Application of yeast-based biocontrol agents against fruit spoilage moulds

Ethical clearance number: 219180334/02/2024

Fruits such as apples, oranges, strawberries and grapes are of commercial importance, serving as primary sources of essential growth factors, including vitamins and minerals. However, their production, safety, and economic contributions to the agricultural sector are severely impacted by mould-induced spoilage. Pathogens such as Botrytis cinerea and Penicillium spp. cause substantial losses during pre- and post-harvest stages, with over 50% of fruit losses in developing countries attributed to these fungi. In South Africa, annual fruit losses due to mould spoilage exceed 60%.

Synthetic chemical fungicides are widely used to manage fruit spoilage fungi, but their prolonged application raises concerns regarding environmental safety, consumer health, and the development of fungicide-resistant strains. Consequently, non-Saccharomyces yeasts have emerged as promising, eco-friendly biocontrol agents. These yeasts utilise diverse mechanisms such as nutrient competition, parasitism, and the secretion of antimicrobial compounds to inhibit fungal growth. This study aimed to evaluate non-Saccharomyces yeasts for extracellular enzyme production, antifungal activity against key fruit spoilage fungi, and their viability and stability assessments on fruit surfaces under post-harvest conditions.

Among 23 yeast isolates screened for extracellular enzyme activity, five were selected for further analysis: Aureobasidium melanogenum (Y6), Suhomyces pyralidae (Y63), Pichia kluyveri (Y64), Meyerozyma guilliermondii (Y88) and Zygoascus hellenicus (Y89). These yeasts were tested in vitro using radial inhibition, dual culture, and double Petri dish assays, as well as in vivo post-harvest trials on apples, strawberries, and oranges. The yeasts were evaluated for antagonistic effects against three B. cinerea strains (B05.10, IWBT-FF1, PPRI 30807) and three Penicillium species (Penicillium expansum PPRI 5654, P. italicum PPRI 10380 and P. digitatum PPRI 30517). Compatibility and potential synergistic effects were assessed through yeast-yeast interaction assays.

Extracellular enzyme production varied among the isolates, with Aureobasidium melanogenum demonstrating robust activity for proteases, glucanases, chitinases, cellulases and pectinases. This yeast achieved 55%, 52% and 40% inhibition against B. cinerea strains B05.10, IWBT-FF1 and PPRI 30807, respectively. Pichia kluyveri and M. guilliermondii showed 100% inhibition of B. cinerea spore germination, while S. pyralidae exhibited 100% inhibition for two strains (B. cinerea B05.10 and IWBT-FF1), and 87% for B. cinerea PPRI 30807. Volatile organic compounds (VOCs) such as isobutanol, 2-phenylethanol, and isoamyl acetate, identified using solid-phase microextraction coupled with gas chromatography–mass spectrometry (SPME-GC-MS) were found to contribute to mould inhibition.

During post-harvest trials, S. pyralidae achieved the highest inhibition of B. cinerea on apples with a mean inhibition of 43%, while M. guilliermondii was most effective against P. digitatum and P. italicum on oranges. Commercial fungicides demonstrated higher efficacy in some instances, though yeast treatments provided viable alternative control. Stability and viability assays revealed varying levels of yeast survival on fruit surfaces, with a decrease in yeast cell concentrations observed after oven drying, while stability was maintained during the storage period. The study concludes that the selected non-Saccharomyces yeasts hold significant potential as biological control agents against fruit spoilage moulds. While post-harvest trials demonstrate promising results, further optimisation and field applications are recommended to enhance their efficacy and adoption in agricultural practices.