This is a soft, smooth curd cheese with a refreshingly light acidity. It ranges from low-fat quark made with skimmed milk to very rich varieties to which cream is added.
Originally and properly, quark is the German name for soft, acid-curd cheese; that means the milk might be curdled by the addition of lemon juice or vinegar but often milk was allowed to curdle through the development of natural acidity (souring) and then gently heated to stop this process and to make the curd slightly more solid. Draining of the curd is minimal, keeping the cheese soft and moist.
These days quark might be helped along by rennet which curdles milk by a different process, and that essentially keeps it sweet. Generally quark is sold with a good amount of liquid whey still present, but this can be drained away if it’s too wet for your purposes.
Speciality shops and supermarkets often sell quark made in Germany. However, any sort of soft cheese can be substituted for it, whether as a spread, an ingredient or in baking. The differences will be of acidity, fat content and moisture content.
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Quark and its equivalents are all meant to be used quite fresh, when the acidity offers a pleasant, refreshing tang. Modern packaging extends life but only until opened, after which it should be used quickly.
Keep refrigerated and sealed but if using it as a spread it is less acidic tasting when at room temperature or slightly below.
Quark can be eaten as it is. It's a substitute for other spreads and an excellent base for dips or sandwich fillings, with herbs or with puréed or chopped vegetables, nuts or fish; by adding a little sugar or cream it makes a delicious accompaniment to fruit.
In cooking, quark is classically used in baked German cheesecakes and thus can be used in whatever recipes you use, baked or unbaked. You might have to adjust other ingredients according to the wetness or acidity of the quark.