Pichia and Related Non-Saccharomyces Yeasts in Solid-State Fermentation: What Coffee Can Learn and What Still Needs Testing

Coffee fermentation is a critical yet under-optimized stage of the coffee value chain, with significant implications for beverage quality, process consistency, and smallholder livelihoods. Pichia species, together with several non-Saccharomyces yeasts historically associated with the genus, are frequently detected in natural and semi-dry coffee fermentation; however, their functional roles remain insufficiently characterized, constraining evidence-based starter-culture design. Direct coffee evidence implicates P. kudriavzevii, P. kluyveri, and Meyerozyma guilliermondii (formerly Pichia guilliermondii) in substrate transformation, fermentation dynamics, and sensory quality development. However, these findings are derived exclusively from studies conducted on single farms during single harvest seasons, without independent validation across multiple farms, harvest seasons, origins, or production environments, limiting their broader applicability. In contrast, ecologically prevalent species, including P. fermentans, Wickerhamomyces anomalus (formerly Pichia anomala), and Debaryomyces hansenii (formerly Pichia hansenii), remain largely unexplored in the context of coffee fermentation. To contextualize the current knowledge and identify promising avenues for future research, this review synthesizes evidence from cocoa, Baijiu, bakery, vinegar, dairy, and other solid-state fermentation systems. Across these diverse matrices, Pichia and related non-Saccharomyces yeasts frequently exert a disproportionate influence on flavor development, microbial succession, and process stability relative to their population abundance. The reported mechanisms include ester and higher-alcohol production, modulation of bacterial community dynamics, extracellular enzymatic activity, and bioprotective effects. Notably, functional impact often occurs without numerical dominance; ester production can increase alongside declining ethanol yield, and ecological persistence may increase despite declining absolute abundance, implicating community-level interactions in addition to direct metabolic activity. However, both the mechanisms and the magnitudes of these effects vary across fermentation systems, underscoring that functions demonstrated in one matrix cannot be assumed to translate directly to coffee without experimental validation. Future progress will require strain-level characterization, mechanistic studies, deliberate multi-species consortium design, process optimization, and field-scale validation in diverse coffee-producing environments. Such efforts will provide a scientific foundation for evidence-based starter-culture development capable of improving coffee quality, consistency, and producer value.