What soured borshch
Kvas is the oldest souring agent in borshch — first from wild herbs, later from beetroot and bread. Tomato products displaced beet kvas across the first third of the twentieth century, a shift complete by the 1920s and 1930s, and home-fermented beet kvas is "practically unknown" today. (Lylo & Braichenko, Ukraine. Food and History, Ukrainian Institute, 2020; L. F. Artiukh, commentary to the 1991 reprint of Klynovetska.)
The culture of Ukrainian borscht cooking was inscribed in 2022 on UNESCO's List of Intangible Cultural Heritage in Need of Urgent Safeguarding — the urgent list, with the displacement of people and bearers named as the threat. UNESCO's own summary names fermented beet juice among the bases. (UNESCO ICH file 01852, decision 5.EXT.COM 5.)
The 1913 method, as written
Zinaida Klynovetska's Страви й напитки на Україні (1913), recipe No. 2, «Буряковий квас на борщ»: peel and wash the beets, slice some into rounds and layer them with whole beets in a wooden tub under cold water. «Через два тижні можно вживать» — usable after two weeks. Keep it in the cellar all winter; every time some is drawn off, top it up with water at once, and keep it clean. Five large beets to a bucket of water.
Her Kyiv-style borshch, recipe No. 7, is then built on that ferment: three cups of beet kvas and one cup of bread kvas, let down with water and salted, before anything else goes into the pot. (Klynovetska, 1913, recipes 2 and 7, from the scanned edition at archive.org.)
Garlic, rye crusts, caraway and a seven-to-ten-day ferment all appeared on this page before 1 September 2026. They came from us; Klynovetska’s text has none of them, and they are gone.
What we say about it, and what we leave out
It is a sour, lightly fizzing ferment. The Ukrainian Institute calls it a source of live bacteria, and that is as far as any source we hold will go. Health claims about digestion, immunity or "winter vitality" appeared here once and have been removed.