Solid by-products from the orange essential oil industry: could they be a source of antimicrobials?

Nowadays there are several methods for extracting orange essential oils and cold pressing is the most commonly used technique for large-scale production, resulting in a significant amount of residue. However, there is limited research on the antimicrobial potential of these residues remaining after oil extraction. Therefore, this study aimed to evaluate the antimicrobial properties of extracts from orange by-products, particularly against Staphylococcus aureus and Escherichia coli , and to investigate whether fermentation may help to release additional antimicrobial compounds through the metabolism of the residual matrix. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were approximately 84 and 42 mg/mL for unfermented and fermented by-products, suggesting that antimicrobial compounds remain in the orange by-products after extraction. Despite the relatively high MIC and MBC values, even sub-inhibitory concentrations markedly affected microbial growth, decreasing growth rate and reducing overall growth capacity. Extract concentrations significantly affected the growth rate of target pathogens, contrary to the fermentation process. The results of this study open up new possibilities for the recovery of antimicrobial extracts from the solid by-products of the orange essential oil industry, highlighting the antimicrobial activity retained in cold-pressed residues and offering promising prospects for sustainable waste management and resource recovery. • Antimicrobial extracts are obtained from orange byproducts left after essential oil extraction. • Sub-inhibitory concentrations markedly affected S. aureus and E. coli growth. • Sub-inhibitory concentrations decrease the growth rate of S. aureus and E. coli . • Extract concentrations significantly affected the growth rate of target pathogens.