From dairy by-products to bioproducts: A PRISMA-guided review of cheese whey permeate valorisation using the non-conventional yeast Kluyveromyces marxianus

Cheese whey permeate (CWP), a lactose-rich effluent obtained after whey protein recovery, is commonly regarded as a low-value by-product. Nevertheless, its high lactose content makes it a promising yet underexploited substrate for bioprocess development. Within the framework of modern integrated biorefineries, the non-conventional yeast Kluyveromyces marxianus has attracted notable interest due to its efficient lactose metabolism and its suitability as a microbial cell factory for industrial applications. This review provides a systematic assessment of current knowledge on CWP conversion by K. marxianus , highlighting its potential role as a niche feedstock within circular bioeconomy strategies. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology was applied to identify relevant studies indexed in PubMed, Scopus and Web of Science reporting the conversion of CWP into ethanol, organic acids, aroma compounds, bioactive molecules, and single-cell proteins. Although ethanol production is the most mature and scalable valorisation pathway, alternative routes remain fragmented and insufficiently explored, due to limited and heterogenous nature of available literature. This scarcity underscores the need for structured and systematic evaluations capable of consolidating existing evidence and identifying critical knowledge gaps. Overall, the findings indicate that CWP should not be considered a mere waste stream, but a versatile bioresource for bioprocess development. K. marxianus is a robust microbial platform for converting CWP into a broad spectrum of value-added products. Using a PRISMA-based approach, this review provides a rigorous framework for benchmarking current processes and guiding the future optimisation of cascading biorefinery strategies integrating CWP within advanced circular bioeconomy models. • PRISMA-based review of cheese whey permeate valorisation using K. marxianus . • Cheese whey permeate is a valuable feedstock for bioproducts by fermentation. • K. marxianus shows robust traits for industrial biotechnology. • Production of proteins, bioaroma and probiotics has low technology readiness level. • Ethanol production is the most mature and scalable bioprocess.