnish, Irish or UK stores I've been to. To my knowledge it is only available in Germany, Estonia and Eastern Europe.
I am not going to go into commercial production methods because they won't apply to anyone wanting to make some themselves. I've found three recipes for making quark at home that sound plausible and have tested one. It yielded grainy quark that tasted like the real thing, had the same mouth feel, smell and texture. I'd meant to make it into a quark cake, but ate it all up after tasting because it was perfect.
These are the plausible recipes I've found:
How to make quark curd from sour milk (method I've tested).
How to make quark curd from buttermilk.
How to make quark curd from milk NB! this is a PDF.
My Mum tried the sour milk method as well and reported that her results with store bought sour milk were not as spectacular as mine. I suspect it is because I used organic sour milk and she used ordinary sour milk. I am very interested how quark made from unprocessed milk soured at home would taste like. Richer, probably. Milder, possibly. Delicious, certainly.