The Potential of Plant Secondary Metabolites as Bread Mould Inhibitors: Exploring Their Individual and Combined Antifungal Effect

Plant secondary metabolites are an interesting source of natural antifungals and offer an alternative to synthetic preservatives. In this study, the activity of 218 secondary metabolites was evaluated against nine <i>Penicillium</i> species and one <i>Aspergillus</i> species, isolated from spoiled par-baked bread. By comparing agar and liquid-based assays, it was found that the hydrophobic nature of these compounds led to an underestimation of the activity in agar-based assays. In liquid medium, it was possible to evaluate the effect quantitatively and differentiate between strong and weak inhibitors. Of the most interesting compounds, the minimal inhibitory concentration (MIC) was determined, and synergistic interactions were studied. This revealed an interesting interaction between benzyl isothiocyanate and carvacrol, which was further investigated through validation in par-baked bread. Antifungal efficacy was assessed in a shelf life and challenge test, revealing that spray application of 200 to 400 µg/mL benzyl isothiocyanate and 1000 to 2000 µg/mL carvacrol significantly increased shelf life. Furthermore, application of benzyl isothiocyanate and carvacrol was as effective as 0.15% propionic acid was incorporated in the dough. A sensory triangle test indicated that benzyl isothiocyanate and carvacrol influenced the flavour of fully baked bread; however, the effect was not perceived negatively.