What Is Zakwas? Fermented Starter for Sour Soups

Part of our Heritage Kitchen Glossary – short guides to the ingredients and techniques behind Eastern European cooking.

The short answer

Zakwas is a Polish sour starter: rye flour or beets left to ferment in water for several days until the liquid turns sour. Two Polish soups are soured with it. Rye zakwas goes into żurek, and beet zakwas goes into barszcz czerwony.

How a zakwas is made

For the rye starter behind żurek, wholemeal or dark rye flour is stirred into water that has been boiled and cooled completely, or filtered, with a few crushed garlic cloves and, if you like, bay leaves and allspice. The jar stands somewhere warm and out of the sun, ideally around 25°C (77°F), and most recipes stir it once a day. After four to seven days it should smell pleasantly and distinctly sour, but not sharp. A pale foam on top is normal and can be skimmed off; mould or a bad smell means the batch should be thrown away. Strained, the starter keeps in the fridge for up to two weeks.

The beet starter, zakwas buraczany, is made from sliced beets covered with salted water, with garlic, bay leaves and allspice, and left in a cool, dark place for five to seven days. The liquid turns deep red and smells pleasantly sour and earthy. Once strained, it keeps for weeks in the refrigerator.

Two zakwas, two soups

Zakwas żytni, the rye starter, goes into Traditional Żurek. It is poured in gradually near the end of cooking and tasted as it goes in, until the soup is as sour as you like. Zakwas buraczany, the beet starter, goes into Traditional Barszcz Czerwony. It is combined with a separately simmered and strained vegetable broth in a ratio of roughly three parts broth to two parts zakwas, then seasoned with a little sugar and a splash of vinegar or lemon juice.

On the table the two soups look nothing alike. Easter żurek is creamy and pale, served with white sausage and a halved hard-boiled egg. Christmas Eve barszcz is a clear, ruby-red broth, served in a bowl or even a teacup, and it comes with mushroom-filled uszka.

Why fermentation, not vinegar

Vinegar makes a soup sour at once, but zakwas gives a softer, rounder sourness that vinegar alone cannot. In barszcz czerwony a little vinegar or lemon juice goes in only at the end, to balance the soup. Everyday barszcz can be made without zakwas and soured with vinegar or lemon juice instead, though Polish cooks advise against vinegar as a substitute. For the Christmas Eve bowl, the starter is made ahead.

Where you will meet it on this site

Rye zakwas is the base of our Traditional Żurek, including its meatless mushroom żur for Lent, and of our Healthy Żurek, which keeps the same starter with half the sausage, no bacon and Greek yogurt in place of cream. Beet zakwas sours our Traditional Barszcz Czerwony, and the dumplings served in it on Christmas Eve are in our Traditional Uszka. Start the rye starter four to seven days before you cook, and the beet one five to seven days ahead.

Frequently Asked Questions

What is zakwas in Polish cooking?

Zakwas is a sour starter made by fermenting rye flour or beets in water for several days. Rye zakwas sours żurek, and beet zakwas sours barszcz czerwony.

What is the difference between zakwas żytni and zakwas buraczany?

Zakwas żytni is wholemeal or dark rye flour fermented in water, the starter for żurek. Zakwas buraczany is sliced beets fermented in salted water; it sours barszcz czerwony and turns deep red as it ferments.

How long does it take to make zakwas?

Rye zakwas takes four to seven days in a warm place, and beet zakwas five to seven days in a cool, dark one. Strained, the rye starter keeps in the fridge for up to two weeks and the beet starter for weeks.

Can I substitute vinegar for zakwas?

For everyday barszcz you can cook fresh beets in the broth and sour it with vinegar or lemon juice at the end. Polish cooks advise against vinegar as a substitute, because zakwas gives a softer, rounder sourness.

What water should I use for zakwas?

Published rye starters use water that has been boiled and cooled completely, or filtered water. For beet zakwas, use spring water or tap water that has been boiled and cooled.

What Is Zakwas? The Fermented Starter Behind Sour Rye Soup