ABSTRACTIn the present work, simulated cocoa fermentation was investigated at the level of metabolic pathway fluxes (fluxome) of lactic acid bacteria (LAB), which are typically found in the microbial consortium known to convert nutrients from the cocoa pulp into organic acids. A comprehensive13C labeling approach allowed to quantify carbon fluxes during simulated cocoa fermentation by (i) parallel13C studies with [13C6]glucose, [1,2-13C2]glucose, and [13C6]fructose, respectively, (ii) gas chromatography-mass spectrometry (GC/MS) analysis of secreted acetate and lactate, (iii) stoichiometric profiling, and (iv) isotopomer modeling for flux calculation. The study of several strains ofL. fermentumandL. plantarumrevealed major differences in their fluxes. TheL. fermentumstrains channeled only a small amount (4 to 6%) of fructose into central metabolism, i.e., the phosphoketolase pathway, whereas onlyL. fermentumNCC 575 used fructose to form mannitol. In contrast,L. plantarumstrains exhibited a high glycolytic flux. All strains differed in acetate flux, which originated from fractions of citrate (25 to 80%) and corresponding amounts of glucose and fructose. Subsequent, metafluxome studies with consortia of differentL. fermentumandL. plantarumstrains indicated a dominant (96%) contribution ofL. fermentumNCC 575 to the overall flux in the microbial community, a scenario that was not observed for the other strains. This highlights the idea that individual LAB strains vary in their metabolic contribution to the overall fermentation process and opens up new routes toward streamlined starter cultures.L. fermentumNCC 575 might be one candidate due to its superior performance in flux activity.
A texture-based pixel labeling approach for historical books
