My version of borscht doesn't call for an acid like lemon juice or apple cider vinegar. But if that is your personal preference, you may do so at the end of the cooking. Adding an acid balances the sweetness in the red beets. It also preserves a deep red color often associated with red borscht. I prefer to add a fermented brine from fermented jalapenos, sauerkraut, or pico de Gallo for extra nutrition and flavor.