Advances in Coffee Drying: A Comprehensive Review of Traditional, Solar, Mechanical, Hybrid, and Emerging Methods

Drying is a critical stage in the postharvest chain, shaping product stability, quality, and economic value. Freshly harvested beans contain high moisture, and inadequate drying can lead to microbial growth, physical deterioration, and loss of key sensory attributes. In recent decades, diverse technologies have been developed to enhance drying efficiency while preserving flavor, improving consistency, and reducing environmental impacts. This review adopts a systematic and comparative approach, synthesizing peer-reviewed literature on conventional practices, advanced solar dryers, mechanical systems, hybrid configurations, and emerging techniques such as microwave, infrared, and desiccant-assisted drying. Emphasis is placed on heat and mass transfer mechanisms, the influence of environmental and operational parameters, and the role of varietal and processing differences. Comparative analyses reveal trade-offs between energy consumption, drying kinetics, and impacts on physical and chemical quality. Sustainability aspects are also examined, including energy use, carbon footprint, water consumption, and scalability for smallholders. Finally, key research gaps are identified, particularly in multiscale modeling, real-time monitoring, and integration with renewable energy and smart control systems. The review highlights pathways for achieving greater consistency, lower environmental burdens, and stronger value chains in producing regions worldwide.