Single-cell proteins: fermentation pathways, nutritional quality and digestibility, and a techno-economic and environmental outlook for sustainable scale-up

The growing global population and rising living standards are driving an increased demand for protein-rich animal-derived foods. However, traditional livestock-based protein production poses serious environmental concerns, including greenhouse gas emissions, land degradation, and high-water usage. Single-cell proteins (SCPs), defined as protein-rich microbial biomass derived from organisms such as bacteria, yeasts, fungi, and microalgae, have emerged as a promising alternative. SCPs offer high nutritional value with essential amino acids such as lysine, methionine, and threonine, along with additional nutrients including carbohydrates, fats, vitamins, and minerals. This review aims to provide a comprehensive evaluation of SCPs as a sustainable and scalable protein source, with a specific focus on their production through diverse fermentation strategies including solid-state, semisolid-state, liquid, and gas fermentation. Particular attention is given to the use of agro-industrial by-products and renewable gases as cost-effective substrates that align with circular economy principles. In addition to outlining the nutritional and environmental advantages of SCPs, this review critically examines their current limitations, including production scalability, potential safety concerns, and consumer acceptance. By integrating recent findings on life cycle assessments, nutritional profiling, and digestibility, this work highlights key technological innovations needed to accelerate industrial adoption of SCPs. The purpose of this review is therefore to synthesize current advances while identifying the challenges and research priorities that must be addressed for SCPs to achieve widespread application in global food systems.