Abstract
Silage is a critical global feedstock, but is prone to aerobic deterioration. The dominant mechanism of O2 transport into silage remains unresolved. Here, multiple sensors tracked O2 and CO2, gas pressure (ΔP) between internal silage and ambient air, pH and silage temperature (Tsi) during the ensilage of maize and ryegrass. We report the first observation that CO2 produced from microbial respiration was partially dissolved in silage water, with evidence of negative or positive ΔP depending on the changing balance between CO2 production and dissolution. The ΔP < 0 reflected an apparent respiratory quotient (RQ) < 1. Net CO2 production was much greater in anaerobic fermentation stage than in initial aerobic phase or later aerobic feed-out phase. O2 transport into silage is intimately linked to the dynamics of net CO2, ΔP, microbial activity, pH and Tsi. These results suggested that both gas diffusion (based on Fick’s law) and advective transfer (Darcy’s law) play equally important roles in governing the complex temporal progression of inward and outward gas fluxes to and from the silage interior. Even though low pH suppressed microbial activity and supported aerobic stability, the negative ΔP increased the risk of O2 entry and aerobic deterioration during feed-out phase.
Highlights
PH suppressed microbial activity and supported aerobic stability, the negative ΔP increased the risk of O2 entry and aerobic deterioration during feed-out phase
Silage is susceptible to oxidation because it contains aerobic bacteria, yeasts and molds that are dormant under anaerobic conditions
Two physical mechanisms of O2 transport to the silage interior during ensilage and storage have been identified as gas diffusion, which is governed by the concentration gradient between the silage pore space and ambient air, and advective transfer, driven by differential gas pressure (ΔP = Pair − Psilage) across the
Summary
PH suppressed microbial activity and supported aerobic stability, the negative ΔP increased the risk of O2 entry and aerobic deterioration during feed-out phase. The silage-making process is generally divided into four temporal phases: an initial aerobic phase, a fermentation phase, a prolonged storage phase, and a final feed-out phase[3]. The fermentation and storage phases are dominated by anaerobic processes. Silage is susceptible to oxidation because it contains aerobic bacteria, yeasts and molds (fungi) that are dormant under anaerobic conditions. While the biochemical process of silage-making has been mostly characterized, determination of conditions within the developing silage has been challenging. For gas composition and concentration inside the silage, a common method has been to extract gas samples from the silage for later analysis using a laboratory gas chromatograph[8,18,19]. Measurement of pH within the silage during its solid-state fermentation has been difficult to achieve[20]
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