Greek yogurt vs Quark: interchangeable in recipes?

I live in the USA, and I just started making and eating quark myself, making the switch to it from "Greek"/strained yogurt very recently. I don't really use either in recipes much, usually just eating it plain or with some fruit. But I've done a fair amount of web-searching about both, since I started making both myself. For both products, I think the things to remember about their natures is that (1) they are milk products, so the relative percentages of protein, fat, and carbs (lactose) depends on what milk (and any additives) was used to make it; and (2) they are strained cultured milk products, emphasis on strained, so the thickness and moisture percentage largely depends on the degree of straining, which totally varies maker to maker, especially for quark/tvorog/topfen. (The standard degree of straining for Greek yogurt is to about half of the original cultured milk volume; quark varies more but usually is at least that much, sometimes much more than that, often to a more cream-cheese-like consistency.) So how much quark and Greek yogurt are interchangeable depends partially on those factors. If you are concerned about moisture content for recipe substitution, perhaps you might adjust quark to more closely match Greek yogurt by mixing a little milk or water (or whey, if you have it) into the quark.

Regarding fat content, keep in mind that the US and German conceptualizations of fat percentage differ. A US fat percentage refers to the entire weight of the product, so for example 100g of US Lidl whole-milk Greek yogurt ("5% fat") has 5g of fat in it; whereas I think German fat percentages refer only to the dry mass, not to weight from water. I know the fat in regular quark is supposed to be 20% of the dry mass, and I would tentatively estimate it is about equivalent to full-fat Greek yogurt. Magerquark should be comparable in fat to nonfat Greek yogurt, whereas Sahnequark is much higher in fat than any Greek yogurt I know.

Now, another similarity between Greek/strained yogurt and quark, for cooking and nutrition, derives from that they are both curdled milk products: they have been thickened from plain milk by acidification, and that means they should be fairly interchangeable in recipes that use that acidity (e.g. tzatziki sauce and baking). If the milk for the quark is curdled by culturing (some varieties, especially German, are instead curdled by rennet, adding acid, adding heat, or a combination of those methods), and has not since been heat-treated, that quark also shares another characteristic with Greek yogurt: live probiotic content. But whichever curdling method is used, quark usually tastes different from Greek yogurt, either because no bacteria were used or because the bacteria used to culture the quark are different from those for yogurt; quark bacteria are mesophilic, while yogurt's are thermophilic. I think this explains quark's somewhat cheesier taste, which I imagine could be a factor in recipe substitution.

Although I just started making and eating quark myself, I concur that it is a worthy food! It's easy and cheap to make, too, even easier than yogurt because it cultures at room temperature instead of 100-115 Fahrenheit (F). A US gallon of milk makes roughly a half-gallon of whey and a half-gallon of Greek yogurt or quark, so even if you discard the nutritious whey you are still getting almost two quarts (64 oz) for the cost of a gallon of milk! This is my process for cultured quark: 1. Gently heat the milk to 185 degrees F (a simmer) (this de-natures the proteins a bit). 2. Let the milk cool to somewhere between 95 degrees F/35 Celcius and room temperature (minimum of 68 F/20 C). 3. Gently mix in (roughly room-temperature) starter culture - (cultured) buttermilk or your own soured milk, in a ratio of about 4 parts milk to 1 part starter. 4. Let this mixture sit/incubate at room temperature or warmer until the whey is visibly separated to your liking, about 1-3 days for a half-gallon. 5. Strain the result (soured milk/Dickmilch) to your liking, probably either for about 2-3 hours at room temperature or overnight in the refrigerator; there are many different things you can use for this, including the traditional cheesecloth layers, paper towels, coffee filters, bouillon strainers, men's handkerchiefs, old T-shirts or pillowcases, nut milk bags, paint strainers, or my favorite, a pair of laundry fine-mesh bags. 6. (Optional) if the quark is thicker than you want, mix liquid (whey, water or milk) back in after you put the quark in your storage container. That's all you need to do to make quark!

Making Greek yogurt is not too much harder, the process differences being (1) that you mix the starter (yogurt, and a much smaller amount) in once the yogurt reaches 116 F, (2) you have to keep it warmer for the incubation, ideally 105-115 F, and (3) a shorter incubation duration, from as few as 3 hours to 24 hours depending on incubation warmth and on your own taste. Or you could make Greek yogurt simply by straining store-bought plain yogurt.

To calculate nutrition info for homemade Greek yogurt or quark, I use a spreadsheet to subtract the whey nutrition info from the info for the milk and starter I used, to give me my quark or strained-yogurt info, which I then use to create a Food in MyFitnessPal.

Hopefully this is helpful