Microwave Versus Conventional Acid-Free Thermohydrolysis of Maize Silage: Effects on Methane Yield and Yeast Fermentation

Hydrothermal pretreatment is routinely judged by how much organic matter it releases into solution, yet the value of that release depends on which microbial system consumes it—a premise that is rarely tested because pretreatment is almost always evaluated against a single downstream process. Two independent maize-silage batches were subjected to the same acid-free thermohydrolysis protocol with explosive decompression under conventional and microwave heating (110, 120, and 130 °C, 20 min holding time) and were then evaluated in two conversion routes of contrasting substrate specificity: mesophilic anaerobic digestion of the whole slurry, measured as cumulative methane yield at day 14 across the full temperature matrix, and alcoholic fermentation of the clarified hydrolysate by Saccharomyces cerevisiae KKP 669, applied to the two extreme temperatures only. The two routes responded in opposite directions. Temperature raised the cumulative methane yield from 99 to 241–270 NmL CH4 g−1 volatile solids (VS), with little further gain above 120 °C, while the same protocol reduced fermentative activity by up to 61%. Heating mode had no detectable effect on methane yield: two-way analysis of variance attributed 64% of the variance to temperature (p = 0.001) and 1.7% to heating mode (p = 0.446). Since the same reactor and feedstock gave a clear microwave advantage when an acid catalyst was present, this is consistent with, though it does not by itself prove, a catalyst-dependent heating-mode effect. Microwave heating nevertheless cut the energy input almost fourfold by reaching temperature three to four times faster, bringing the incremental balance close to break-even. Furanic by-products were unlikely to account for the suppression of the yeast, remaining two to three orders of magnitude below reported inhibitory concentrations; the suppression instead tracked the titratable weak-acid load, which rose from 2.0 to 8.8 g acetic acid equivalents L−1 (r = −0.984). Acetic acid is simultaneously an inhibitor of yeast and a direct precursor of methane, so one protocol plausibly released compounds acting as substrate for one route and as toxicant for the other. Solubilisation indices are therefore route-specific and unreliable as general measures of pretreatment effectiveness.