 The dominant tastes in borscht are sweet and sour . This combination is traditionally obtained by adding beet sour . It is made by covering sliced beetroots with lukewarm preboiled water and allowing bacteria to ferment some of the sugars present in beetroots into dextran ( which gives the liquid a slightly viscous consistency ) , mannitol , acetic acid and lactic acid . Stale rye bread is often added to hasten the process , but usually omitted in Jewish recipes , as chametz ( leavened bread ) would make the sour unfit for Passover meals . Sugar , salt and lemon juice may be also added to balance the flavor . After about 2 – 5 days ( or 2 – 3 weeks without the bread ) , the deep red , sweet and sour liquid may be strained and is ready to use . It is added to borscht shortly before the soup is done , as prolonged boiling would cause the tart flavor to dissipate . The beet sour is known in Slavic languages as kvas ( literally ' sour , acid ' ; compare kvass ) and in Yiddish as rosl ( from a Slavic word originally referring to any brine obtained by steeping salted meat or vegetables in water ; compare Russian rassol ' pickle juice ' , Polish rosół ' broth ' ) . Apart from its employment in borscht , it may be also added to prepared horseradish or used as pot roast marinade . 
