Identity
Saccharomyces ferment filtrate — INCI Saccharomyces Ferment Filtrate — is the filtered liquid from fermenting yeast (Saccharomyces), sold as a conditioning "essence." It sits at the centre of the "miracle water / pitera" mythology, but chemically it is a variable fermentation broth: amino acids, sugars, vitamins, and organic acids whose proportions depend on the strain and the fermentation.
Two separations matter. First, it is a blend, not a single molecule, so it is graded on its own evidence, not on any one constituent. Second — and the point marketing most often blurs — it is a different ingredient from Galactomyces ferment filtrate, Bifida ferment lysate, and Lactobacillus ferment. Those are other micro-organisms with their own (also limited) evidence; "yeast ferment" is not a single interchangeable thing.
Mechanism (as proposed)
On skin, the filtrate acts as a humectant, amino-acid-rich conditioner: it holds water and leaves a smooth, hydrated, more radiant-looking surface. That is the supportable, appearance-level benefit, graded C given the limited independent human evidence.
What the evidence does not support is the headline promise — "renewal", "anti-ageing", the "miracle" transformation. Radiance from a ferment essence is a conditioning, surface effect, not cellular renewal, which would be a treatment claim outside cosmetic scope. Ferment essences are an established, pleasant part of K-beauty and J-beauty routines; judged honestly, they hydrate and condition rather than renew.