The Proto-Indo-European word *kwat-
Word *kwat-
Meaning sour
Synonyms *suhₓros, *h₂emros

Comments
Skr. kvathas, Lat. cāseus, OCS kvasŭ, Russ. kvas, Ltv. kūsāt, Alb. cos, Polish kwas, Goth. ƕ aþō, Eng. hwaþerian/

Other words that might be of interest
*h₃em- ‘bitter’, *sweh₂d-u- ‘sweet’, *h₂em- ‘raw, bitter’, *(s)terh₁- ‘stiff’, *st(h₂)eug- ‘stiff’