Evaluation of Tea Tree Essential Oil (Melaleuca alternifolia) as a Multifunctional Antioxidant and Antimicrobial Additive for Diesel Fuel

ABSTRACT: Melaleuca alternifolia is a plant native to Australia, and its essential oil is known for its biological properties, including antimicrobial, anti-inflammatory, and antioxidant effects; as a result, it is used in cosmetic formulations and in the pharmaceutical industry. Few studies have examined the use of these properties as an additive in diesel fuel. In this study, the essential oil inhibited visible fungal growth against two of the main fungi that degrade diesel/biodiesel—Bacillus subtilis with a minimum inhibitory concentration (MIC) of 0.5 μL/mL, and the fungi Aspergillus niger and Aspergillus fumigatus with similar MIC values of 0.25 μL/mL. Based on this, the same potential was tested in Melaleuca oil/diesel blends at various concentrations, demonstrating inhibition of visible fungal growth at blend concentrations of 1% (v/v) and above, whereas the control (without essential oil) and concentrations of 0%, 0.25%, and 0.50% allowed fungal growth. Tests for density and viscosity remained within the quality limits established by ANP Resolution No. 968/2024. The addition of Melaleuca alternifolia essential oil also produced a concentration-dependent reduction in insoluble oxidation products, with the 2% blend showing the highest oxidative stability.