Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage

Natural preservatives are gaining attention as chemical-free solutions to extend produce shelf life and prevent microbial spoilage. Therefore, sweet potato peel (SPP) was investigated as a source of antifungal bioactive compounds in this study. We evaluated essential oils and, for the first time, a bound terpene (BT) concentrate extracted from SPP against Aspergillus flavus, using both in vitro and in vivo assays. Murasaki organic Japanese sweet potato (Ipomoea batatas L.) peels, A. flavus AF13, a highly aflatoxigenic fungus, and Creole tomato (Solanum lycopersicum) fruits were used in the study. Essential oils were extracted by hydrodistillation (HD) and vacuum distillation (VD), while the BT fraction was isolated and concentrated. HD and VD yielded 19 and 10 terpenes, respectively, with linalool and α-terpineol dominating and representing more than 50% of total terpenes in both distillates. The BT concentrate demonstrated significant inhibition of A. flavus growth at concentrations starting from 12.5 µL/mL. The strongest effect was observed at 100 µL/mL, with a 26.0 ± 1.0 mm inhibition zone and 55.56 ± 4.53% growth reduction. In contrast, HD and VD distillates showed no antifungal activity in either in vitro or in vivo assays. Consistently, the BT concentrate-treated tomatoes reduced fungal growth and spoilage, with lesion diameters less than 10 mm after 7 days of storage, while the HD and VD distillate treatments had diameters over 20 mm, and the untreated control had diameters over 60 mm. These findings highlight that SPP waste could be an economical and bio-based source for developing natural antifungal ingredients. The success is anticipated to offer a potential alternative to current synthetic fungicides in preventing fungi A. flavus-induced spoilage of nightshade vegetables.