TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The method comprises, detecting at least one property of the fermentation broth in each of the fermenters, and analyzing at least one property of the fermentation broth in each of the fermenters, prior to step (c). Detecting step comprises measuring a concentration of the nutrient and/or carbon source, or fermentation product in the first and second fermenter broth and analyzing comprises determining the difference in the concentration of the nutrient and/or carbon source, or fermentation product in the first and second fermenter broth. The detecting step and analyzing step comprise determining the difference in the concentration of nutrient and/or carbon source, or fermentation product in the first and second fermenter broth. The method comprises determining the difference in ratio of the concentration of the nutrient to the concentration of the carbon source in the first and second fermenter broth. The nutrient comprises introducing undigested biomass. The method comprises: introducing the nutrient to the first fermenter and introducing the carbon source to the second fermenter; introducing the nutrient to the second fermenter and introducing the carbon source to the first fermenter; and introducing the nutrient and carbon source at a predetermined ratio. In method (I), the detecting step further comprises: comparing at least one detected property of each fermentation broths against each other and against a predetermined optimization of the at least one detected property; and determining which fermentation broth to introduce to which digested biomass. Preferred Components: The fermentation product comprises a carboxylate product. The nutrient comprises undigested biomass and essential components for life processes. The carbon source comprises undigested biomass and any biologically available carbon source for essential life processes. In the method (I): at least one property comprises pH, nutrient concentration, carbon source concentration and/or nutrient concentration to carbon concentration ratio; and the first biomass comprises undigested biomass, and the second biomass and third biomass comprise at least partially digested biomass.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: In the fermenter system, many conduits comprise: a first portion of many conduits configured to convey biomass between each of the fermenters; and a second portion of many conduits configured to convey fermentation broth between each of the fermenters. The fermenter system further comprises a control system, where in response to the sensor system, the control system is configured to control flow through many conduits and the nutrient supply. The nutrient supply comprises at least one undigested biomass, nutrient-rich (140) supply and/or carbon-rich (130) supply. EXAMPLE - No suitable example given.