Valorization of tropical fruit seed wastes for biodiesel production: A review of extraction technologies, catalytic systems, and biorefinery perspectives

Biodiesel is emerging as a promising renewable energy source; however, the primary feedstock resources for biodiesel are edible oils, which creates a conflict between food security and fuel production. Although non-edible and waste-derived feedstocks for biodiesel production have been widely studied, a comprehensive review of tropical fruit waste seeds as seasonally available, low-cost feedstocks, encompassing an assessment of extraction technologies and catalytic performance, remains unexplored. This review concentrates on the utilization of tropical fruit waste seeds, including rambutan ( Nephelium lappaceum ), papaya ( Carica papaya ), tamarind ( Tamarindus indica ), watermelon ( Citrullus lanatus ), orange ( Citrus sinensis , Citrus reticulate , Citrus tangerine , and Citrus aurantium ), mango ( Mangifera indica ), and jackfruit ( Artocarpus heterophyllus ), as sources for biodiesel production. It provides a comprehensive review of various biodiesel feedstocks, oil extraction methodologies, and catalytic transesterification processes, highlighting their respective advantages and disadvantages. The review also offers insights into current trends and prospective developments in the field. Additionally, it summarizes the valorization of solid residues generated by fruit processing industries. The findings emphasize the potential of tropical fruit waste seeds as alternative raw materials for biodiesel synthesis and propose a sustainable approach to waste management and renewable energy demands.