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The utilization and conversion of glucose to volatile acids were monitored in anaerobic digestors by 13C-nuclear magnetic resonance. Glucose was converted to lactate and acetate. Lactate was subsequently converted to propionate. The distribution of the labeled carbons in propionate suggested that minor amounts were produced via the randomizing pathway and that the major portion of propionate was derived from lactate.

Anaerobic digestors were fed daily with dairy cattle manure (5% total solids) augmented with 0-20 mM glucose and were monitored daily for gas volume and composition and volatile acid content. Propionate accumulated in digestors that were fed glucose at initial digestor concentrations of 10 mM or more. Digestors that received 14-20 mM glucose failed, but identical digestors that received 20 mM glucose plus 10 mM acetate or HCO-3 did not fail. The sparing effect of HCO-3 was primarily buffering and the similar behavior of digestors that received acetate suggest that acetate metabolism perhaps provided additional HCO-3 for buffering. Analyses of H2 and volatile acid concentrations during a 6-8-h period following feeding in digestors fed glucose or glucose plus acetate showed that propionate and H2 accumulated simultaneously and that H2 concentrations were 3 microM or less. Monitoring 13C-labeled glucose metabolism via 13C nuclear magnetic resonance indicated that glucose was primarily converted to lactate and that the major product from lactate was propionate in both glucose and glucose plus acetate fermentations.