Domesticating extracellular electron transfer processes to extract value from cheese whey: opportunities and challenges

Cheese whey (CW), a major by-product of the dairy industry, is characterized by high organic load and large production volumes, creating significant environmental and management challenges. Conventional biological treatments such as anaerobic digestion (AD) and dark fermentation (DF) enable partial resource recovery, but their efficiency is often limited by process instability, intermediate accumulation, and weak metabolic cooperation within mixed microbial communities. In this context, extracellular electron transfer (EET) has emerged as a promising approach to enhance microbial interactions and process performance. This review explores current CW valorization strategies, focusing on the role of EET in regulating metabolic pathways, shaping microbial community dynamics, and improving system efficiency. DF, AD, and bioelectrochemical systems are discussed as the main valorization routes, with particular attention to electrically conductive materials (ECM) and electrochemical strategies that stimulate direct interspecies electron transfer (DIET), improve redox balance, and promote the production of hydrogen, methane, and other value-added compounds. EET-based strategies can also reshape microbial community dynamics by promoting electroactive and syntrophic interactions and enhancing lactate conversion and carbon/electron flux distribution. Overall, integrating EET with microbial community control could significantly advance sustainable CW valorization within a circular bioeconomy framework.