Heap fermentation as a case-based ecological context for interpreting variability in solid-state fermented foods

Heap fermentation is a characteristic stage of sauce-flavor Baijiu production and represents a complex open solid-state fermentation system shaped by gradients in temperature, oxygen availability, and moisture. Rather than a universal model, it is treated here as a well-documented case through which observations from an industrial fermentation can be organized and evaluated. This review synthesizes current knowledge on microbial succession, functional groups, and metabolite formation during heap fermentation and discusses these observations in relation to ecological assembly perspectives. The evidential scope and limitations of commonly used approaches are examined, including amplicon sequencing, quantitative assays, multi-omics analyses, spatial measurements, and modeling methods such as network analysis and reaction-diffusion simulations. Particular attention is given to uncertainties in linking community composition with process behavior and to practical constraints of applying advanced analytical tools in open fermentation environments. Studies have reported that environmental gradients are associated with shifts in microbial populations. Open inoculation, however, contributes variability among batches. Across published reports, some studies report relatively stable fermentation outcomes, although the mechanisms remain unclear. Accordingly, heap fermentation is discussed as a case-based context for a broader question relevant to many solid-state fermentations: why similar operating conditions yield reproducible outcomes in some cases but variable results in others. The review organizes available evidence, highlights current limitations and identifies key questions that may require further investigation.