matsoni or plain yogurt
Georgian cultured milk, thick and mildly tangy, used here as a starter for home-made cultured milk or as an ingredient; a plain yogurt with live cultures does the same job.
Matsoni, also spelled mazoni, is Georgia's cultured milk, a little thicker and rounder in flavour than ordinary yogurt. It is most likely found in Russian or Eastern European grocery shops, but a plain supermarket yogurt works perfectly well.
Substitute:
Plain yogurt without additives that contains live cultures; heat-treated yogurt will not work as a starter, and the result will be slightly tangier and less thick than true matsoni.