NOVELTY - Fermenting biomass comprises: (a) fermenting biomass in a first fermenter (110 a) to form digested biomass and a first fermentation broth; (b) introducing the digested biomass from the first fermenter to a second fermenter having a second fermentation broth; and (c) introducing a nutrient and/or carbon source to at least one of the fermenter.    USE - The method is useful for fermenting biomass (claimed), which is useful for producing chemicals and fuels.    ADVANTAGE - The method utilizes carboxylate source, which economically and favorably provides high carboxylate product yields in the fermentation broth.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) method (I) for fermenting biomass comprising: (a1) fermenting a first biomass in a first fermenter to form a first digested biomass and a first fermentation broth, (b1) fermenting a second biomass in a second fermenter to form a second digested biomass and a second fermentation broth, (c1) fermenting a third biomass in a third fermenter to form third digested biomass and a third fermentation broth, (d1) detecting at least one property of each of the fermentation broths for analysis, (e1) introducing the first digested biomass to the second fermentation broth in the second fermenter, (f1) introducing the first fermentation broth to the third digested biomass in the third fermenter, and (g1) introducing the third fermentation broth to the second digested biomass; and    (2) a fermenter system (100) comprising: many fermenters, having a first fermenter, a last fermenter, and at least one intermediate fermenter, where the first fermenter comprises the inlet for biomass, and the last fermenter comprises the inlet for fermentation broth; many conduits disposed between each of the fermenters; a sensor system, having a sensor (150) positioned in each of the fermenters; and a nutrient supply, fluidly connected with each of the fermenters.    DESCRIPTION OF DRAWING(S) - The figure shows a fermenter system.    Fermenter system (100)    First fermenter (110 a)    Carbon-rich sources (130)    Nutrient-rich sources (140)    Sensor (150)